The Gender Blur

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Thus the issue becomes not whether there is a biologically influenced sex difference in aggression—the answer being a solid, technical "You betcha"—but rather how rigid that difference is. The best science, in my opinion, tends to align with basic common sense. We all know that there are extraordinarily gentle men and murderous women. Sex differences are always generalizations: They refer to a behavior, with some evolutionary rationale behind it. They never define, entirely, an individual. And that fact alone should tell us that there's always—even in the most biologically dominated traits—some flexibility, an instinctive ability to respond, for better and worse, to the world around us.

This is true even with physical characteristics that we've often assumed are nailed down by genetics. Scientists now believe height, for instance, is only about 90 percent heritable. A person's genes might code for a six-foot-tall body, but malnutrition could literally cut that short. And there's also some evidence, in girls anyway, that children with stressful childhoods tend to become shorter adults. So while some factors are predetermined, there's evidence that the prototypical male/female body design can be readily altered.

It's a given that humans, like most other species—bananas, spiders, sharks, ducks, any rabbit you pull out of a hat—rely on two sexes for reproduction. So basic is that requirement that we have chromosomes whose primary purpose is to deliver the genes that order up a male or a female. All other chromosomes are numbered, but we label the sex chromosomes with the letters X and Y. We get one each from our mother and our father, and the basic combinations are these: XX makes female, XY makes male.

There are two important—and little known—points about these chromosomal matches. One is that even with this apparently precise system, there's nothing precise—or guaranteed—about the physical construction of male and female. The other point makes that possible. It appears that sex doesn't matter in the early stages of embryonic development. We are unisex at the point of conception.

If you examine an embryo at about six weeks, you see that it has the ability to develop in either direction. The fledgling embryo has two sets of ducts—Wolffian for male, Muellerian for female—an either/or structure, held in readiness for further development. If testosterone and other androgens are released by hormone producing cells, then the Wolffian ducts develop into the channel that connects penis to testes, and the female ducts wither away.

Without testosterone, the embryo takes on a female form; the male ducts vanish and the Muellerian ducts expand into oviducts, uterus, and vagina. In other words, in humans, anyway (the opposite is true in birds), the female is the default sex. Back in the 1950s, the famed biologist Alfred Jost showed that if you castrate a male rabbit fetus, choking off testosterone, you produce a completely feminized rabbit.

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