How strange it is that people of honest feelings and sensibility, who would not take advantage of a man born without arms or legs or eyes—how such people think nothing of abusing a man born with low intelligence.

Es asombrosa la cantidad de mujeres que prefieren una conversación inteligente a una musculatura sólida.

I have received the favor of your letter of August 17th, and with it the volume you were so kind as to send me on the “Literature of Negroes.” Be assured that no person living wishes more sincerely than I do, to see a complete refutation of the doubts I have myself entertained and expressed on the grade of understanding allotted to them by nature, and to find that in this respect they are on a par with ourselves. My doubts were the result of personal observation on the limited sphere of my own State, where the opportunities for the development of their genius were not favorable, and those of exercising it still less so. I expressed them therefore with great hesitation; but whatever be their degree of talent it is no measure of their rights. Because Sir Isaac Newton was superior to others in understanding, he was not therefore lord of the person or property of others.

The idea that women and men might actually think differently, that is have different preferred modes of thinking or different thinking abilities, came up in both classes. At the time, it seemed clear to me that any between-sex differences in thinking abilities were due to socialization practices, artifacts and mistakes in the research, and bias and prejudice. After reviewing a pile of journal articles that stood several feet high and numerous books and book chapters that dwarfed the stack of journal articles, I changed my mind. The task I had undertaken certainly wasn’t simple and the conclusions that I had expected to make had to be revised.

The literature on sex differences in cognitive abilities is filled with inconsistent findings, contradictory theories, and emotional claims that are unsupported by the research. Yet, despite all of the noise in the data, clear and consistent messages could be heard. There are real, and in some cases sizable, sex differences with respect to some cognitive abilities. Socialization practices are undoubtedly important, but there is also good evidence that biological sex differences play a role in establishing and maintaining cognitive sex differences, a conclusion that I wasn’t prepared to make when I began reviewing the relevant literature.

Far from being the smartest possible biological species, we are probably better thought of as the stupidest possible biological species capable of starting a technological civilization—a niche we filled because we got there first, not because we are in any sense optimally adapted to it.

The juvenile sea squirt wanders through the sea searching for a suitable rock or hunk of coral to cling to and make its home for life. For this task, it has a rudimentary nervous system. When it finds its spot and takes root, it doesn’t need its brain anymore, so it eats it! (It’s rather like getting tenure.)

Shortly after Charles Spearman’s key work in 1904, intelligence became the best-studied, best-established trait in psychology. Higher intelligence predicts higher average success in every domain of life: school, work, money, mating, parenting, physical health, and mental health. It predicts avoiding many misfortunes, such as car accidents, jail, drug addiction, sexually transmitted diseases, divorce, and jury duty. It is one of the most sexually attractive traits in every culture studied, for both sexes. It is socially desired in friends, students, mentors, co-workers, bosses, employees, housemates, and especially platoon mates. It remains ideologically controversial because its predictive power is so high, and its distribution across individuals is so unequal.

Consider two recent high-profile cases. In 2005, Harvard’s then-president Lawrence Summers suggested gender differences in intrinsic ability as one cause of the dearth of women in the top tier of science, rather than espousing the popular view that women’s under-representation results from biased hiring, discriminatory tenure practices and negative stereotypes. Summers’s insinuation of biologically-based sex differences in cognitive ability was radioactive, setting off debates on campuses and outpourings of editorials. Despite apologizing for reckless language — which his supporters felt research supported — he later resigned.

James Watson is the most illustrious scholar to have his career ended for reckless language. Watson’s downfall was his assertion that “all our social policies are based on the fact that [African] intelligence is the same as ours — whereas all the testing says not really”. Although he hoped everybody was equal, “people who have to deal with black employees find this is not true”. Watson instantly plunged from A-list Nobelist to outcast, and was suspended from his chancellorship of Cold Spring Harbor Laboratory. Watson later clarified in a statement that he does not believe Africans to be genetically inferior, but this had little impact on the controversy.

Watson’s first assertion could be read as scientifically supported: black Africans’ IQ scores are lower than those of white Europeans. But Watson’s use of ‘intelligence’ was interpreted as meaning ‘intrinsic cognitive ability’, ignoring how unfamiliarity with testing format, low quality of schooling, or poor health might depress IQ scores. There have been analyses showing average national IQs for sub-Saharan Africa to be approximately 30 points lower than average IQs for predominantly white European nations, and drawing a racial conclusion from those results. A refutation of these analyses would provide an opportunity to advance understanding. Sadly, although these analyses can be refuted, as we and others have done, most of those who scorned Watson never knew they existed.

Attacks on Watson and Summers extinguished discussion by making moral attributions about their presumed character flaws rather than debating facts. But character attacks lead to a one-party science that squelches divergent views.

Some scientists hold more ‘acceptable’ views, ourselves included. We think racial and gender differences in IQ are not innate but instead reflect environmental challenges. Although we endorse this view, plenty of scholars remain unpersuaded. Whereas our ‘politically correct’ work garners us praise, speaking invitations and book contracts, challengers are demeaned, ostracized and occasionally threatened with tenure revocation.

The lavish attention devoted to intelligence (raising it, praising it, worrying when it is low, etc.) seems wasteful in light of the fact that we choose to virtually ignore another set of mental skills with just as much social consequence—rational thinking mindware and procedures. Popular books tell parents how to raise more intelligent children, educational psychology textbooks discuss the raising of students’ intelligence, and we feel reassured when hearing that a particular disability does not impair intelligence. There is no corresponding concern on the part of parents that their children grow into rational beings, no corresponding concern on the part of schools that their students reason judiciously, and no corresponding recognition that intelligence is useless to a child unable to adapt to the world.

[D]eification of intelligence can have a truly perverse moral consequence that we often fail to recognize—the denigration of those low in mental abilities measured in intelligence tests. Such denigration goes back to the very beginnings of psychometrics as an enterprise. Sir Francis Galton would hardly concede that those low in IQ could feel pain: The discriminative facility of idiots is curiously low; they hardly distinguish between heat and cold, and their sense of pain is so obtuse that some of the more idiotic seem hardly to know what it is. In their dull lives, such pain as can be excited in them may literally be accepted with a welcome surprise. Milder and subtler version so f this denigration continue down to the modern day. In 2004 author Michael D’Antonio published a book titledThe State Boys Rebellion about the ill treatment of boys in the Walter E. Fernald School for the Feebleminded and how a group of boys residing at the school rebelled against this treatment. Disturbingly, however, reviews of the book tended to focus on the stories of those boys who later were found to have normal IQs. TheThe York Times Book Review (June 27, 2004) titled its review “A Ledger of Broken Arms: Misdiagnosis and Abuse at a School for the ‘Feebleminded’ in the 1950s.” We might ask what in the world does “misdiagnosis” have to do with the issue of highlighting the ill treatment in these institutions? The implication here is that somehow it was less tragic for those “properly diagnosed”—whatever that may mean in this context. Shades of Galton, and of the dark side of the deification of intelligence, are revealed in the reactions to this book.

Let us suppose, unrealistically, that IQ tests really measure intellectual ability. Let us in fact assume, even more unrealistically, that they measure the intellectual abilities that are relevant to success in metaphysics. Why should we suppose that a species with a mean IQ of 100—our own species—isable to solve the problems of metaphysics? Pretty clearly a species with a mean IQ of 60 wouldn’t be in a position to achieve this. Pretty clearly, a species with a mean IQ of 160 would be in a better position than we to achieve this. Why should we suppose that the “cut-off-point” is something like 90 or 95? Why shouldn’t it be 130 or 170 or 250? The conclusion of this meditation on mystery is that if metaphysics does indeed present us with mysteries that we are incapable of penetrating, this fact is not itself mysterious. It is just what we should expect, given that we are convinced that beings only slightly less intellectually capable than ourselves wouldcertainly be incapable of penetrating these mysteries. If we cannot know why there is anything at all, or why there should be rational beings, or how thought and feeling are possible, or how our conviction that we have free will could possibly be true, why should that astonish us? What reason have we, what reason could we possibly have, for thinking that our intellectual abilities are equal to the task of answering these questions?

If I had doubts of being able to light up a room by quicksilver intelligence, something I both envied and suspected, I was confident of having an ability to find my way to clear things of my own to say, some of which might produce a longer light. But what was definitely also needed in order to produce these goods was the onward marching.

Intelligent life on a planet comes of age when it first works out the reason for its own existence. If superior creatures from space ever visit earth, the first question they will ask, in order to assess the level of our civilization, is: ‘Have they discovered evolution yet?’ Living organisms had existed on earth, without ever knowing why, for over three thousand million years before the truth finally dawned on one of them.

Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices, but honestly and courageously uses his intelligence.