From Steven Pinker's The Blank Slate: The Modern Denial of Human Nature:

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But of course the minds of men and women are not identical, and recent reviews of sex differences have converged on some reliable differences. Sometimes the differences are large, with only slight overlap in the bell curves.

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With some other traits the differences are small on average but can be large at the extremes. That happens for two reasons. When two bell curves partly overlap, the farther out along the tail you go, the larger the discrepancies between the groups. For example, men on average are taller than women, and the discrepancy is greater for more extreme values. At a height of five foot ten, men outnumber women by a ratio of thirty to one; at a height of six feet, men outnumber women by a ratio of two thousand to one. Also, confirming an expectation from evolutionary psychology, for many traits the bell curve for males is flatter and wider than the curve for females. That is, there are proportionally more males at the extremes. Along the left tail of the curve, one finds that boys are far more likely to be dyslexic, learning disabled, attention deficient, emotionally disturbed, and mentally retarded (at least for some types of retardation).

At the right tail, one finds that in a sample of talented students who score above 700 (out of 800) on the mathematics section of the Scholastic Assessment Test, boys outnumber girls by thirteen to one, even though the scores of boys and girls are similar within the bulk of the curve [NOTE this was from the pre-1994 SAT where the math section was harder and not truncated at the top. Today the ratio is a bit less than 2-1 at the top end.]. With still other traits, the average values for the two sexes differ by smaller amounts and in different directions for different traits. Though men, on average, are better at mentally rotating objects and maps, women are better at remembering landmarks and the positions of objects. Men are better throwers; women are more dexterous. Men are better at solving mathematical word problems, women at mathematical calculation. Women are more sensitive to sounds and smells, have better depth perception, match shapes faster, and are much better at reading facial expressions and body language. Women are better spellers, retrieve words more fluently, and have a better memory for verbal material.

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Nonetheless, discussions of the leaky pipeline in science rarely even mention an alternative to the theory of barriers and bias. One of the rare exceptions was a sidebar to a 2000 story in Science, which quoted from a presentation at the National Academy of Engineering by the social scientist Patti Hausman: "The question of why more women don’t choose careers in engineering has a rather obvious answer: Because they don’t want to. Wherever you go, you will find females far less likely than males to see what is so fascinating about ohms, carburetors, or quarks. Reinventing the curriculum will not make me more interested in learning how my dishwasher works."

An eminent woman engineer in the audience immediately denounced her analysis as “pseudoscience.” But Linda Gottfredson, an expert in the literature on vocational preferences, pointed out that Hausman had the data on her side: “On average, women are more interested in dealing with people and men with things.” Vocational tests also show that boys are more interested in “realistic,” “theoretical,” and “investigative” pursuits, and girls more interested in “artistic” and “social” pursuits.

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The most dramatic example comes from an analysis by David Lubinski and Camilla Benbow of a sample of mathematically precocious seventh-graders selected in a nationwide talent search. The teenagers were born during the second wave of feminism, were encouraged by their parents to develop their talents (all were sent to summer programs in math and science), and were fully aware of their ability to achieve. But the gifted girls told the researchers that they were more interested in people, “social values,” and humanitarian and altruistic goals, whereas the gifted boys said they were more interested in things, “theoretical values,” and abstract intellectual inquiry. In college, the young women chose a broad range of courses in the humanities, arts, and sciences, whereas the boys were geeks who stuck to math and science. And sure enough, fewer than 1 percent of the young women pursued doctorates in math, physical sciences, or engineering, whereas 8 percent of the young men did. The women went into medicine, law, the humanities, and biology instead.

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Gottfredson points out, “If you insist on using gender parity as your measure of social justice, it means you will have to keep many men and women out of the work they like best and push them into work they don’t like.” She is echoed by Kleinfeld on the leaky pipeline in science: “We should not be sending [gifted] women the messages that they are less worthy human beings, less valuable to our civilization, lazy or low in status, if they choose to be teachers rather than mathematicians, journalists rather than physicists, lawyers rather than engineers.” These are not hypothetical worries: a recent survey by the National Science Foundation found that many more women than men say they majored in science, mathematics, or engineering under pressure from teachers or family members rather than to pursue their own aspirations— and that many eventually switched out for that reason. I will give the final word to Margaret Mead, who, despite being wrong in her early career about the malleability of gender, was surely right when she said, “If we are to achieve a richer culture, rich in contrasting values, we must recognize the whole gamut of human potentialities, and so weave a less arbitrary social fabric, one in which each diverse human gift will find a fitting place.”

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When you combine the imbalance in high-end math test scores, with the imbalance of innate preference for abstract work versus people work, it is not at all surprising that only 20% of Pinterest would be female.

If Pinterest feels like its hiring practice is not meritocratic, it should fix that.

If Pinterest feels like it is missing out on an untapped candidate pool, it should figure out how to tap that pool.

But there is zero reason to try to make the female ratio 30% for the sake of making it 30%. There is no moral reason, there is no practical reason. Males and females are different, twas ever thus. This obsession with equalizing employment number in all spheres is a bizarre (and destructive) fashion of our age.