Centrality as an Explanation for Racial Differences in Intelligence
of Economics and Finance
of New Orleans
Orleans, La. 70148
Access to Intelligence Raising Mutations Determines Population
Diffusion of Genes
Foraging Populations with Population Density Varying
The Role of Agriculture
Boat Migrations and Trading
Negative Selection for Intelligence
Implications for Other Genes
Implications for Variability in Intelligence
is affected by many different genes. It has also plausibly been subject to unidirectional
selection. Calculations show that favorable mutations would move at a rate that
was slow relative to the time since modern human symbolic culture emerged. This
makes it very likely that geographical differences in the frequencies of various
intelligence related genes exist. With unidirectional selection in a polygenetic
system, it is meaningful to talk about some areas being more advanced than others
(since there is a direction in which all are moving). Centrally located populations
will normally be more advanced. Genes will move faster in thinly populated areas.
The thinly populated areas can serve as genetic freeways that carry genes rapidly
including agriculture, the horse and the ship, accelerated the spread of mutations.
The horse caused the Eurasian steppes to become a genetic highway that transported
favorable mutations across Eurasia. This probably caused these areas to reach
high levels of intelligence ahead of other areas. Areas without horses or ships,
such as sub-Saharan Africa lagged. Peripheral areas such as Australia and the
Americas also lagged due to isolation from the large populations of Eurasia.
Keywords: Intelligence, race, population genetics, unidirectional selection.
survey of the intelligence of the world's peoples Lynn (1991a) found that the
highest levels were found in people that evolved in Eurasia (Mongoloids and
Caucasoids), with low values found for those that evolved in Africa (Negroids).
explanations that have been offered for the evolution of racial differences
in intelligence have involved differing strengths of selection for intelligence
in various regions. Climate has been the most common source for differential
selection for intelligence.
These theories have argued that the intellectual demands of life in cold climates
was greater than in warm climates. Lynn (1991b) has placed emphasis on the intellectual
abilities needed to survive cold, to build fires, and to hunt in groups. Rushton
presented a theory involving r versus K selection. Miller (1991) has pointed
to the need to store food to survive the winter and how this may have selected
for intelligence. He has also (Miller, in press) argued that one of the advantages
of intelligence was that it helped in detecting deception in mates and potential
mates, and that this ability was more important in cold climates than in warm
ones. The implicit assumption in these models is that the same alleles were
present in virtually all populations. Thus, intellectual differences between
populations must reflect differences in the strength of selection for intelligence.
to be presented here is that some populations were reached more quickly by more
of the mutations that produce high intelligence. These became the more intelligent
populations. Other populations, those that were less accessible to intelligence
increasing mutations, lagged in intelligence. Thus, the populations reached
by the largest numbers of such mutations would have the highest average intelligence.
Most populations experienced selection for intelligence, although its strength
may have differed.
several stylized facts (well established generalizations) that will be used
in the argument.
of the current human variation in intelligence is genetic (Bouchard, 1993; Bouchard,
Lykken, McGue, Sega, & Tellegen, 1990; Jensen, 1981, Plomin, & Loehlin,
1989; Rowe, 1994).
genetics research suggests the absence of a single gene for intelligence. Instead,
intelligence is affected by a large number of different genes (Plomin, Owen,
& McGuffin,1994; Plomin, McClearn, Smith, Vignetti, Chorney, Venditti, Kasarda,
Thompson, Detterman, Daniels, Owen, & McGuffin, 1994; Plomin R., McClearn,
G., Smith, D., Skuder, P., Vignetti, S., Chorney, M., Chorney, K., Kasarda,
S., Thompson, L., Detterman, D., Petrill, S., Daniels J., Owen, M., & McGuffin
P., 1995; Skuder, P., Plomin R., McClearn, G., Smith, D., Vignetti, S., Chorney,
M., Chorney, K., Kasarda, S., Thompson, L., Detterman, D., Petrill, S., Daniels
J., Owen, M., & McGuffin P., 1995).
suggests 50 genes, each contributing about 3 IQ points is of a plausible order
of magnitude. Jinks & Fulker (1970, p. 343) conclude that at least 22 loci
seem to be controlling IQ. Later (p. 344), using data for inbreeding depression,
they conclude that about 100 genes seem to be showing dominance for high IQ.
has been unidirectional selection for intelligence in much, if not all, of the
world. The fact of inbreeding depression suggests that many of the alleles that
contribute to low intelligence are recessive, with the alleles contributing
to high intelligence being dominant. This suggests that the genes for high intelligence
have been the subject of a continual process of directional selection (Jinks
& Fulker, 1970, p. 343). Because directional selection acts very slowly
in eliminating recessive genes, but quickly to increase the frequency of dominant
alleles, a high average level of dominance suggests long continued directional
selection for a trait. It is not known exactly what selected for intelligence
in the course of human intelligence, but plausible candidates include the intellectual
requirements of survival, and the social needs to get along with fellow humans.
4. It will
be presumed that each individual has an equal probability of experiencing an
intelligence raising mutation (regardless of the population they live in). This
is standard genetic theory, since no population differences in vulnerability
to mutations are known. Weakening this assumption would not change the nature
of the argument.
genes spread slowly under prehistoric conditions in which humans were divided
into tribes that only occasionally exchange genes with each other. There is
evidence that humans have built in mechanisms that make them ethnocentric and
suspicious of outsiders (Reynolds, Falger, & Vine, 1987). While this does
not completely prevent contact and interbreeding between human populations,
it does greatly reduce it. Hiorns
& Harrison (1977) compute gene frequencies for 300 generations for 10 populations
in a row with one starting out with a gene frequency of 100% and the other zero.
Their graphs show that for a migration rate of 5% each generation (the percentage
of the populations that marries into the adjacent tribe) and a selection factor
of .01, even after 300 generations, the populations are still easily distinguishable.
Their analysis led the authors to conclude (pp. 440-441), RThis analysis clearly
stresses the limitations of migration and reinforcing selection as homogenizing
influences on between-population variety in short-term evolution. It seems unlikely,
for instance, that should some populations have become fixed for a Tgeneral
improvementU gene since the end of the Palaeolithic, or even since the advent
of the species Homo sapiens as we know it, gene flow and selection would have
distributed the genes very far in space or that it would have achieved an appreciable
frequency in many of the populations it had reached, through these agencies
(1989) uses Fisher's (1937) wave of advance model for the spread of advantageous
genes to make this point. Fisher showed, by using a diffusion model, that after
a gene was established in a deme there would be a wave of advance for which
V= (1/2)s(s)1/2, where V is the velocity of the wave, and s is the selective
advantage of the allele. The measure of the parent offspring distance is provided
by s which in turns equals me2, where e2 is the area of the deme, and m is the
migration rate between demes. The parent offspring distance depends on the average
distance between centers of adjacent demes, and the percentage of the population
that migrates between demes. Increasing the area of the demes increases the
distance between them. This increases the average parent offspring distance,
and hence the rate of gene flow. This effect will be important later in the
argument when the effects of introducing the horse and the ship are considered.
uses plausible parameters (selection coefficient of .01, 5% migration between
demes, demes 500 people and 5000 square miles) based on characteristics of current
hunter-gatherers, and concludes that advantageous genes would advance at .8
miles per generation. For a favorable mutation to go from South Africa to the
China coast would require 400,000 years. Weiss & Maruyama (1976) and Livingstone
(1992) use similar assumptions to arrive at somewhat faster rates of advance,
but still quite a slow one.
be noticed that this is a long period of time relative to the time that many
authorities believe has passed since humans left Africa. For instance, if humans
reached China 100,000 years ago, a favorable mutation that occurred in China
would not have reached Europe or Africa yet, nor would mutations occurring in
the latter areas have reached China. Thus, if the original hunter-gatherer social
pattern had remained in place, there could be many favorable mutations that
are found in only certain parts of the world, simply because there has not been
time for them to spread to other parts of the world. This makes regional variation
in frequencies of intelligence relevant genes virtually certain.
one possibility that should be noted. If one population had a sufficient advantage
over other populations, possibly because of their intelligence, or the weapons
or organization it made possible for them, this population could expand at the
expense of the other populations. Their expansion just distributes the genes
for intelligence faster than they would diffuse in the standard stepping stone
models. The replacement of the Neandertals by anatomically modern humans could
be such an episode. Such replacement can be seen in the fossil record (although
even here disproving evolution in place is difficult).
differences between populations left no evidence in the fossil record (and the
differences between more and less intelligent individuals in modern populations
are typically not the sort that would be apparent in skeletal remains), several
such rapid replacements could have occurred that left no fossil record, and
these may have played a role in disseminating genes for intelligence. Later
in this paper, dispersals due to the coming of agriculture, the horse, and ocean
going ships will be discussed. The period since the emergence of modern symbolic
intelligence is short relative to the time required for mutations to spread
around the world (see below).
from when they adopted their current symbolic culture, humans have had their
current level of intelligence for a relatively short period of time. Of course,
there is no direct measure of early human intelligence. However, Stringer &
Gamble (1993) document the absence of modern symbolic culture before the upper
Paleolithic. Noble & Davidson (1991) argue that there are no signs of symbols
in the archaeological record before 32,000 years ago. White (1982) summarizes
the differences between the middle and upper Paleolithic, most of which can
be interpreted as evidence for greater intelligence in the upper Paleolithic.
Binford (1982, p. 178) states his impression that "the ability to anticipate
events and conditions not yet experienced was not one of the strengths of our
ancestors prior to the appearance of clear evidence for symboling." Intelligence
might almost be defined as "the ability to anticipate events and conditions
not yet experienced".
culture of prehistoric man was at a very low level before the emergence of anatomically
modern man, and gradually increased. The rate of progress was very slow. Although
it is just barely possible that humans had a high level of intelligence long
before they developed evidence of a sophisticated material culture, and merely
did not display their intelligence, the simplest explanations for the long period
of a primitive, non-symbolic culture is that humans had not yet developed sufficient
intelligence to do more (again see discussion in Miller, 1995). The
interesting thing is that the period in which humans have had a symbolic culture
appears to be short in comparison with the time required for genes to diffuse
around the world. If high intelligence is recent, the mutations that produced
the high intelligence would not have time to reach all populations.
above discussion shows why genes, even subject to favorable selection, are likely
to diffuse slowly, with at any given time
there being many favorable mutations that have reached some populations but
the nature of the argument to come clearer, imagine that each favorable mutation
raises the IQ of the individual carrying the mutation by one IQ point. The IQ
of an individual will then be determined by the number of favorable alleles
he has inherited. The average intelligence of a population will then be determined
by the number of favorable alleles that have reached it, weighted by the percentage
of the population that has inherited each allele. If, as appears to be the case,
genes can go to fixation in one population before they have even reached other
populations (see the simulations in Livingstone, 1992), the intelligence of
a population can be conceptualized as determined by the number of favorable
mutations that have reached it. Let us explore the implications of this simple
to Intelligence Raising Mutations Determines Population Average Intelligence
what the above stylized facts imply for the distribution of intelligence among
populations. With selection for intelligence, a major determinant of the average
intelligence level in a population will be the number of genes favorable for
intelligence that have
reached the population. As will be shown below, a major determinant of the number
of favorable alleles that reach a population is that population's location,
with the more peripheral populations receiving fewer favorable alleles.
about the diffusion of advantageous genes on a straight line, it would seem
that the middle would be likely to have been reached by more favorable genes
than either end. As
the diagram shows, the largest number of advantageous mutations would be expected
to have reached the center. The easiest way to see this is to consider a period
of time just sufficient for a gene originating at either end (i.e. either A
or C) to have reached the center. Now consider a point not at the center, say
at B. The genes originating at the left side B would still have reached it,
but there would be an area on the extreme right, near C, from whence mutations
would not yet have been received. Since this argument can be made for all points
not at the center, it follows that the population with the highest expected
number of favorable mutations will be the one located at the center. The highest
value for a polygenetic trait such as intelligence subject to unidirectional
selection is expected to be at that point.
point is simple, but most discussions of the evolution of human traits seem
to have presumed that the lines in the above diagram would be horizontal, and
that human intelligence was rising in a uniform manner. The slowness of gene
flow makes this unlikely. If we think of the world as three lines joined at
the Middle East (Africa, Asia, Europe), the Middle East would be expected to
have received the largest number of advantageous mutations, and the peripheral
regions to have received the fewest. The prediction is that during foraging
times, the Middle Eastern populations would have had the largest number of alleles
conducive to high intelligence. If agriculture had not come, one might have
expected this pattern to have persisted into modern times.
For a flat
plane the highest level of intelligence would be expected at the center. The
argument can be seen on Figure 2. It shows how a favorable mutation originating
at C spreads outwards in concentric circles. The diagram can also be interpreted
as showing the areas from which which favorable mutations will have been received.
At point C, all mutations occurring before T1 will have been received from the
area encompassed by the circle labeled T1. All mutations occurring since time
T2 inside the circle labeled T2 will have reached point C. The further back
one looks, the larger the area there is to draw on for mutations. Now imagine
a continent shaped like the ellipse. Consider points A and C. All mutations
arising within the areas encircled will have reached both points. The circles
are the same size, with the result that the maximum distances from which mutations
can reach them are the same.
for the hypothetical population at C, the land area within its circle (the circle
labeled with B and D) is much less than for the circle centered at A. It follows
the peripheral population at C will have benefitted (on average) from fewer
mutations than the population at point A. Thus, it would be expected that the
population at point A would have a higher intelligence than the population at
this is a simple point but the prediction is that the more centrally located
populations will have been reached by the most mutations. Hence, they will be
the most intelligent. It
should be noticed that the above prediction that intelligence should be higher
in the more centrally located regions is a logical deduction from several generally
accepted facts: that intelligence is affected by numerous genes, that intelligence
has been selected for during relatively recent prehistoric times, and that favorable
mutations diffuse slowly. Anyone wishing to dispute the conclusion, that the
centrally located populations should have a higher frequency of the genes for
intelligence, has to dispute one of the generally accepted facts, or dispute
the logic. Either is hard. The predicted forager pattern might roughly fit the
data if Australia and America are thought of as peripheral regions with small
populations (i.e. few advantageous mutations) and slow diffusion of advantageous
genes originating elsewhere. Both Australia and America have lower intelligences
than Eurasia (Lynn, 1991a). Before
discussing the implications of agriculture, let us discuss further how fast
favorable genes might migrate in a foraging population. This speed should not
be taken to be constant.
Populations with Population Density Varying
tribes are roughly constant in population size (as they are believed to be),
the distance to the boundaries of a tribe
will be much less in low population density areas. In such low density regions
the population ranges over larger areas, and bands
will be separated by larger distances. Those of marriageable age have to look
much further to find mates. Thus genes would actually flow fastest through such
areas. In essence, they would travel many more miles before they hit a tribal
A few wide
ranging tribes could pick up the genes at one end, have them increase in frequency
within their populations, and then
transmit them to the other end of their territory. Once this had been done,
many more miles would have been covered than would have happened in a more densely
populated region, where many tribal boundaries would have had to be crossed.
One implication is that thinly populated areas like the Sahara may have been
less of a barrier to the flow of favorable genes than thought (they could remain
a barrier to neutral genes) (Miller, 1994a). This
argument of course makes the differences observed between the north and south
of the Sahara desert more puzzling Cavalli-Sforza, Menozzi, & Piazza (1994).
Probably most of the genes for which it appears a barrier are selectively neutral,
or subject to only weak selection. With only diffusion, a small desert population
can have little impact on the adjacent populations. Thinly
populated areas actually may act as genetic freeways.
the situation shown in Figure 3:
is a tribe. The wide rectangles (only one of which is completely shown) represent
low population density tribes. A
mutation appearing in any square is equally likely to be transported to any
adjacent square, where it then proceeds towards fixation. After a period of
time, the mutation is carried to an adjoining tribe, represented by a rectangle.
Thus, the arrows leaving square A show how a favorable mutation might be transported.
After the first period, the mutation is present in adjacent populations only.
If the whole diagram was like the lower two rows, there would be waves of mutations
moving across the diagram in the manner Fisher described, but it would take
a long time for a mutation arising at one end to reach the other end.
the situation with low density populations to the north. Mutations arising in
the lower squares will quickly reach the
upper rectangles. They will then move rapidly east and west. These low density
regions then serve as freeways transporting advantageous mutations to other
regions. As illustrated, the mutation reaches B and D in the first period. In
the second period, the mutation is transported to each of the rectangles adjacent
to those reached in the first period. It can be seen that the distant rectangle
E is reached at the same time as the much closer rectangle C. The mutation was
transported to the low density area D, where in the course of dispersing within
the tribe it was quickly carried to the far boundaries. From there, it readily
diffused to the population E. Thus, the low population density area served as
a genetic freeway transmitting the favorable mutation from A to E, much quicker
than if it had to traverse the intervening series of small squares. The tribal
boundaries are major obstacles to gene flow. The number of boundaries to be
crossed determines the speed with which favorable mutations move. The number
of boundaries to be crossed in going A, B, C is the same as the number crossed
going A, D, E. Thus, tribes C and E will receive the mutations at about the
same time, even though E is much further away.
might be even more rapid if the populations were very mobile themselves, as
might happen for pastoralists (see
discussion below), or northern groups following mobile herds of game (i.e. reindeer).
The steppes of Eurasia might have been such a freeway, picking up genes from
populations around the boundaries of Eurasia and carrying them to the other
ends. They may have delivered them to Europe and North Asia at either end.
evidence on the size of tribes is thin, but it does appear that the area occupied
by a tribe increases as the environment
deteriorates. There is a tradition in anthropology that tribes have about 500
members. If tribes have an equilibrium size of about 500 (see Kelly, 1994, for
an evaluation of this tradition), in low density areas the tribes must occupy
larger areas in order to reach
this population size (which is argued to be roughly the number required to provide
adequate mating opportunities).
Birdsell's (1953) examination of the relationship between the area occupied
by an Australian tribe and the rainfall showed that tribal areas were larger
in the drier climates. After excluding the tribes which depended on resources
that were not likely to be affected by local rainfall (island tribes, coastal
tribes, and tribes on large rivers fed from outside the tribal territory) and
R by the elimination of tribes in which cultural factors modify the size of
the population from the assumed constant of 500 persons,S the area occupied
by a tribe (Y) and the rainfall (X) was: Y=7,112.8 X-1.58451
goes up as the square of a linear dimension, the linear dimensions of the territory
of the tribe (L) will vary as approximately X-3/4. Thus, the distance between
tribal boundaries increases as the rainfall declines. In very dry areas, such
as the interior of Australia or the prehistoric Sahara, distances between tribal
boundaries would be appreciably greater.
In a low
density area, like the Sahara, the genes should have to cross fewer tribal boundaries
to cover any given distance. As Birdsell (1951: 282) put it in discussing Australia
in an earlier paper, considered
in terms of the rate and ease of gene flow, the great, forbidding, arid desert
spaces of the central portion of the continent represent freeways, rather than
obstacles, to gene exchange between distant populations.
evidence, from the spread of Carpentarian characteristics in Australia, that
genetic diffusion is indeed as would be predicted from a model where tribal
boundaries are an obstacle to gene flow, and the boundaries are further apart
in areas of low rainfall. This leads to the hypothesis that in Eurasia, as in
Australia, the central part of the continent with its low population densities
may have constituted a freeway that permitted favorable mutations to move faster
than they would have if population densities were higher. For instance, mutations
originating in the densely populated area of China could have moved through
populated Central Asia to Central Europe faster than if they had to diffuse
along the coast of China, reach India, traverse northern
India, and then moved through the Middle East, and the Balkans.
Role of Agriculture
discussed above how Rouhani (1989) estimated that the rate of progress of advantageous
genes would be only .8 miles per generation for a hunter-gatherer population.
This slow speed is predominantly due to the low intermarriage rates across tribal
boundaries, which in turn arises from tribes being endogenous. Genes can move
faster if whole populations move, since even on foot, people can move faster
than .8 miles per generation. It is not known how often favorable genes were
carried forward by the movement of tribes, although it is easy to imagine that
of genes could have been faster with the coming of agriculture. It has been
argued that agriculture was spread by demic diffusion. A settled way of life
increases the population growth rate, and farming populations would be expected
to expand into adjacent areas that were inhabited only by foraging populations.
Menozzi, & Piazza (1994, pp. 108-111) present evidence that the expansion
of agriculture into Europe was at about the rate of one kilometer per year.
The kilometer per year advance of genes by demic diffusion would far exceed
the .8 miles (1.3 kilometers) per generation estimated (see above) for a foraging
population. With a generation of 20 years, this is about 25 times as fast as
genes would diffuse through a foraging population.
suggests genes of the agricultural population were carried along with the expansion
of agriculture. There was a continual mixing of the expanding agricultural population
with the hunter-gather populations of the regions they were moving into. Thus,
any advantageous mutations that had occurred in the hunter- gather populations
would probably be picked up and carried by the expanding farming populations
into new areas. This wave of advance carried both the original genes of the
first population to adopt farming, and of the intervening foraging populations.
The Middle East is likely to have been a central area during the hunter--gathering
period when intelligence was high.
be noticed that the accelerated pace at which genes move during a demic diffusion
of agriculture is a one time effect. After the spread of agriculture, population
density would increase. It would then be expected that tribes would shrink in
size and the distances to be traveled to find a mate would decrease. In addition,
agriculture involves a sedentary life which would reduce the natural movements
of foraging people which might bring them into contact with other groups. The
situation might come to resemble that in New Guinea with a very large number
of tribes each speaking their own languages, and doing relatively little intermarrying
(Cavalli-Sforza et al. 1994). This would cause the post-agricultural revolution
rate of spread of new mutations to decrease to a level below that of the foraging
not all areas would have adapted agriculture. Those areas that were too dry,
or otherwise unsuitable would have retained a foraging life style. These thinly
populated desert and steppe areas would remain areas of low population density
able to serve as genetic freeways moving genes from one area to another. In
particular, the thinly populated Central Asian steppes may have served as a
genetic freeway connecting the densely populated agricultural areas of China,
India, the Middle East, and Europe. With mutation rates being the same in the
different populations, most of the favorable mutations would occur in the agricultural
areas where most of the world's population lived. However, the thinly populated
Central Asian area would have received these genes and transmitted them to other
parts of Asia.
the farming population there was selection for intelligence, the genes once
introduced into a population would move towards fixation. This demic farming
diffusion model predicts higher intelligence levels in the farming communities
after the agricultural expansion, because more of the genes for intelligence
would have reached them.
also be expected that the greatest number of genes for intelligence would have
reached the areas that were settled at the
end of the agricultural expansion. The reason is that the wave of advance would
have picked up the largest number of favorable mutations. To illustrate, consider
a series of tribes arrayed along a straight line A, B, C, D, E, F. An agricultural
among A. Its expansion brings it into C. A favorable gene that had emerged in
C passes into the expanding population, and begins to increase in frequency.
Meanwhile, the favorable gene is carried with the wave of advance. Additional
favorable genes may be picked up from D and E. Thus, when the wave reaches F,
it is likely to have picked up any favorable genes that were in the foraging
populations that the wave traversed. Thus, it is the populations that are last
reached by the wave of advance that are likely to have picked up the largest
number of favorable genes.
would hold even if the first population to adopt agriculture was no more intelligent
than other foraging populations. It could even work if the farmers were less
intelligent. However, the first farmers may have been more intelligent. It is
possible that intelligence was needed to conceive of the idea of planting crops
and cultivating them. This is not to say that the first to conceive of planting
seeds and farming actually did so.
Hunting and collecting takes less work than farming. It is likely that the fact
that seeds grew into plants, and that farming was possible was discovered several
times. It was then promptly forgotten since it was easier to gather what nature
had already planted. However, eventually population may have risen to the point
where adequate food was not gathered by hunting and gathering, and planting
increased the food supply. At that point, someone conceived of farming, implemented
the idea, and encouraged his descendants to do so. The first farmer was very
likely more intelligent than most. He was also likely more farsighted since
he could visualize the harvest vividly enough to inspire him to do the work
of planting for a return that will come only many months from now. Once farming
was adopted by a few pioneers, their descendants were more numerous, and their
genes spread. The initial advantage was partially a better food supply, but
it may have also been the adoption of a sedentary way of live that permitted
women to have the next child before the first was old enough to walk long distances
on its own (Lee, 1972 as cited by Ammerman & Cavalli-Sforza, 1984, p. 64).
The problem of carrying two children at once in a migratory lifestyle is believed
to have limited foragers to having only one child young enough that could not
walk at his parents' pace. Because adopting farming required intelligence, and
because farmers probably out reproduced others, farming's appearance would have
been accompanied by increased intelligence.
even if the initiation of farming did not require any special intelligence,
the first farming population may have been unusually intelligent. The first
farmers are believed to have been Middle Eastern. As discussed earlier, the
Middle East is a central
region receiving favorable mutations from Asia, Europe, and Africa. Thus, at
any given time, populations in this area are likely to have received more favorable
mutations than populations in other regions. Evidence is that farming was carried
into Europe from the Middle East by movement of farming populations. Such movements
would have carried the genes for intelligence that had reached the Middle East
from Africa and Asia into Europe (Europe presumably already had the genes that
had emerged there). This demic expansion from the Middle East into Europe can
explain why modern Middle Easterners do not appear to be more intelligent than
Europeans, even though the earlier theoretical argument suggested that a greater
number of favorable genes should have reached them.
et al. (1994) report that the gene frequency differences between European populations
are relatively small and that European gene frequencies resemble those of the
Middle East. In dendograms (descent trees) the branch leading to Europeans is
often relatively short. A very interesting study discussed by Cavalli- Sforza
et al., (1994) analyzed only a few populations (including Chinese, Europeans,
two populations of African pygmies, and Melanesians), but collected data on
a very large number of alleles. A
tree constructed from this data showed a very short branch leading to the Europeans
(p. 91). Several explanations were considered, but the most plausible was mixture.
Calculations showed that the European gene frequencies could be explained well
by a mixture of Chinese with a smaller percentage of pygmies. Obviously, this
is not the actual racial history of the Europeans (who are both taller and lighter
skinned than either group, for instance). The pygmies are fairly close to other
Africans in the frequency of their measured genes (the set of measured genes
frequencies includes no genes that affect height) according to their data.
surprising result is most easily explained by the current European gene frequencies
reflecting a relatively recent (in prehistoric terms) migration of a Middle
Eastern population that was in turn a mixture of Asians and Africans (or at
least one which regularly received genes from both). After expansion, the gene
frequencies were frozen, and drift did not change them much from those that
had existed in the Middle East. Renfrew
(1991) and Barbujani, Pilastro, Domenico, & Renfrew (1994) argue that not
only do European gene frequencies suggest demic diffusion from the Near East,
but evidence of such demic diffusion can also be found in the areas occupied
by the speakers of Altaic languages, and by the Asian speakers of Indo-European
and Elamo-Dravidian languages, and possibly the Afro-Asian languages. The basic
argument is that agriculture emerged in the Near East among several groups in
the Fertile Crescent. One wave of expansion swept into Europe carrying the Indo-European
languages with it. The existence of this wave is documented by the distribution
of gene frequencies in Europe, and by archaeological evidence which shows a
steady advance of farming at about 1 kilometer per year. It is hypothesized
that another wave, possibly starting in the Zagros foothills of Iran, led to
an expansion of the populations that become Elamo-Dravidian speakers. This wave
reached as far as southern India. Later, Indo-European speakers expanded into
Iran and North India, leaving the Dravidian speakers isolated in South India,
with a couple of relict populations isolated along the expansion path. Of course,
these theories are highly controversial, with most linguists rejecting the idea
that the origins of the language groups go back as far as the origins of agriculture.
speakers are argued to have spread north from the Fertile Crescent area, expanding
all the way to Korea and Japan. Along the way they would have had the chance
to pick up and spread genes for intelligence in the large area from the Middle
East to Japan. Finally, the Afro-Asian speakers spread from the Levant into
Egypt and then on to the rest of North Africa. Even if these arguments are rejected,
the existence of these language groups is generally agreed to be due to the
languages having a common origin. The spread of the language from the area of
common origin must have been accomplished by the movement of people, even if
only small groups of conquerors (see discussion below). Of course, if the movements
were later than the origin of agriculture, there would be less time for favorable
genes to be selected for.
case of all of these expansions, the argument being made is not that the early
farmers were necessarily any more intelligent than the foragers whose territory
they expanded into. Instead, the argument is simply that, due to the mixing
of genes from a larger area, a larger selection of advantageous genes would
have reached the populations affected by the agricultural expansion. If the
genes were merely neutral, the resulting mix would be a weighted average of
the gene frequencies of the constituent populations. However, with directional
selection for higher intelligence, having a wider selection of intelligence
related genes for selection to work on would have resulted in the eventual evolution
of higher intelligence. Thus, even if the evidence of a common language family
reflects only a conquest, a few new genes would have been introduced.
had spread into an area, the rate of gene flow would be expected to decline
again. Agriculture would support a denser population, and one that was less
mobile. In a denser population the distance that must be traveled for a mate
is less, and the average distance between the partners in a marriage is less.
Languages and dialects would differentiate, and these differences would prevent
marriages between different groups. Tribal groups would come to occupy smaller
areas, and would be expected to be endogamous. Thus, the rate of diffusion of
genes would be limited by the boundaries of the tribal groups, and the short
distances from one boundary to the other would again limit the gene diffusion.
The situation might come to resemble that in New Guinea where there are large
number of tribes occupying a relatively small area, with large linguistic and
genetic differences between the tribes. In such an environment new intelligence
raising genes would spread very slowly. (That is, they would spread slowly unless
something again happened to cause large scale migrations of new peoples).
be noticed that if the numerical size of demes remains constant (say at the
traditional 500), changing population density
uniformly does not change the rate of advance of a trait undergoing unidirectional
selection, such as intelligence. The total number of mutations at any given
distance is increased as population density increases, but the number of deme
boundaries to be crossed is also increased as each deme comes to occupy a smaller
area. A way to see this is to think of the demes as being in a hexagonal grid,
with the demes arranged in concentric circles around the deme one is interested
in. At any given time, the mutations (if any) from a certain number of demes
away are reaching the target deme. Changing the size of the demes does not change
the number of boundaries that must be crossed for mutations arising say 20 demes
away to reach the target deme. If, say after 100,000 years, mutations 20 demes
away are just reaching the target deme, it makes no difference how large the
demes are in a model in which members of a deme are equally likely to mate with
any other member of the deme. The distance to the deme that is 20 demes away
just happens to be less when the population density is lower.
as pointed out, where the problem is expressed as time to cross a specified
distance, lowering population density lowers the number of boundaries to be
crossed, thus speeding up the time required for a gene to cross the boundaries.
the heterogeneity in the population densities will decrease the number of boundaries
to be crossed to connect two distant demes, since the gene flow will be through
the low density areas between the centers for high population. The coming of
agriculture probably did increase the heterogeneity of population density. The
areas that adapted agriculture were the areas of higher rainfall, which were
probably already areas of relatively high population density. Agriculture just
increased their population densities further. The areas of low rainfall, which
were already areas of low population density, would have remained foraging areas
of low population density. Thus, the heterogeneity increased.
evidence for several migrations after the early spread of agriculture. These
are the migrations that are usually interpreted as giving rise to various major
language groups (The Renfrew hypothesis discussed earlier that agricultural
expansion gave rise to the Indo-European language groups is a minority view).
For instance, there is a linguistic similarity between the various languages
of the Indo-European group which extend from India to Western Europe. This is
usually explained by these languages having a common origin, implying that the
speakers of the proto-Indo- European language once lived in an area small enough
to have a common language, estimated by Mallory (1989, p. 146) at 250,000 to
1,000,000 square kilometers. Obviously, for the Indo-European language to now
cover the very large area they do cover, there must have been an expansion of
the language, which was almost certainly caused by a movement of at least some
people, even if just a few conquers. Mallory estimated that the proto-Indo-Europeans
were in their homeland 4500-2500 years B. C. They expanded from this homeland.
Why they expanded is not definitely known, but one plausible explanation is
the domestication of the horse, and the advantage this gave them in warfare.
of the horse occurred around 5,000 BC or earlier. This innovation cut traveling
times by a factor of five or more, nullifying whatever territorial boundaries
had previously existed... Riding provided the ability to strike out over great
distances, instigated cattle looting or horse-stealing raids, the accumulation
of wealth, trading capacities, and the development of violence and warfare.
Material remains of the first half of the 5th millennium B. C. show that in
an enormous territory east of the Don River and between the Middle Volga, the
Caucasus Mountains, and the Ural Mountains there spread a uniform culture.S
(Gimbutas, 1991, p. 354). Very likely this uniform culture arising from the
mobility horses permit mixed the genes thoroughly, and much more quickly than
normal diffusion could have mixed them. Gimbutas and others have argued that
the advantage of the horse would have led to the expansion of the first peoples
to have mastered it. This would have rapidly spread intelligence promoting genes.
The effects of the initial expansion were followed by a period of faster gene
flow resulting from the horse based culture. Not
only is the horse a major asset in warfare, but a pastoral economy seems to
lead to an emphasis on fighting. This is basically because the development of
an economy based on livestock changes the cost benefit-ratio for raiding, making
it a much more economical source of food. Livestock is easily driven away. In
contrast grains and tubers must be carried away (and perhaps even harvested).
Foraging people seldom have much worth raiding for (other than women). Faced
with the threat of raids, those owning livestock are forced to develop fighting
skills to defend their livestock. Even today, herding people seem more oriented
towards fighting. Since the horse domesticators were probably a pastoral people,
they would be expected to have developed a livestock raiding culture. With the
military advantage of horseback riding adding to culture oriented toward fighting,
they very plausibly could have expanded into surrounding peoples, as Gimbutas
(1991), Mallory (1989), Anthony (1986) and others have argued. The case for
such an expansion is based on both archaeological evidence, and the widespread
prevalence of the Indo-European languages. Such an expansion presumably carried
for the spread of Indo-European languages, it is not necessary for the original
inhabitants of an area to be displaced. Conquest by a relatively small group
can lead to the adoption of the conquerors language. The classic example is
the adoption of Turkish in what is now Turkey, which is known to be the result
of a conquest by a relatively small number of Turks.
the introduction of a small number of advantageous genes would not require many
people, especially if the leaders of the
conquering army were more likely to be carrying the desirable genes. It is very
likely that achieving and retaining leadership of a conquering army was facilitated
by intelligence. It is also very likely that the conqueror's leadership had
an above average chance
of leaving their genes, through either marriage or rape. Once the genes had
been introduced into a population, if there was selection for such genes, they
would gradually increase in frequency. Genetic evidence for an expansion from
the steppes exists.
et al. (1994, p. 293 and fig. 5.11.3) found that the third principal component
for European gene frequencies showed an area of extreme values north of the
Black Sea, with what appear to be roughly concentric circles around this area.
They point out that this is consistent with an expansion of the Kurgan culture
from the steppes of Europe such as Gimbutas argued for. They also note that
Scythians were in the same area later, and also invaded Europe. It could also
have been the original homeland, or an intermediate long-term homeland for some
of the other barbarian populations that later invaded Europe.
original homeland is a subject of disagreement. For the sake of discussion,
imagine it was in the steppes north of the black Sea (as above) or north of
the Caspian Sea as hypothesized by Gimbutas, with expansion from here going
into Europe and further east into Asia. Any mutations for intelligence between
the homeland and Western Europe would have been swept up by the migrating populations
and spread into Europe. In the long period of time since 4500-2500 years BC
there would have been time for these genes to benefit from selection, and to
increase in frequency. Similar
effects could occur with other homelands, although the magnitude of the effect
might vary. For instance, if nomadism was introduced into the steppes from the
farming populations on its western edge (in the Ukraine or Rumania), the steppe
populations might initially have had gene frequencies similar to those populations
and a later movement into parts of Western Europe might have brought fewer new
genes. However, they still might have picked up genes from further east in the
course of subsequent movements, and then brought these into Western Europe.
Of course, with a more western origin of the steppe nomads, the latter movement
into Iran and India would have brought into these areas genes originating in
western Europe. Thus, regardless of where the population that spoke proto-Indo
European is believed to have originally lived, the movements of the parts of
this population that spread the Indo-European languages would have spread intelligence
promoting mutations, the more favorable of which would have been selected for.
for the above effect to occur, it need not be argued that the new arrivals were
more intelligent that the conquered. They may have been less intelligent, with
the conquest's immediate effect being to lower the average intelligence. However,
if the new arrivals had genes for intelligence that had not yet reached the
original population, the net effect after a long period of time, could have
been to have raised the intelligence of the combined populations above that
of any of the original populations. For instance, consider the unrealistic case
where genes have become completely fixed in a population. The settled farmers
have two favorable mutations fixed, and the invading populations one. The invaders
are lower in intelligence. The immediate effect of mixing the populations lowers
the intelligence below that of the original residents. However, with selection
for intelligence after many generation the newly merged population may come
to have all three alleles in high proportion, and to have an intelligence higher
than that of either of the predecessor populations. Thus,
to argue that the Indo-European expansions contributed to raising intelligence
one does not necessarily have to argue that the Indo-Europeans were themselves
superior in intelligence. They may have merely played a role in spreading desirable
mutations widely. Of course, the original Indo-European expansion is just one
of many expansions by steppe horse riding populations.
of steppe peoples are known to have occurred in history such as the barbarians
that invaded the Roman Empire
(Cavalli-Sforza et al. 1994, Fig. 5.2.6), and later the Magyars and Mongols.
It is very likely that the steppes of Eurasia were traversed in both direction
several times by horse mounted conquerors. China was repeatedly conquered by
horseman from the steppes of Asia (Cavalli-Sforza et al. 1994, p. 201-202; Darlington,
1966). In particular the Hun that sacked Rome, under Attila, those that attacked
India under Mihirakula, and the Hsuing-nu who threatened China were apparently
the same group, spreading their genes over this vast Eurasian area (Kust, 1983,
p. 36). Any favorable mutations arising between China at one end, and Europe
at the other were probably diffused throughout the Eurasian region. Similar
arguments could be made for these other expansions, although after the later
expansions there would be less time for natural selection to increase the frequency
of any desirable genes introduced. Again, if the conquerorsU leaders were more
likely to carry genes for intelligence, and these leaders fathered many offspring,
newly introduced genes might have had a head start that was more important than
might have been guessed from the numbers in the conquering army.
horse borne long distance movements are known, including that of the Turks (see
Mallory, 1989, p. 147 map), or that of the Arabs out of Arabia into North Africa,
Spain, etc. Each of these could have spread favorable genes.
It is also
very likely that horse riding societies permitted choice of spouses over relatively
large areas. The horse provided mobility,
and the lack of the attachment to a fixed place that farming people had would
permit groups to gradually change their locations.
Pastoralism often leads to seasonal movements which may cover from 20 to 1000
miles or more (Cavalli-Sforza et al. 1994, p. 200). These may bring the nomads
into contact with different sedentary populations at different times, or into
contact with other nomadic populations. Hence, genes can be presumed to have
moved very rapidly in the areas of the world populated by horse mounted people.
Thus, the great steppes of Eurasia appear to have been a freeway that transported
desirable mutations from one end of Eurasia to the other end. This happened
long enough ago so that there were often several hundred generations for selection
to increase the frequency of the genes that made for intelligence. The result
was the evolution of high intelligence within the peoples within the reach of
the horse riding Eurasian populations. These appear to have extended from Japan,
Korea, and China at one end to Western Europe and North Africa at the other
of access to the horse have slowed down the spread of genes? Many areas of the
world lacked access to horses till recently. The Americas, Australia, and New
Guinea lacked horses because they were isolated by water from Eurasia, where
the horse was domesticated. Sub-Saharan Africa is contiguous to areas that used
horses, but because of the tsetse fly did not have domesticated horses. In these
areas populations would have moved on foot. Tribal size would have been smaller.
Favorable mutations would have moved slowly. Over time they would come to have
a lower level of intelligence than the lands whose gene flows were facilitated
Migrations and Trading
method that could carry favorable mutations over long distances is boats. Once
long distance boat transport had emerged, mutations could cover long distances
without having to diffuse slowly through populations.
example of such long distance boat based migrations is that of the Phoenicians
who settled such places as Carthage. This could have moved mutations at an even
more rapid rate than horse based migrations.
be noted that the Phoenicians were located at the Middle Eastern crossroads
where they may have received genes from Asia, Europe, and Africa. Their early
colonies could have transported these genes to distant places from whence they
spread. This may have contributed to the spread of Middle Eastern genes throughout
with other innovations, it is plausible that the people that first perfected
long distance ship transport may have been above average in intelligence. If
this was so, the ability of the ship to carry them long distances would have
dispersed their genes widely. From the initial colonies, the genes would have
spread to adjacent peoples, and then spread rapidly in frequency.
after the Phoenician era, the Greeks established a far flung set of colonies,
extending to the Black Sea Coast and around the Mediterranean. After that the
Roman Empire emerged. It was centered on the Mediterranean and experienced large
scale migrations of peoples. This could have easily transported favorable mutations
from one end of the Empire to the other. With a large number of generations
since this era, natural selection could have served to raise the frequency of
the desirable genes throughout the Mediterranean region, and areas in contact
be noticed that the ship also led to extensive long distance trade. If ancient
sailors and merchants were like modern sailors, they left genes behind them.
From the ports, genes could have easily spread inland.
the traders and ship captains were well above average in intelligence. It is
very likely that soon after a intelligence increasing gene reached a population,
some of those receiving the gene went into the intelligence requiring profession
of trading (where those with the high IQ have a comparative advantage). Since
the traders tended to be travelers, the gene may have been at an over 50% frequency
among those going on trading trips even when it had a much lower frequency in
the population as a whole. This could speed up the diffusion of the gene among
people who did long distance trading.
It is also
very likely that offspring of traders became traders themselves, and that the
traders in a community intermarried extensively. This could make an advantageous
gene move faster than with random marriage. Suppose for instance an advantageous
gene emerged in Central Asia and was carried to the Black Sea's east coast.
One could easily imagine a Phoenician trader bringing home a concubine or slave
carrying the gene. This match could easily give rise to a son who then signed
on to participate in a trading trip to England's Cornwall tin mining district,
where he mated with a prostitute. Thus, in two generations a gene could make
it half way across Eurasia. This sequence of events is much more likely if traders
were drawn from sons of traders, than if they were randomly selected from the
whole Phoenician population. In the latter case, it might take many generations
for the gene to slowly increase in frequency before it reached someone who was
making a Cornwall trip. Such long distance gene transport would be of little
importance for neutral genes, since the percentage of genes in Cornwall that
could be traced to the Black Sea Coast would be small. However, if the gene
did raise intelligence and was hence selected for, the gene could come to have
a high frequency in Cornwall, and diffuse from there to the rest of England.
The parts of Eurasia (and North Africa) connected by long distance trading routes
would tend to have their intelligence raised as intelligence promoting genes
were spread over long distances.
Selection for Intelligence
discussion has been on the assumption that high intelligence was selected for.
Such positive selection for intelligence is plausible in many societies. Intelligence
would help in attracting mates, and in achieving positions of leadership that
led to mating opportunities. It probably also assisted in earning a living and
hence promoting the feeding and survival of one's offspring, as well as in arranging
advantageous matches for them. However, in more recent times intelligence may
not have contributed to reproductive success. In most modern industrial societies,
the high socioeconomic status and educated individuals have fewer children than
those with low status and poor educations (Herrnstein & Murray, 1994; Itzkoff,
1994). This seems to occur especially with females. Females postpone marriage
and child bearing to obtain an education. They find a conflict between a high
status occupation and child rearing. (Women also find a conflict between low
status occupations and child rearing, but find it easier to sacrifice a low
status, uninteresting occupation for child rearing). Also, low intelligence
seems to lead to more failures at contraception and additional births. However,
these conditions appear to have risen only recently with changing status for
women, and the emergence of modern contraception.
have been earlier selection against intelligence. Most likely, the population
of cities in Medieval Europe and early modern times failed to reproduce themselves,
primarily because the high population density facilitated the spread of disease.
The population was maintained by continual immigration from the surrounding
In at least
some circumstances, it is likely that those that immigrated to the cities (and
remained there) were of higher intelligence than those that remained in the
cities. The cities probably had a higher proportion of occupations for which
high intelligence was an asset, including craftsmen, traders, and government
officials. Intelligence was probably not as much of an asset in peasant agriculture.
It is possible (but unproven) that high intelligence by encouraging movement
to a city (and being able to earn enough to stay there) was actually selected
against in some times and places. If the selection for intelligence disappeared,
the areas of the world where genes were being well mixed would no longer have
a tendency to have their intelligence raised.
for Other Genes
has been developed for genes that raise intelligence because that is a socially
desirable trait that many believe to have been subject to unidirectional selection
for most of human history. However, the principle is perfectly general that
producing the long distance importation of new alleles lead to the increase
in the receiving area of any trait subject to
In a region
where malaria was endemic and malaria resistance was being selected for, one
would expect more mutations resistant to malaria to have reached the areas that
were exchanging genes over long distances than the areas that were relatively
be other traits that have been subject to selection in much of the world. In
northern climates, a common strategy for getting through the winter was storage
of food. It was also desirable to devote effort to the building of homes that
would protect from the cold. In modern economies, this ability leads to saving
and investment in productive resources. In prehistoric tropical societies there
was little opportunity for planning ahead. Thus, the populations that moved
out of the human cradle in Africa have probably been under selection for the
ability to defer gratification. This trait would be expected to be most common
in the areas that have received genes from a large part of the world.
account appears to be congruent with what is known about the worldwide distribution
of intelligence. High intelligence is reported for the populations of Europe
and Northeast Asia (China, Japan, Korea) which are at each end of the Eurasian
steppes (Lynn, 1991a). Areas that are isolated from Eurasia by water, and of
smaller populations (Australia and the Americas) have lower scores even when
the populations are living in relatively cold areas that might be thought to
have selected for intelligence.
above model, Australia and the Americas are peripheral areas. Their populations
were probably too small (in relation to the Eurasian populations) to generate
many mutations, and they were probably too far, or too isolated to receive many
mutations from the Eurasian land mass. Elsewhere (Miller 1995), I argued that
these continents were sufficiently isolated that mutations occurring on the
Eurasian land mass since their initial settlement probably had not reached them.
Even if I am wrong, and there has been some gene flow since initial settlement,
it is likely that their positions far from the Eurasian population centers caused
them to be very peripheral, limiting their access to intelligence increasing
mutations. Thus, they would have been expected to have lagged behind Eurasia
in the development of intelligence, as the data shows them to do (see Miller,
1995 for detailed documentation).
is found in Africa, and among those of African descent. This can be plausibly
argued to be due to weaker selection for intelligence in a tropical climate
(Miller, 1991, in press) along with the isolation caused by lack of horses and
poorer access to water borne trade (and traders) in earlier eras. The poorer
access to water borne trade would be due to poor harbors on the coast, a lack
of inland seas, and a lack of navigable rivers flowing down to the water.
predictions emerge? Right now, while the evidence is quite strong that there
are genes that contribute to intelligence,
exactly what these genes are and where they are located is unknown. However,
evidence has recently been presented that certain genetic markers are statistically
more common in those of high intelligence than in those of low intelligence
(Plomin, et al, 1994; Plomin et al, 1995; Skuder, et al, 1995), and one has
been found that appears to affect spatial ability without affecting intelligence
(Berman, & Noble, 1995). Given the rate of progress in molecular genetics,
it is likely that several alleles that have a positive or negative effect on
intelligence will soon be located. If the above theory is right, not only will
these genes prove to differ in frequency between populations in different parts
of the world, but some of the ones identified in European or northeast Asian
populations (the populations most commonly studied, simply because they are
the populations that are most convenient to the leading laboratories) will be
found to be essentially absent (a low frequency may be the result of recent
mixing with Europeans) in the original aboriginal populations in such areas
as Australia and the Americas.
theory raises the possibility that certain alleles with a favorable effect on
intelligence may have become fixed in European or Northeast Asian populations
if they originated in these regions, (and possibly even if they originated elsewhere
but reached these populations early enough for natural selection to fix them).
Studies that are limited to just one group (such as Caucasians or Japanese)
may not detect a correlation of these genes with intelligence.
example is provided by the high-affinity aldehyde dehydrogenase gene, which
comes in two versions in Orientals (Tu & Israel, 1995). One version provides
protection against alcoholism because they cannot easily digest the aldehyde
that is produced
after alcohol consumption. The aldehyde makes them mildly sick. This simple
genetic difference can explain most of the difference in drinking within the
Oriental population in North America. However, the allele that is common in
Orientals is virtually unknown in Caucasians. Studies limited to Caucasians
would not have discovered this genetic effect.
argument would suggest that mixed populations (such as American blacks, or those
of mixed Australian aboriginal and
Caucasian descent) might very profitably be investigated. A finding that possession
of a particular genetic marker was correlated with intelligence would suggest
that that marker either directly affected intelligence, or was close to a gene
that affected intelligence. Of course, in populations that are a mixture of
two populations that differ in intelligence, any gene that differs in frequency
may be merely serving as a marker for the extent of admixture (not to mention
for the extent of acculturation).
be necessary to control for this. For instance, if there were other genes that
were believed to be unrelated to intelligence (possibly from studies in other
ethnic groups), but which did differ in frequency between the two parent groups,
these could be used to estimate the degree of admixture. Many genetic markers,
including blood group, human leukocyte antigen genes, and restriction length
polymorphisms, are known to differ between populations (Cavalli- Sforza et al.
1994). Thus, it should be possible to estimate the extent of admixture independently
of the genes believed to be linked with intelligence. Independent measures of
acculturation would have to be sought as a control. This differs from the procedure
of the major quantitative tract loci study of intelligence so far (Plomin, et
al. 1994), which limited itself only to Caucasians.
argued that some isolated areas such as Australia may have received few new
mutations after settlement. However, if they
experienced continued selection for intelligence, some of the alleles that the
population arrived with may have become fixed, or nearly fixed in their populations.
In this case, the standard deviation of intelligence should be smaller in such
populations than in the populations that have been continually receiving new
genes from other populations. This is a testable prediction.
are generally found to have somewhat lower standard deviations for intelligence
than Caucasians (Jensen, 1980). This might be explained if a slower migration
of alleles into Africa and within Africa had resulted in African populations
having fewer polymorphic intelligence relevant genes. Many intelligence relevant
alleles would have reached them so long ago that they had become fixed, and
many other alleles would not have reached them yet, even if they accounted for
appreciable variation in other populations. In the areas that have had continual
access to new mutations there will be more alleles that have not become fixed,
causing a greater standard deviation of intelligence.
for Variability in Intelligence
this ongoing process of new mutations coming into a population followed by selection
for them may be the way to resolve the paradox of why there is so much genetic
variation for intelligence (g), if g is a trait that is beneficial. Some have
pointed out that variables that are subject to strong selection normally show
little variability. 'sually such variables reach their equilibrium values quickly,
and are now observed in the process of reaching equilibrium.
Patterson (1995, p. 210) gives great weight to Vale's (1980, p. 435) rhetorical
question, "If IQ is a fitness character, why should the additive variance
be anywhere near .71?". Vale goes on to argue, "The answer of course
is that it should not, if indeed IQ is closely related to fitness. If it is
not so related, then presumedly it has not been selected for throughout human
evolution. If it has not been selected for, then it evidently has not played
a very great role in that evolution."
a trait can be contributing to fitness and be being selected for without the
trait having reached its genetic limit, although powerful selection makes it
more likely that the limit will be rapidly approached, making it harder to observe
the organism in the process of being selected. For a trait subject to the type
of selection in which one animal having the trait increases the benefit of a
even higher level of the trait in another individuals (the so called arms race
or red queen effect, see Ridley, 1994), the period of adaptation is increased.
If intelligence is subject to unidirectional selection in which people with
a higher intelligence benefit reproductively from being able to outwit those
of lower intelligence, it is likely that at any given time there will be some
of higher intelligence than others, thus solving the problem. Still, in general
Vale and Patterson have a point for virtually all traits except intelligence.
since it is needed to discover its own existence, occupies a special position
among all traits. As intelligence gradually increases, it is to be expected
that a few individuals with sufficient intelligence to do psychometrics, and
discover the concept of g will emerge. When the distribution of intelligence
has risen to the point where some individuals investigate intelligence, others
individuals will be of much lower intelligence. At this time, only a small fraction
of the population is likely to have sufficient intelligence to do psychometrics
and to understand the concept of g. Thus, the finding of a wide range in intelligence,
a variable that contributes to fitness, is perhaps not as surprising as it might
appear at first.
(1995, p. 196) argument "The problem which Herrnstein, Jensen, and all
hereditarian psychologists face them, from the discipline on which they have
so heavily drawn, is that IQ scores are too hereditary if they are to sustain
the claim that these tests have any significance beyond the test center and
classroom." This would be a much more powerful argument if applied to any
trait other than intelligence. The
same argument can be extended to populations. Because of the wide geographical
area Homo sapiens occupies, its long generations, and the obstacles to gene
flow across tribes, there are likely to be differences in the intelligence of
different populations at any time. When some populations have reached the point
of having the technology to explore the world, they are likely to discover that
other populations have not yet developed to this point, and they can be expected
to conclude that there are differences between the world's various populations
is a genetically influenced variable that is affected by many different genes.
It has also plausibly been subject to unidirectional selection. Calculations
indicate that for a small hunter-gatherer population that genes would move at
a rate that was
slow relative to the time since modern human symbolic culture emerged. This
makes it very likely that geographical differences in
the frequencies of various intelligence related genes will exist. With unidirectional
selection in a polygenetic system, it is meaningful to talk about some areas
being more advanced than others (since there is a direction in which all are
moving). Centrally located populations will normally be more advanced. Genes
will move faster in thinly populated areas. The thinly populated areas can serve
as genetic freeways that carry genes rapidly across continents.
very slow progress of genes with a stable population structure, occasional waves
of advance caused by new technologies or the movement of populations can greatly
accelerate the movement of mutations. The spread of agriculture was one such
event. The coming of the horse and the ship were other similar events. The horse
caused the steppes of Central Asia to become a genetic highway that transported
favorable mutations from China, Europe, India, and the Middle East to other
areas. This caused these areas to reach high levels of intelligence before other
areas. Areas without the horse, such as sub-Saharan Africa, would have lagged.
areas such as Australia and the Americas probably also lagged due to isolation
from the large populations of Eurasia.
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A. J. & Cavalli-Sforza, L. L. (1984). The neolithic transition and the genetics
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Princeton University Press.
D. W. (1986). The "Kurgan culture," Indo-European origins, and the
domestication of the horse." Current Anthropology, 27, 291- 304.
G., Pilastro, A., Domenico, S. D., & Renfrew, (1994). Genetic variation
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