The Consequences of Variable Intelligence By: Tatu Vanhanen

University of Helsinki, Finland

[The following article originally appeared in Mankind Quarterly, Volume XXXV, Number 4, Summer, 1995.]

BOOK REVIEW ARTICLE

Human Intelligence and National Power:
A Political Essay in Sociobiology
Seymour W. Itzkoff
New York: Peter Lang Publishing, 1991.

The Road to Equality: Evolution and Social Reality
Seymour W. Itzkoff
Westport, Connecticut: Praeger, 1992.

The Decline of Intelligence in America:
A Strategy for National Renewal
Seymour W. Itzkoff
Westport, Connecticut: Praeger, 1994.

Seymour W. Itzkoff argues in his three books published in 1991-94 that there are significant hereditary intellectual differences between individuals and groups and that as a consequence of this variation there are very large differences in educability, social status, and economic achievements of individuals and groups. According to him, intelligence is part of each individual's inheritance, as much as one's height and personality. Therefore,

"the issue of intellectual variability in humans and the consequent variability in average intelligence between groups of individuals, and their ethnic, racial, religious, and national identities, is the Copernican problem of our time" (1991, 10).

He challenges the egalitarian dream of socialists, sociologists, and liberal egalitarians, according to which intelligence is uniformly distributed in all populations and all humans were equal to any social and intellectual task if only they were not held down. Itzkoff points out and enumerates great failures of social policies based on these unrealistic views of human nature. The theme is the same in all three books, but he discusses it from different perspectives.

In Human Intelligence and National Power. A Political Essay in Sociobiology (1991), he focuses on the evolution of human intelligence and the emergence of intellectually different human groups, as well as on various consequences of the variability of human intelligence, including the European florescence, the failure of communism, the rise of Japan's power, the decline of the United Sates, and the Third World debacle. He emphasizes the significance of intellectual homogeneity in ethnically homogeneous nation-states and examines the ways to raise the level of general intelligence "g".

In The Road to Equality: Evolution and Social Reality (1992), Itzkoff focuses on the failure of Marxists and liberal egalitarians to create an egalitarian and classless society and argues that their basic assumptions of human nature were wrong. They failed to recognize that human beings are endowed with differing quantities and qualities of intelligence and that the same concerns ethnic groups. From this perspective, he examines the hallucinations and misfortunes of our evil century, the methods to achieve classlessness and to end oppression and degradation, the ethic of intervention, the democratic quest, essential feminism, the mysterious ethnicity, and the significance of the wealthy. His message is that America's social dilemmas are in part due to hereditary intellectual differences between individuals and groups.

In the latest book, The Decline of Intelligence in America: A Strategy for National Renewal (1994), Itzkoff analyses the problems and social pathologies of America and claims that they are related to the decline of general intelligence. His central idea is that new generations are coming from the lower end of the intellectual, and thus the social, scale. As a consequence, a population of permanently poor Third World Americans is emerging. In the second part of the book, he recommends policies intended to turn the trend. The solution proposed in this book is simple: the government should stimulate the finest to form families of the traditional sort in which children are conceived, born, raised, and educated to the highest levels for which they are capable, and the helpless should be encouraged and guided not to have children that they cannot rear and educate to functional cultural levels.

The problems analyzed in Itzkoff's books are extremely important. He has had courage to take up issues that have not been discussed because it has not been politically correct to assume that there might be intellectually different human groups and that social inequalities might in part be due to variable intelligence in humans. It has been difficult even for evolutionary biologists to accept the idea that humans vary in general intelligence (see, for example, Gould 1981; Lewontin 1982). Even more difficult it has been to accept the claim that there are hereditary intellectual differences between ethnic groups (see Vine 1994). I try in this essay to tell about Professor Itzkoff's central ideas, arguments, evidence, examples, and renewal proposals and to evaluate the practical significance of his theoretical insights and reform proposals.

The Evolutionary Roots of Intellectual Differences

Let us start from his central idea concerning hereditary differences between individuals and groups. How to explain the origin of assumed group differences?

He traces the origin of intellectual variability of human groups to the geographical dispersion of early humans and to the variation in their environmental circumstances. According to him, Homo erectus originated in Africa, but it possibly split into modern geographical races of man already one or 1,5 million years ago when some groups emigrated from Africa to the other Old World continents.

At this stage of human evolution, from about 1,5 to 0.5 million years ago, various groups of humans, whether races or ethnic groups, seem to have had similar levels of intelligence. There were not many differences in tools used by them. However, after 500,000 B.P., a revolution begins to occur in the North, in Europe and western Asia among Caucasoids during the Pleistocene Ice Ages. Intelligence helped the survival of people in harsh and variable environmental conditions. High intelligence was useful. The average brain size and intelligence increased in Caucasoid populations through natural selection. He says that

"in the challenging environment of the north, a big brain had extraordinary selective value. These humans could think deeply and analytically" ( 1992, 37).

Finally, about 35,000 B.P., Cro-Magnon appeared in Europe. His assumption is that Homo sapiens sapiens evolved in Europe as a consequence of adaptation to harsh and variable environmental conditions:

"the northern quadrant of humanity subject to the flow and ebb of the glaciers inhabited a far more challenging and dangerous environment than those living in the tropical south" (1991, 194).

There was not similar pressure for intellectual evolution among the human populations living in "millions-of-years-old tropical garden of Eden." Consequently, northern populations achieved a higher level of general intelligence than tropical populations.

Itzkoff assumes that the ability of large-skulled, adaptively able northern sub-species of Homo to handle this ferocious Ice-age environment and even prosper probably forced them to migrate for more space. Over the period of 150,000 years, they moved east and south and spread their genes. He further assumes that

"modern blacks originated in Western Africa after 10,000 B.P. as a result of mixtures between indigenous proto-Negroids and Pygmies, and incoming Caucasoids" (1991, 40).

[As a consequence,]

"Negroid and Caucasoid races have biologically more in commo
n with each other than they do with any of the other races" (1991, 42).

The original Mongoloid descendants of Homo erectus pekinensis along the Yellow River Valley also absorbed a steady stream of Caucasoid wanderers across the Siberian and Kazakhistan plains. The same concerns the Koreans and Japanese,

"who speak a Uralic/Altaic language related to the hybrid Siberian steppe peoples and thence to the Estonians and Finns" (1991, 42).

[In this way the Cro-Magnon people wandered from their unknown Eurasian homeland to the other parts of the world]

"hybridizing with the existing transitional erectine-sapiens humans all over the world."

[The New Guinea, Australian and Tasmanian Australid populations are possible exceptions (1991, 18, 39). Today's]

"racial divisions are the remnant memories of ancient human separations that go back several million years"
( 1992, 7).

This is a very interesting assumption on the origin of intellectual differences between human populations and of geographical races. It differs radically from interpretations, according to which the evolution of modern people took place in Africa.

C. B. Stringer, for example, claims that

"all living people are closely related and share a recent common ancestor who probably lived in Africa. From that African ancestral group, all the living peoples of the world originated. "

He continues that the ancestors of Europeans , Asians and the populations of the American and Australian continents probably share common ancestors within the past 60,000 years. This idea does not presuppose any significant intellectual differences between human populations. In fact, Stringer emphasizes their similarity:

"What is certain is that the early modern peoples of each part of the world were all similar in basic anatomy and behavior, but regional differences in physique and culture rapidly developed subsequently" (Stringer 1992, 249. See also Howells 1992; Ritter 1981, 98-101).

Stephen Jay Gould, similarly, assumes that Homo sapiens

"is tens of thousands, or at most a few hundred thousand, years old, and all modern human races probably split from a common ancestral stock only tens of thousands of years ago" (Gould 1981, 323).

Itzkoff's assumption differs from the "Out of Africa" hypothesis in two important points: (1) he claims that human populations have racially differed from each other one or 1.5 million years, although there have been new mixtures later on, and (2) he provides a plausible explanation for the origin of intellectual differences between human populations. The alternate hypothesis would be unable to provide any explanation for intellectual differences between the northern and tropical populations. The crucial question is whether such differences really exist.

General Intelligence "g"

Itzkoff's claims that individuals vary in intelligence and that such variation is principally due to hereditary factors. What kind of evidence does he provide to support this claim?

He refers to intelligence tests (I.Q.) that have been carried out in various countries since the beginning of this century. They indicate consistently that humans vary in intelligence. A heated debate has continued on the question whether such variation is more due to hereditary or environmental factors and whether there is any "general intelligence" that could be measured (see Gould 1981; Lewontin 1982; Itzkoff 1987).

Itzkoff refers to evidence of the existence of general intelligence "g" and of its hereditarian character. According to him, 50-80 percent of general intelligence seems to be due to hereditarian factors. Innumerable studies of monozygotic and dizygotic twins have provided evidence on the hereditary nature of intelligence. For example, he says,

"identical twins reared apart in differing life circumstances are much more similar intellectually than fraternal twins reared under the same roof" (Itzkoff 1987, 142; cf. 1991, 27).

[ Worldwide studies of sibling adaptation, he continues,]

"regardless of the race or ethnicity involved, reveal that a sociologically uplifting environment has no long-term impact either on the personality or the intellectual profile that the children bring with them from their biological heritage" (1992, 88).

The results of these studies also imply that the genetic variation in intelligence depends on a relatively small number of genes because the possible variability between even closely related individuals seems to be enormous (see 1992, 31-32; 1994, 101). Itzkoff comes to the conclusion that it

"should be clear to all but the most ideologically and theologically devout environmentalists that human achievement and personality have a dominating biological and thus hereditary component" (1992, 31).

I think that it would be difficult to disprove his argument that human intelligence varies and that hereditary component is dominating in this variation. If we accept the argument on the hereditary intellectual differences between individuals, it becomes difficult to deny the possibility that there might be hereditary intellectual differences between ethnic groups, too.

This is a much more inflammatory proposition than the claim of individual intellectual differences. Everybody has probably made observations of great individual differences in intelligence, but it is more difficult to make observations of the average intelligence of ethnic or racial groups. Therefore, it has been easy to deny the existence of such differences and to argue that there cannot be any significant differences in the average intelligence of ethnic or racial groups. And if all human races separated from a common ancestral stock in Africa only some tens of thousand years ago, it would be difficult to find any plausible explanation for the emergence of such differences. However, Itzkoff has a plausible explanation for the origin of intellectual differences between human groups, as mentioned above, and he provides data that indicate the existence of such differences among contemporary ethnic groups His evidence is based on the consistency of the results of intelligence tests (I.Q.) carried out in many countries.

According to the results of intelligence tests given in his books, the average I.Q. for American whites is 100, for African-Americans 82-85, for Hispanics somewhere in between, and for native Americans in the low to mid-90s, whereas it is 103-107 for Japanese and probably more than 100 for Han Chinese, too. Itzkoff stresses that they are ethnic groups that differ from each other in intelligence, not racial groups, but, on the other hand, he emphasizes the difference between northern and tropical populations. In general

"the northern peoples of the world, the residue of the original Caucasoids and Mongoloids have more on average brain power" (1992, 50).

This is probably the most controversial part of his argumentation, but because his conclusions and policy recommendations are based on it, those who disagree with him should try to show that he is wrong. It is not enough to say that it is not politically correct to make such propositions. In open society, people should be prepared to discuss and examine also the ideas that contradict their own convictions and belief systems.

Itzkoff provides additional support for his thesis from empirical data on educational and economic achievements of different ethnic and national groups. According to him, it was natural that the technological civilization emerged in the North, in the area of Caucasoid Eurasians. The present great economic inequalities between the north and the south are related to intellectual differences. Therefore, it has been difficult to equalize economic conditions between the industrially developed north and the Third World countries. It has succeeded only in the parts of the world where national ethnic groups have been intellectually approximately equal with Caucasoids. This concerns particularly northern Mongo
loids, Japanese, Koreans and Han Chinese.

On the other hand, development aid from the north has not been enough to generate and maintain technological development in Africa. Itzkoff finds further evidence for his thesis from the fact that all immigrant groups have not succeeded equally in America. According to his data, more intelligent ethnic groups have succeeded much better than less intelligent groups.

Social Consequences

We come to the social consequences of variable intelligence. They are enormous. For example, Itzkoff refers to many types of social facts and problems connected with variable intelligence in humans. He argues that social inequalities are persistent because humans vary in intelligence. He accuses the ideology of egalitarianism for the genocides and holocausts of this century. Communists killed tens of millions of people of higher intelligence to further equality. The failure of communism was caused, according to his interpretation, by their erroneous assumption that intelligence is distributed homogeneously among individuals. They believed that the masses could easily be educated to fill the vacuum created by the destruction of the bourgeoisie establishment.

It was not so. Marxists had forgotten Marx's refutation of those sections of the Gotha Program (German socialist parties) that asserted the absolute uniformity of human abilities. Marx himself believed in the existence of intellectual differences in human beings.

Itzkoff further argues that Japan's economic success story has been powered by the high intelligence of the ethnically homogeneous Japanese people. Because of universally high intelligence of its ethnically homogeneous population, the Japanese state does not need to subsidize any permanently "catch-up" portions of the nation, and because there is a rich supply of talent ready to step in, the salaries of executives remain relatively low. In Japan, the average chief executive earns about eight times the average of his workers; in the United States the average chief executive earns about 160 times the worker average (1992, 152).

Itzkoff presents an extremely inflammatory and important explanation for the failure of modernization in most parts of the Third World. According to his assumption, it is due to clear differences in average intelligence between the northern and southern populations. Northeast Asia, including mainland China, Taiwan, Hong Kong, Singapore, and South Korea, are rapidly rising from war and political chaos because of the high intelligence of their populations, whereas in Africa, Central and South America, and many parts of southern Asia, the pace of modernization has been at best slack. Many hundreds of billions of dollars in loans have simply gone down the drain in corruption and incompetence. The reason for the Third World debacle is in the fact that the level of intelligence is lower in the tropical south. He supports this assumption by the experiences gathered from minority populations of the tropical south living in the north. Some members of such minorities thrive and prosper, but the majority falls into despair.

On the other hand,

"ethnic Han Chinese living in either Indonesia, Malaysia, or the Philippines achieve at levels parallel to their Chinese compatriots in the U.S. or Hong Kong, despite extensive negative discrimination." And Japanese, Germans, and Italians born in Brazil achieve as their confreres do in their respective homelands (1991, 195).

The Decline of the United States

The major problem examined by Itzkoff concerns the decline of the United States and its causes. He complains of the lack of open discussion and warns that never

"in history has a society that has blocked the open search for truth survived to prosper. "

[ In America, the intellectual leadership of the great public media institutions and the universities has effectively handcuffed the elected political representatives and prevented them from considering solutions:]

"The taboo word is, of course, race. Because so much of our internal tragedy does involve the minorities of color, the stereotyped excuse is that discussions about biological intelligence and the variable behavior that it elicits will militate against the interests of these minorities."

[He does not accept this argumentation, and he tries to show that it is in the interest of all Americans to think deeply ]

"about this reality of variable human intelligence and whether there might be a connection between this issue and the fact that our national profile is sinking so rapidly" ( 1994, 6).

What does he mean by "decline of the United States?" Itzkoff claims that this decline

"can be confirmed by any of the criteria that historians have ever used to measure the state and condition of a nation and its people" (1994, 3).

The indicators of decline used by Itzkoff include the rise of criminality in American cities, the status change from a great creditor nation to the world's largest debtor nation, the enormous loss of high-wage jobs, the fact that some 50-80 percent of the workforce is not able to work and produce at an internationally competitive level, the decline in educational standards and achievements of the public schools as indicated by the quarter-century decline of SAT (the Scholastic Aptitude Test) scores, social disintegration, and the expanding poverty populations at the bottom of society. It seems to me that he has presented enough empirical evidence on the decline of the United States compared to Japan or to some European nations.

Itzkoff explains the economic and educational sinking of the United States by the decline of the average intelligence. The welfare policies encouraged the poorest, least capable sectors of the population, from all the races and ethnic groups to have children. However, he does not provide much statistical evidence for his claim that poor sections of the population have produced relatively more children than more wealthy and educated ones. It is not self-evident that this claim should be true.

According to sociobiological theories, wealthy and dominant sections of the population are expected to have been reproductively more successful than poor ones, at least until modern times (see Betzig 1986; Rogers 1990, Roskaft et al. 1992). Therefore, I would like to see more statistical evidence. One example, to which he refers, concerns the blacks. The proportion of black citizens in the U.S. grew from 9.8 percent in 1940 to 12 percent in the mid-1980s.

Itzkoff sharply criticizes the welfare-policies that have produced a new human zoo. He says:

"Like animals whom we now have trained to reproduce in captivity, there is a new and growing class of Homo sapiens living within the ostensibly modern societies."

[He assumes that even Marx would look at this new and classically unrecognized situation with horrified wonder:]

"What he would see would be the public welfare hospitals where they are born, the flocks of social workers who minister to their dole, the Head Start teachers, then the special education and remedial classes in the state schools, the drug clinics, probation officers, public health nurses, the police and the jails, the crime-ridden public housing projects, the food-stamps, the underground subways, bus terminals, and railway stations and the spaces over the heating vents on the public streets that serve as sleeping places, the municipal hospital emergency rooms, and then the AIDS wards and hallways where they die" (1992, 90-91).

On the other hand, the invention of "the pill" and feminist ideas decreased the birth rate among educated and more intelligent sectors of the population. Liberal equalitarians told the people that it was not important who had the babies. The children could easily be educated to high levels of social productivity, they preached. To the educated classes,

"both men and women, they urged liberation, careerism, and material consumption, heaven forbid c
onceiving, bearing, and raising large families" (1992, 91).

[As a consequence,]

"the United States mean I.Q. has dropped about five points over the last several generations, the result of this differential birth rate" ( 1991, 163-187).

Briefly stated, Itzkoff argues that the poor and intellectually lower sections of the population have been reproductively much more successful than the wealthy and more intelligent sections of the population and that it has caused the fall of national intelligence. He estimates that already by 1994, roughly half of the American population can be seen to be sinking below international levels of intellectual and educational achievement needed to maintain competitive production. And he asks, what is "to become of these individuals, and then of the formerly wealthy nation that encouraged their coming into being?" (1994, 107). He assumes that they will be pushed deeper and deeper into the culture of poverty. What to do?

Remedies Proposed

Professor Itzkoff argues that because social pathologies and other problems of the United States have been aggravated by the decline of general intelligence of its population, the best remedy would be to increase the level of general intelligence. He stresses that it is not a purely racial or ethnic issue because those at the bottom of the intellectual pyramid come from all groups, white, African-American, Latino, and others. It is clear, however, on the basis of his books that the problem focuses on African-Americans and other ethnic groups originated from the south.

The remedy proposed by Itzkoff is simple: the most intelligent and educated men and women should bear and raise many more children than those from the bottom of the economic and educational social class structure. Besides, the traditional nuclear heterosexual family should be saved. He accuses liberal egalitarians for hating monogamy and the nuclear family:

"They fear and despise men as heads of household, and thus with a woman actively raising her brood of children in the home, the kids not out in day care or with illegal aliens acting as 'foster' parents. The idea that males and females differ in any important bio-cultural manner, physical or intellectual, is anathema to their unisex ideology, and their despising of historical male and female values."

[ As a consequence of liberal policies, Itzkoff continues, we]

"have lost the children of almost two generations of our educated and liberated women. It has had almost the same effect as if it had been genocide" (1994, 126, 133).

According to his interpretation, it will depend on the policies of the government whether the reproduction trends change to the proposed direction or not. The government should pass

" social policy legislation aimed at creating inducements, as well as legal protections, that will lead to the wealthy and successful having more than their share of children and the poor limiting their procreative activity in the interest of their own individual social and economic aspirations" (1992, 160).

The prescription is clear, but it seems to me that he does not yet have any clear idea what such "social policy legislation" should include and how the government could carry out such policies. However, he makes some proposals.

• First, people should be reeducated.
• Second, job priorities should be given to married men with families.
• Third, all births should require the identification of the father.
• Fourth, men and women at the top of social scale without children should be punished through the tax system.

The government should try

"to establish a long-term social policy that will `encourage' the birth of 50 percent more children from the upper half of the social and income brackets than from the lower."

[It is not clear how it could be done, although he says that we]

"must persuade the potentially parasitic classes at the top and at the bottom of society to act appropriately. The wealthy educated will have to validate their socially acquired assets by bearing their own offspring or adopting needy children. Those at the bottom should be humanely persuaded, with generous gifts if deemed appropriate but for one generation only to refrain from conceiving and having children" (1994, 192-195).

Itzkoff makes several other interesting reform proposals. I refer to only two of them. He would like to decrease the relative number of African-Americans because their average level of general intelligence "g" is low. The discouragement of illegitimate births would serve this purpose.

On the other hand, he suggests that the "talented tenth" of the African-Americans should produce many more children than the less intelligent majority. In this way it would be possible to raise the general intelligence of the African-American minority.

Besides, the United States should change its immigration policies radically. No more illegal immigration, he says, and

"those who are here in violation of our laws, along with the children that have been born here in the interim," must return to their homelands (1994, 161).

Only talented people, irrespective of their race, should be allowed to immigrate to the country.

Itzkoff is deeply worried about the declining intelligence in America because he would like to retain his country among the first class nations in the competitive world of the twenty-first century, which is not possible without a highly intelligent population. America's crisis is a natality crisis, he says, but the leadership of the United States is indifferent to this issue. It does not care who is having the children.

Discussion

I agree with Professor Itzkoff in most points of his analysis. Evidently humans vary in intelligence, and this variation is principally due to hereditary factors. He has convinced me that ethnic groups may also vary in general intelligence "g". I agree with him that social consequences of intellectual variability are enormous and that they can be seen in all areas of human life. The origin of social inequalities is in the fact that humans are not similar in their intelligence and other capabilities. It is also quite probable that a significant part of the persistent poverty in the Third World is related to intellectual differences between ethnic groups. He is probably right in his central assumption that the level of general intelligence would increase if the upper half of social and income brackets could produce 50 percent more children than the lower half. The problem is how to get people to follow his advice.

According to the sociobiological inclusive fitness hypothesis, all organisms are programmed to further their own reproductive interests and not to concern themselves about others (see, for example Dawkins 1976; Alexander 1980). Therefore, I assume that it would be extremely difficult or impossible to persuade the members of any minority ethnic group to sacrifice their own reproductive interests for the assumed higher interests of the nation.

It might be possible to achieve some results by economic and other inducements, but it is quite possible that coercion and even force would be needed to achieve substantial results. Itzkoff has not proposed or discussed the use of coercion, although he proposes that the births should be reduced at the bottom of the social and economic scale and that all births should require the identification of the father. Is this a case in which the government might use coercion and even force to carry out its family policy?

If the father cannot be identified and made responsible for the child, the state might require the prevention of the birth by compulsory abortion. However, if coercion and force become necessary to prevent the births of unwanted children, we have to ask whether the aims are worthwhile enough to justify such policies. Is the maintenance of intelligence so important that it justifies the use of coerc
ion and force against women who break the legal rules of reproduction? I do not know, and Itzkoff has not discussed this problem. It should be discussed because I do not believe that his radical reproductive reforms could be carried out without coercion.

It is true that African-Americans are at the bottom of the social and economic scale, but I would like to point out that they have not been losers in the Darwinian struggle for existence. In fact, according to the data given by Itzkoff, they have been even more successful than the whites because their relative number has increased in the United States since the 1940s. It means that in some way they have become better adapted to their social environment than the white majority. Despite their poverty, they have borne and raised children more than their share, whereas many wealthy and educated and probably also highly intelligent whites feel themselves so poor and insecure that they cannot afford to have children.

We should remember that in the Darwinian struggle for existence reproduction is the only criterion of success, not wealth, education, or intelligence. By this criterion the American blacks have been more successful than the whites.

Itzkoff has brought into discussion the issue of variable intelligence in humans and indicated through extensive evidence and examples its crucial importance in national and international politics. I think that it is time for us to take biological factors seriously and examine their relevance from various perspectives.

As Itzkoff says, the scientific evidence for the biological roots of our social behavior continues to accumulate (1994, 5). It is becoming clear that environmental egalitarians were wrong in their traditional assumption that human behavior and social structures are principally, if not completely, shaped by our environment. Human nature matters probably more than we can imagine. Itzkoff has focused on one very important aspect of human nature, to hereditary intellectual differences between individuals and groups, and he has disclosed its social and political relevance in superb manner.

References

• Alexander, Richard D, 1980 Darwinism and Human Affairs. London: Pitman Publishing.
• Betzig, L. L. 1986 Despotism and Differential Reproduction: A Darwinian View of History. New York: AIdine.
• Dawkins, Richard 1979 The Selfish Gene. London: Granada Publishing.
• Gould, Stephen Jay 1981 The Mismeasure of Man. Harmondsworth, Middlesex; Penguin Books.
• Howells, W. W. 1992 "The dispersion of modern humans," in Steve Jones, Robert Martin and David Pilbeam (eds.), The Cambridge Encyclopedia of Human Evolution. pps. 389-401. Cambridge: Cambridge University Press.
• Itzkoff, Seymour W. 1987 Why Humans Vary in Intelligence. Ashfield, Massachusetts: Paideia Publishers.
• Lewontin, Richard 1982 Human Diversity. New York: Scientific American Books.
• Ritter, Horst 1981 Humangenetik: Grundlagen - Erkenntnisse - Entwicklungen. Breisgau: Herder Freiburg.
• Rogers, Alan R. 1990 "Evolutionary Economics of Human Reproduction," Ethology and Sociobiology Vol. 11 , No. 6.
• Roskaft,Eivin, Annelise Wara, and Auslaug Viken 1992 "Reproductive Success in Relation to Resource-Access and Parental Age in a Small Norwegian Farming Parish During the Period 1700-1900, " Ethology and Sociobiology Vol. 13, Numbers 5/6.
• Stringer, C. B. 1992 "Evolution of early humans," in Steve Jones et al. (eds), The Cambridge Encyclopedia of Human Evolution. ss. 241-251. Cambridge: Cambridge University Press.
• Vine, Ian 1994 "The Political Abuse of Sociobiology - A Test Case" (a book review of Itzkoff's "Human Intelligence and National Power: A Political Essay in Sociobiology"), ESS Newsletter No. 33, January 1994, 13-31