When Two Minds Think Alike

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When Two Minds Think Alike

Simon Baron-Cohen discusses how a powerful new idea may give us

valuable insights into the cause of autism.

by Simon Baron-Cohen • Posted November 10, 2006 12:45 AM

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.

Assortative mating is a term borrowed from the field of genetics

that refers to a long-recognized aspect of animal behavior: the sim­

ple 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.

All human brains have a systemizing mechanism that is set at

different levels. The extreme behavior of children with autism

(whose systemizing mechanism may be set too high) may seem like a

far cry from the more moderate behavior seen in their parents and

grandparents, but this new theory proposes that across the

generations these are only differences of degree. Think of a child

with autism, who appears lost in his or her own world, totally

focused on lining up Lego bricks into colorful patterns for hours at

a time. This is hypersystemizing. Then think of the parents we

talked about at the outset: a fascination with weather reports is

one example of systemizing; a fascination with model trains is

another, albeit milder, example.

Evidence from parents in the general population suggests there is

assortative mating for systemizing, such that people who are

attracted to systems are more likely to have a partner who shares

this characteristic. Combined with the fact that both parents of

children with autism are likely to be very detail-oriented, highly

analytic, and to have a father who worked in a field requiring good

systemizing skills, this suggests that the genes involved in

systemizing may be linked to the genes that cause autism.

Although these genes remain to be identified, the assortative mating

theory throws up some testable predictions: First, autism should be

more common in families where both parents are strong systemizers.

For example, some media reports have claimed that autism is more

prevalent in areas like Silicon Valley, but we need well-controlled

tests to see if this is true. Second, since the drive to systemize

is stronger in males than in females, it should be the case that

both mothers and fathers of children with autism are more likely to

have strongly " male " interests and behaviors. Finally, if

systemizing is linked in part to prenatal testosterone levels (which

studies from our lab suggest may be the case), then mothers of

children with autism may be more likely to have testosterone-linked

medical conditions. Again, a highly testable prediction.

Autism has skyrocketed from a rare condition of only four in 10,000

children in the 1970s to an astonishing 1 percent of children today.

This massive change undoubtedly reflects improved recognition of the

condition and consequent growth in services. In addition, even the

way in which we conceptualize autism has changed, from being a

discrete category in the 1970s—you either had autism or you did not—

to a spectrum condition that acknowledges degrees of autism.

Therefore, the boundary between those who have the diagnosis and

those who do not is a fuzzy one, leaving room for clinical

interpretation.

But could the apparent rise in the prevalence of autism also in part

be the result of assortative mating of two strong systemizers? Is it

possible that over and above the effects that social and clinical

factors have had, assortative mating of two strong systemizers has

become more frequent?

Consider that in the late 1950s, less than 2 percent of

undergraduates at MIT (a university that caters to people with good

systemizing skills) were women. Today female enrollment has jumped

to 50 percent. This microcosm is just one example of how society has

changed in ways that would bring strong systemizers into greater

proximity. Over the same period, air travel has also meant far

greater opportunities for people from widely differing backgrounds

to meet, possibly brought together by their common interest in

systems. Finally, over this same timeframe, individuals who are

systemizers have enjoyed new employment opportunities as the result

of the digital revolution. Where 50 years ago a strong systemizer

might have found a job as an accountant, today every workplace needs

computer-savvy employees, and the financial rewards for good

systemizing skills can be immense.

The contribution of assortative mating to the changing prevalence

rate of autism is difficult to study. It is also controversial: the

idea that a child with autism is the result of the unique mixing of

genes from parents who share a common feature sits uncomfortably

with those who want to believe the cause of autism is purely

environmental. But I believe the preliminary evidence in support of

the theory warrants further research. We stand to gain valuable

insight into the cause of autism; and given the links between autism

and the capacity to systemize, a trait that has helped humans to

dominate the planet, we stand to gain a better understanding of

human nature.

—Simon Baron-Cohen is professor of developmental psychopathology and

director of the Autism Research Centre at Cambridge University.