Over the years I've been struck by a pattern among the parents of children with autism. The mothers often say things like "my child is a lot like my husband—just writ large. My husband has to watch the weather forecasts every night, and my son has to watch them every hour." When I ask about their parents, the mothers comment, "Well, my father was rather similar to my husband—he collected model trains and knew everything there was to know about each one."
Such observations don't amount to evidence about the cause of autism, but they do give us clues about where to look. Autism is at root genetic, but new research from my lab at Cambridge University implicates genes inherited from both parents. From this and other observations, we've formulated the "assortative mating theory." Its central idea is that both mothers and fathers of children with autism (or its milder variant, Asperger Syndrome) share a common characteristic and have been attracted to each other because of their psychological similarity.
Advertisement
Assortative mating is a term borrowed from the field of genetics that refers to a long-recognized aspect of animal behavior: the simple idea that mate selection is not random. Anmals, including human animals, do not mate with just anyone.
Darwin theorized that two kinds of selection operate to ensure that some animals have better reproductive success than others: natural selection and sexual selection. Deer with large antlers, for example, are more likely to reproduce—not just because they can defeat weaker males in contests over females (natural selection), but also because the females themselves tend to prefer males with the largest antlers (sexual selection). Animals are finely tuned to external indicators of fitness, and these indicators influence whether or not they will mate with a potential partner.
Assortative mating goes one step further by noting that two animals of the same species often end up mating when they have a common or similar trait. A clear example in humans is that taller men tend, on average, to have female partners who are also above average in height. Unconsciously, we seem to select partners who are similar to ourselves in at least one respect. Other studies in humans have shown assortative mating for physical characteristics as subtle as eye color and for psychological characteristics such as personality.
So what has all of this got to do with autism? We know that autism runs in families, and that if a child with autism is a twin, the chances of the other twin also having autism is much higher if the twins are identical. This tells us that genes are likely to be an important part of the explanation, and that one should look at the parents of children with
autism for clues. Furthermore, our studies have uncovered four findings that implicate assortative mating in autism. First, both parents of children with autism are likely to be super-fast on attention tasks, in which the aim is to spot a detail as quickly as possible. Second, both parents have an increased likelihood of having had a father who worked in the field of engineering. Third, both parents are more likely to have elevated scores on subtle measures of autistic traits. And fourth, both parents show a trend toward a more male pattern of brain activity when measured using MRI.
The chances of both parents displaying these similarities are vanishingly small. Something must be causing two such individuals to be attracted to one another. I propose that "something" is strong systemizing—the drive to analyze the details of a system in order to understand how it works.