What of Mr. Summers's other claim, that such sex differences are innate? We know that culture plays a role in the divergence of the sexes, but so does biology. For example, on the first day of life, male and female newborns pay attention to different things. On average, at 24 hours old, more male infants will look at a mechanical mobile suspended above them, whereas more female infants will look at a human face.

It has also been found that the amount of prenatal testosterone, which is produced by the fetus and measurable in the amniotic fluid in which the baby is bathed in the womb, predicts how sociable a child will be. The higher the level of prenatal testosterone, the less eye contact the child will make as a toddler, and the slower the child will develop language. That is connected to the role of fetal testosterone in influencing brain development.

Males obviously produce far more prenatal testosterone than females do, but levels vary considerably even across members of the same sex. In fact, it may not be your sex per se that determines what kind of brain you have, but your prenatal hormone levels. From there it's a short leap to the intriguing idea that a male can have a typically female brain (if his testosterone levels are low), while a female can have a typically male brain (if her testosterone levels are high). That notion fits with the evidence that girls born with congenital adrenal hyperplasia, who for genetic reasons produce too much testosterone, are more likely to exhibit "tomboy" behavior than girls with more ordinary hormone levels.

What does all this have to do with autism? According to what I have called the "extreme male brain" theory of autism, people with autism simply match an extreme of the male profile, with a particularly intense drive to systemize and an unusually low drive to empathize. When adults with Asperger's syndrome (a subgroup on the autistic spectrum) took the same questionnaires we gave to non-autistic adults, they exhibited extreme Type S brains. Psychological tests reveal a similar pattern.

And this analysis makes sense. It helps explain the social disability in autism, because empathy difficulties make it harder to make and maintain relationships with others. It also explains the "islets of ability" that people with autism display in subjects like math or music or drawing -- all skills that benefit from systemizing.

People with autism often develop obsessions, which may be nothing other than very intense systemizing at work. The child might become obsessed with electrical switches (an electrical system), or train timetables (a temporal system), or spinning objects (a physical system), or the names of deep-sea fish (a natural, taxonomic system). The child with severe autism, who may have additional learning difficulties and little language ability, might express his obsessions by bouncing constantly on a trampoline or spinning around and around, because motion is highly lawful and predictable. Some children with severe autism line objects up for hours on end. What used to be dismissed by clinicians as "purposeless, repetitive behavior" may actually be a sign of a mind that is highly tuned to systemize.

One needs to be extremely careful in advancing a cause for autism, because this field is rife with theories that have collapsed under empirical scrutiny. Nonetheless, my hypothesis is that autism is the genetic result of "assortative mating" between parents who are both strong systemizers. Assortative mating is the term we use when like is attracted to like, and there are four significant reasons to believe it is happening here.