Starting with the Minnesota twin study thirty-five years ago, and now with a mountain of data from twin, sibling, and adoption studies, we know that pretty much anything you can quantify about human personality, behavior, and intelligence is to some degree heritable, average-average at around the fifty percent level but sometimes much higher. We now in fact have a busy and exciting field of study called “behavioral genetics.” Given that all these traits are heritable, it follows from the ordinary laws of biology that different races will exhibit different statistical profiles on them. That’s not astonishing, mysterious, or horrible: It’s just first-floor-level science.

Errors of reductionism and biodeterminism take over in such silly statements as “Intelligence is 60 percent genetic and 40 percent environmental.” A 60 percent (or whatever) “heritability” for intelligence means no such thing. We shall not get this issue straight until we realize that the “interactionism” we all accept does not permit such statements as “Trait x is 29 percent environmental and 71 percent genetic.” When causative factors (more than two, by the way) interact so complexly, and throughout growth, to produce an intricate adult being, we cannot, in principle, parse that being’s behavior into quantitative percentages of remote root causes. The adult being is an emergent entity who must be understood at his own level and in his own totality. The truly salient issues are malleability and flexibility, not fallacious parsing by percentages. A trait may be 90 percent heritable, yet entirely malleable.

[T]he psychic and physical characteristics of human beings, and the differences between individuals, are the consequence of an interaction between the genes that are present in the fertilized egg and the sequence of environmental circumstances that the developing organism experiences during its entire life history. With a few exceptions, like cystic fibrosis where possession of the defective genotype leads ineluctably to the disease, or language acquisition where the language spoken depends only on experience and not at all on genotype, human characteristics are all subject to this interaction of forces. There are, moreover, random events in cell growth and differentiation that are neither genetic nor environmental in the usual sense, and which play an extremely important part in development, especially in behavioral traits.

I have always been interested in family history. Chromosomes are funny things, aren't they? They may skip a generation and you can find children who resemble the grandfather, rather than either parent. Heredity is more important than environment. Blood will tell. For example, a man is either musical by heredity or he is not. You can't make a man musical by the environment. You can find a person who is very musically inclined and be puzzled because neither parents nor grandparents had any ear for music. But if you trace it back, you will find that the great-grandfather was a musician. But the environment plays a great part in the development of a man. It is significant whether a man is brought up in the city or in the country, near a lake or on the shores of the ocean.

In fact, happiness is one of the most highly heritable aspects of personality. Twin studies generally show that from 50 percent to 80 percent of all the variance among people in their average levels of happiness can be explained by differences in their genes rather than in their life experiences. 28 (Particular episodes of joy or depression, however, must usually be understood by looking at how life events interact with a person’s emotional predisposition.)

...the results have consistently suggested that introversion and extroversion, like other major personality traits such as agreeableness and conscientiousness, are about 40 to 50 percent heritable.

As you grow up, you gradually express your own innate intelligence and leave behind the influences stamped on you by others. You select the environments that suit your innate tendencies, rather than adjusting your innate tendencies to the environments you find yourself in. This proves two vital things: that genetic influences are not frozen at conception and that environmental influences are not inexorably cumulative. Heritability does not mean immutability.

I had begun by trying, for the sake of scholarly thoroughness, merely to write a short chapter for my book on the ‘culturally disadvantaged’ that I expected would succinctly review the so-called nature-nurture issue only to easily dismiss it as being of little or no importance for the subsequent study of the causes of scholastic failure and success. I delved into practically all the available literature on the genetics of intelligence, beginning with the works of the most prominent investigator in this field, Sir Cyril Burt, whom I had previously heard give a brilliant lecture entitled The Inheritance of Mental Ability’ at University College, London in 1957. The more I read in this field, the less convinced I became of the prevailing belief in the all-importance of environment and learning as the mechanisms of individual and group differences in general ability and scholastic aptitude. I felt even somewhat resentful of my prior education, that I could have gone as far as I had—already a fairly well-recognized professor of educational psychology—and yet could have remained so unaware of the crucial importance of genetic factors for the study of individual differences.

When certain hereditary tendencies are almost indelibly ingrained, environment will have a hard battle to effect a change in the child; but that a change can be wrought by the surroundings we all know. The particular subject may at first be stubborn against these influences, but repeated application of the same modifying forces in succeeding generations will at last accomplish the desired object in the child as it does in the plant.

The study of inbreeding depression seems to me especially important in the study of human abilities, because inbreeding depression indicates genetic dominance, and the presence and degree of dominance are related to natural selection for the trait in the course of its biological evolution. It was of great interest to me to discover, for example, that of the several ability factors that can be extracted from the various subtests of the Wechsler Intelligence Scale for Children, the one that shows the greatest susceptibility to inbreeding depression is the g factor (Jensen, 1983b). This finding indicates that one of our most widely used standard psychometric tests of intelligence yields scores that reflect some part of the variance in the biological intelligence that has developed in the course of human evolution.

The conclusion that all these studies converge upon is that about half of your IQ was inherited, and less than a fifth was due to the environment you shared with your siblings – the family. The rest came from the womb, the school and outside influences such as peer groups. But even this is misleading. Not only does your IQ change with age, but so does its heritability. As you grow up and accumulate experiences, the influence of your genes increases. What? Surely, it falls off? No: the heritability of childhood IQ is about forty-five per cent, whereas in late adolescence it rises to seventy-five per cent. As you grow up, you gradually express your own innate intelligence and leave behind the influences stamped on you by others. You select the environments that suit your innate tendencies, rather than adjusting your innate tendencies to the environments you find yourself in. This proves two vital things: that genetic influences are not frozen at conception and that environmental influences are not inexorably cumulative. Heritability does not mean immutability.

We may compare this sum of the life forces, which we call heredity, to the character of a sensitive plate in the camera. Outside pictures impress themselves more or less distinctly on the sensitive plate... Stored within heredity are all joys, sorrows, loves, hates, music, art, temples, palaces, pyramids, hovels, kings, queens, paupers, bards, prophets and philosophers, oceans, caves, volcanoes, floods, earthquakes, wars, triumphs, defeats, reverence, courage, wisdom, virtue, love and beauty, time, space, and all the mysteries of the universe. The appropriate environments will bring out and intensify all these general human hereditary experiences and quicken them again into life and action, thus modifying for good or evil, character—heredity—destiny.

While it is necessary to point out the importance of "heredity" as a determining factor in human life, it is fatal to elevate it to the position of an absolute. As with environment, the concept of heredity derives its value and its meaning only in so far as it is embodied and made concrete in generations of living organisms. Environment and heredity are not antagonistic. Our problem is not that of "Nature vs. Nurture," but rather of Nature x Nurture, of heredity multiplied by environment, if we may express it thus. The Eugenist who overlooks the importance of environment as a determining factor in human life, is as short-sighted as the Socialist who neglects the biological nature of man. We cannot disentangle these two forces, except in theory. To the child in the womb, said Samuel Butler, the mother is "environment." She is, of course, likewise "heredity." The age-old discussion of "Nature vs. Nurture" has been threshed out time after time, usually fruitlessly, because of a failure to recognize the indivisibility of these biological factors. The opposition or antagonism between them is an artificial and academic one, having no basis in the living organism.

We now know that DNA differences are the major systematic source of psychological differences between us. Environmental effects are important but what we have learned in recent years is that they are mostly random – unsystematic and unstable – which means that we cannot do much about them.