Even with barriers stripped away, they don’t behave like male clones. So I began to wonder what would happen if all the “shoulds” — the policy and political agendas — were shifted to the side for a moment to examine the science. Would female really look like an alternative version of male? As a developmental psychologist, I could see that males were hardly a neutral, homogeneous group. Instead of being what de Beauvoir called “the absolute vertical with reference to which the oblique is defined,” it was clear that boys and men demonstrate a wide range of biologically based foibles that make many unpredictable, others fragile, and still others reckless or even extreme. If anyone is oblique, it’s males.

For me, the question of whether males really fit our expectation of the standard, neutral gender — what I’ll call the “vanilla” gender — started in my pediatric clinic waiting room. Over twenty years of clinical practice and teaching as a child psychologist, I had seen mostly males. Boys and men with learning problems, attention problems, aggressive or antisocial boys, those with autistic features, those who didn’t sleep well or make friends, or couldn’t sit still, dominated my practice — and that of every other developmental psychologist I knew. Research confirmed the gender breakdown of my waiting room.

Learning problems, attention deficit disorder, and autism spectrum disorders are four to ten times as common in boys; anxiety and depression twice as common in girls. From the point of view of learning and self-control, boys are simply more vulnerable. Defining their strengths and weaknesses, and teaching others how to, had been the focus of the first half of my working life. I had been at it so long that many of my first charges were now adults, and to my surprise, I began to see some of them featured as success stories in the press. One had become a designer of international renown. Another had made money as a financial analyst and was leapfrogging from one investment bank to another.

A third had become an electrical engineer who had pioneered an invention. A fourth was a chef on his way up. And there were more. These apparently fragile boys had overcome their early difficulties through the support of parents and teachers, who, after all, were attentive and observant enough to seek out a psychologist, presumably only one of many steps they might have taken with that child’s welfare in mind.

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But it occurred to me that there might also be a biological thread. In some, there seemed to be a flip side to early male vulnerability. Many of these initially fragile boys continued to have obsessive interests or an appetite for risk that set the stage for their careers. Meanwhile, many of the girls their age who were light-years ahead of them in classroom learning, language, social skills, and self-control opted for paths that would not necessarily lead them to the highest status or the most lucrative careers. They had other goals. So even if being male made childhood a bumpier road, as adults at work, the situation was reversed.

In The Sexual Paradox I examine the trajectories of these two extreme groups — fragile boys who later succeed, and the gifted, highly disciplined girls who eclipsed them in third grade — as a way of exploring sex differences. These apparent opposites challenge our assumptions. We expect that the fragile boys will continue to struggle. We expect that high-achieving girls will shoot right to the top.

That so many in these groups violate our expectations tells us something important about sex differences. If boys and girls, are on average, biologically and developmentally distinct from the start (and I’ll walk you through some of the more intriguing evidence), wouldn’t these differences affect their choices later? Could men’s and women’s diverging developmental paths and different work priorities be linked?

Excerpted from "The Sexual Paradox" by Susan Pinker. Copyright © 2008 by Susan Pinker. Reprinted by permission of Scribner, an imprint of Simon & Schuster, Inc.

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