Summers proposed "that in the special case of science and engineering, there are issues of intrinsic aptitude, and particularly of the variability of aptitude, and that those considerations are reinforced by what are in fact lesser factors involving socialization and continuing discrimination." In other words, biology outweighs environment. No evidence he presented justifies this hypothesis. So how did he reach it?
First, he rashly extrapolated from the limits of socialization in one area to the limits of socialization in another. "Most of what we've learned from empirical psychology in the last 15 years has been that people naturally attribute things to socialization that are in fact not attributable to socialization," he said. "We've been astounded by the results of separated twins studies. The confident assertions that autism was a reflection of parental characteristics … have now been proven to be wrong." For this reason, he was "hesitant about assigning too much weight" to the idea that girls and boys are socialized differently.
In the Q&A, a questioner pointed out that the environmental differences affecting identical twins (which are always of the same sex) are nothing like the environmental differences affecting boys and girls. Summers replied,
The field of behavioral genetics had a revolution in the last 15 years, and the principal thrust of that revolution was the discovery that a large number of things that people thought were due to socialization weren't, and were in fact due to more intrinsic human nature. And that set of discoveries, it seemed to me, ought to influence the way one thought about other areas where there was a perception of the importance of socialization. I wasn't at all trying to connect those studies to the particular experiences of women and minorities who were thinking about academic careers.
Any Harvard student who gave this answer on an exam would be flunked. If you aren't claiming that a highly abstract resemblance to another subject has any bearing on this one—and you present no evidence to justify the cross-application—you have no business bringing it up.
Second, Summers confused two different causal conflicts. In the course of arguing that socialization was a less persuasive explanation for differential outcomes than biology was, he observed, "When there were no girls majoring in chemistry, when there were no girls majoring in biology, it was much easier to blame parental socialization. Then, as we are increasingly finding today, the problem is what's happening when people are 20, or when people are 25, in terms of their patterns with which they drop out." In other words, even after we've substantially canceled out differences in socialization by getting women to major successfully in sciences, they still drop out of the academic race. Well, yes. But that doesn't show that the alternative factor is biology. It just shows that there's an alternative factor—and Summers had already mentioned two other alternative factors that would more plausibly affect 25-year-old women: bias against women and bias against people who bear and raise children. The limits of egalitarian socialization in controlling a woman's career prove nothing about the limits of sexist socialization in shaping a girl.
At one point, Summers acknowledged, "It's pointed out by one of the papers at this conference that these tests are not a very good measure and are not highly predictive" of academic success. "And that's absolutely right," said Summers. "But I don't think that resolves the issue at all. Because if … there are some systematic differences in variability in different populations, then whatever the set of attributes are that are precisely defined to correlate with being an aeronautical engineer at MIT or being a chemist at Berkeley, those are probably different in their standard deviations as well."
What? This is pure abstract inference at an absurd level. It's also incoherent. You can't presume that men and women differ in the second respect while inferring this presumption from a likeness to their difference in the first. Either you presume similarities, or you presume differences.
Why did Summers make these mistakes? The transcript suggests two conflicting reasons. One is that he's stubborn and argumentative. He repeatedly deflected cultural explanations by saying things like, "No doubt there is some truth in that," "This kind of taste does go on," and "Yeah, look, anything could be social"—and then minimizing these explanations. The consistent tone of his remarks was "Yeah, but …" There are two possible explanations for that tone in this context. One is that he's a sexist. The other is that once he offers a hypothesis, he'd rather defend and extend it than listen objectively to the alternatives. He's got an open mind but not an open heart.
I suspect this, rather than sexism, is the root of Summers' errors, because a sexist wouldn't have said what he said while displaying a second intellectual flaw evident in the transcript. Again and again, Summers warned his listeners to be skeptical of what they'd prefer to believe. We all want to believe socialization explains differences in male and female outcomes, he observed. Therefore, he reasoned, we should distrust that hypothesis and look for evidence to the contrary. He was so busy being skeptical of the popular explanation that he forgot to be skeptical of the unpopular one. He overstated the case for innate sex differences not because he wanted to believe it, but because he didn't.
If you think this explanation is too kind to Summers, ask yourself why he told the story about his daughters. An old-fashioned sexist wouldn't have told that story, because he wouldn't have been surprised at his daughter's maternal behavior—never mind that he wouldn't have given her a truck in the first place. Summers brought up the incident not because it would rock the academic world—it didn't—but because it rocked him. As he put it, the incident "tells me something." He wasn't speaking as the president of Harvard or even as a scholar. He was speaking as a modern dad who thought he could overcome nature and discovered he couldn't.