THE ASSORTATIVE MATING THEORY

Simon Baron-Cohen [4.5.05]
Topic:

My thesis with regard to sex differences is quite moderate, in that I do not discount environmental factors; I'm just saying, don't forget about biology. To me that sounds very moderate. But for some people in the field of gender studies, even that is too extreme. They want it to be all environment and no biology. You can understand that politically that was an important position in the 1960s, in an effort to try to change society. But is it a true description, scientifically, of what goes on? It's time to distinguish politics and science, and just look at the evidence.

Introduction by John Brockman

Simon Baron-Cohen is Professor of Developmental Psychopathology and Director of the Autism Research Centre at Cambridge University. In this Edge feature, he presents his new Assortative Mating Theory which connects his two fields of research: the characteristics of autism in terms of understanding what's going on in the brain and the causes of the condition; and understanding the differences between males and females.

"My new theory is that it's not just a genetic condition," he says, "but it might be the result of two particular types of parents, who are both contributing genes. This might be controversially received. This is because there are a number of different theories out there — one of which is an environmental theory, such as autism being caused by vaccine damage — the MMR vaccine (the measles, mumps, and rubella combination vaccine). Another environmental theory is that autism is due to toxic levels of mercury building up in the child's brain. But the genetic theory has a lot of evidence, and what we are now testing is that if two "systemizers" have a child, this will increase the risk of the child having autism. That's it in a nutshell.

Baron-Cohen realizes that his theory might raise anxieties. "Just because it's potentially controversial," he says, "doesn't mean that we shouldn't investigate it. And there are ways that you can investigate it empirically."

He also expects controversy. Given the continuing public discussing in the US about innate sex differences, he will, no doubt, be challenged when he says "It was interesting for me to discover that there's been a sleight of hand, mostly in the States, such that the word 'sex' has been replaced by the word 'gender'. Baron-Cohen believes that it's time "to distinguish politics and science, and just look at the evidence". Others will feel differently.

— JB

SIMON BARON-COHEN is Professor of Developmental Psychopathology and Director of the Autism Research Centre at Cambridge University. He is also a Fellow of Trinity College, Cambridge. His books include Mindblindness; and The Essential Difference: The Truth about the Male and Female Brain.

SIMON BARON-COHEN's Edge Bio Page

THE REALITY CLUB: Marc D. Hauser, Steven Pinker, Armand Leroi, Carole Hooven, Elizabeth Spelke, Alison Gopnik, David C. Geary, Helena Cronin, Linda S. Gottfredson. NEW Baron-Cohen responds.


THE ASSORTATIVE MATING THEORY

[BARON COHEN:] I've been working on the question of autism, trying to understand what characterizes autism from a psychological perspective and ultimately aiming to understand what's going on in the brain and the causes of the condition. My new theory is that it's not just a genetic condition, but it might be the result of two particular types of parents, who are both contributing genes. This might be controversially received. This is because there are a number of different theories out there — one of which is an environmental theory, such as autism being caused by vaccine damage — the MMR vaccine (the measles, mumps, and rubella combination vaccine). Another environmental theory is that autism is due to toxic levels of mercury building up in the child's brain. But the genetic theory has a lot of evidence, and what we are now testing is that if two "systemizers" have a child, this will increase the risk of the child having autism. That's it in a nutshell.

A systemizer is somebody whose style of thinking is predominantly in terms of understanding things according to rules or laws. You can think of lots of different kinds of systems: mathematical systems (algebra, computer programs), or mechanical systems (computers or cars); natural systems (weather, or rocks, geology); and social systems (businesses, or the military).

In each case, when you systemize what you do, you try to understand the system in terms of the laws that govern the system. Economics would be an example of a system, where people are trying to predict a crash, or predict what's going to happen in terms of stock markets. They are trying to understand things according to laws or rules. The theory we are testing is that if you have a mother and a father who are both systemizers, the risk of the child having autism increases.

Systemizing is expressed in behavior, so, for example, if your hobby is playing with computers, that's the behavior that you see. But obviously such activity reflects your interests, which is what's going on in your mind, not just in your behavior. The mind of a systemizer is drawn to understand systems. Steven Pinker has a nice phrase about spiders, that spiders are just programmed to spin webs. He uses that as an analogy for the way in which a typically developing child is programmed to learn language. These programs are not a hundred percent deterministic; you can intervene, you can change. There's obviously plasticity in the system. In the same way, systemizing isn't going to turn out to be a hundred percent genetic. There are few if any behavioral characteristics in humans that are one hundred percent genetic .

There are five steps for testing this theory. First, we need to establish whether or not systemizing runs in families. Secondly, we need to find out if there are there any genes associated with systemizing. Thirdly, are the parents of children with autism systemizers, defined according to their cognition? Fourthly, do they both carry the genes for systemizing? Finally, when these genes combine, does this raise the risk of their child having autism?

This theory will be controversial, and it might raise anxieties. But just because it's potentially controversial doesn't mean that we shouldn't investigate it. And there are ways that you can investigate it empirically.

How would you investigate it? Well first thing is to look at families where there's already an autistic child, and look at the parent directly. We've already conducted some of those studies, and found that whereas in the general population systemizing is more common among males, in the case of parents of a child with autism, the mother of such a child is also very likely to be a systemizer, with male-typical interests.

One example of how we test this is to give them a task where you have to analyze a visual pattern as quickly as you can to find a component part. In the general population males are quicker at this kind of analytic task, but in the case of parents of children with autism, the mothers are just as fast as typical males. The mothers are showing a typical male profile, and that's counter-intuitive since you would expect them to be showing a more typical female profile. That's just one clue that this theory is worth exploring.

A second clue is that we've looked at the rates of engineering in both fathers and the grandfathers of children with autism. Engineering is an occupation where you have to be a good systemizer, for example, understanding mechanical systems. We found that fathers of children with autism are over-represented in the field of engineering. And what was interesting was that we found exactly that same pattern in the grandparents too.

You start with the child with autism; he or she is the end result of this experiment of nature. And you work backwards to see if there were there clues in the previous generation — or previous two generations. This new theory is called "the assortative mating theory", The clue that both sides of the family are contributing similar genes is that in our study of occupations, grandfathers on the maternal and the paternal sides were both more likely to be working in the field of engineering. So the strong systemizing wasn't coming down just one side of the family. It's called assortative mating because it describes the idea that two individuals might end up in a union because of having similar characteristics. They're selecting each other on the basis of having similar characteristics.

The assortative mating theory connects with the field of sex differences — my other big area of interest . I've been trying to understand the differences between males and females. It was interesting for me to discover that there's been a sleight of hand, mostly in the States, such that the word 'sex' has been replaced by the word 'gender'.

This has happened in a very subtle way over the last century, so that in the States, nobody talks about sex differences; they talk about gender differences. Whenever you want to refer to somebody's sex you refer to their gender. I call it a sleight of hand, because actually 'sex' is the older word. Your sex is either male or female, and in biology your sex is defined by whether you have 2 X chromosomes or an X and Y chromosome. There's been a subtle shift into talking about gender, to whitewash the word sex.

Why has this happened? Presumably, because your sex is determined by your chromosomes. And in the States the ideology is that we shouldn't be determined by anything; we should be able to be anything we choose. The blank slate. Gender refers to how you think of yourself: as masculine, or feminine, It's much more subjective, and is commonly believed to be culturally constructed. Italian male gender behavior is expressed differently from English male gender behavior. This gives the impression that people's gender behaviour can change as they change culture, even if their biological sex is fixed.

Talking about gender is therefore much more optimistic than talking about sex. It's the rags to riches idea — you can become anything. But I've been very interested to go back to the original notion of sex, as a biological characteristic, and to ask if there are there any essential differences between males and females in the mind. And to understand that if there are psychological differences, what are the biological mechanisms that give rise to these? Are they genes, are they hormones?

In our own work, we have been focused very much on fetal testosterone — the hormone that the fetus is producing in the womb, to see whether that has any effect on later behavior. We had a perfectly good word, which was sex. But it's become almost a profane word in the U.S. I recently wrote a journal article on sex differences in the mind. Everywhere I'd written the word sex, the copy editor changed it to the word gender. A systematic change had been introduced, and I asked that the original word be used. The editors asked me to give them a good reason, because they explained in the States the preferred word is gender. I had to explain, a person's gender is different to their sex. It's a distinction that seems to have got lost. It's hard to know whether it was deliberate, or whether it just happened without anybody noticing.

Back to hormones. We've been conducting laboratory studies on the amniotic fluid in the womb — the fetus is effectively swimming in this amniotic fluid. We analyze how much testosterone, the so-called male hormone, is in the amniotic fluid. It's not actually a male hormone, because both sexes produce it, it's just that males produce a lot more than females. That's because it comes from the testes. Females also produce it in the adrenal glands. And even within the boys, or within the girls, you see individual differences in how much is produced.

The question is, does this translate into anything psychological if you follow up those children? We measured the amniotic fluid testosterone, then waited until the baby was born, and then looked at the baby's at 12 months old, 18 months old, two years old. It's a longitudinal prospective study.

What we found is that the higher the baby's level of fetal testosterone, the less eye contact the child makes at 12 months old. And also the slower they are to develop language at 18 months old. To me these are really fascinating results, because we're looking at something biological, in this case a hormone which presumably is influencing brain development to produce these quite marked differences in behavior. We always knew that girls talked earlier than boys — that there is sex difference in language development — and we also knew that there's huge variability at 18 months: some kids have no words at all, and other kids have huge vocabularies, about 600 words. No one's really been able to explain this variability. Why should one kid be almost mute and another kid be very verbal?

People have identified some factors, such as that first-born children talk earlier than later-born children. Obviously there are environmental factors that are relevant. Presumably that's because first-born children get much more attention from their parents. But over and above your birth order, it looks like hormones also explain some of the variability. We've now followed up these kids into school, they're four years old, and we're still finding that the prenatal hormone production levels are influencing behavior in middle childhood. This is just one example of why we shouldn't ignore biology in explaining differences in how the mind works.

I don't argue it's all biology. But for a long time social behavior and language development were seen as purely environmental or learned experience. These hormone studies suggest hormones are also part of the explanation. We also know from medical conditions that if, for example, for genetic reasons you have an overproduction of testosterone, this condition can change your behavior. So if you look at girls with a condition called Congenital Adrenal Hyperplasia (CAH), where they are producing too much testosterone for genetic reasons, they look like tomboys. Their interests are very male-typical interests; they like playing with little toy cars, they like building tree houses, and they perform very quickly on spatial tests, unlike typical girls.

The evidence for my assertions comes from experiments. And in all of these areas I'm not interested in beliefs without evidence.

One experiment we conducted here in Cambridge was at the local maternity hospital. Essentially we wanted to find out whether sex differences that you observe later in life could be traced back to birth, to see if such differences are present at birth. In this experiment we looked at just over one hundred newborn babies, 24 hours old, which was the youngest we could see them, and we presented each baby with a human face to look at, and then a mechanical mobile suspended above the crib. Each baby got to see both objects.

Obviously these objects are different in interesting ways, because the human face is alive, and it can express emotion, it's a natural object. The mechanical mobile is man-made, it's not alive, and obviously it doesn't have emotions. We tried to make the two objects equivalent in some important ways. One is that they were both the same size; another was that they were a similar colour, in order to try and control features that might be grabbing the child's attention. But effectively what we did was film how long each baby looked at each of these two objects.

We asked the mothers not to tell us the sex of their babies, so that we could remain blind to whether this was a boy or a girl. And for the most part that was possible. Sometimes it was possible to guess that this was a boy or a girl, because there would be cards around the bed saying, "Congratulations, it's a boy." That potentially could have undermined the experiment, although we then gave the videotapes to a panel of judges to simply measure how long the baby looked at the face or the mobile. By the time the judges were looking at these videotapes they didn't have any of these potential clues to the sex of the baby, because all you could see was the eyes of the baby.

The results of the experiment were that we found more boys than girls looked longer at the mechanical mobile. And more girls than boys looked longer at the human face. Given that it was a sex difference that emerged at birth, it means that you can't attribute the difference to experience or culture. Twenty-four hours old. Now you might say, well, they're not exactly new-born, it would have been better to get them at 24 minutes old — or even younger. But obviously we had to respect the wishes of the parents and the doctors to let the baby relax after the trauma of being born. And let the parents get to know their baby. So strictly speaking, it might have been one day of social experience. But nonetheless, this difference is emerging so early that suggests it's at least partly biological.

The results were published in 2001 in a scientific journal and the experiment hasn't yet been replicated, and obviously in science what is needed is independent replication. I'll be interested in other labs to attempt to do this. As far as I know there hasn't been any attempt. This may be because it's quite hard work.

To test a hundred babies, you have to hang around hospitals waiting for babies to be born. That sounds pretty straight-forward, because babies are being born every day. In a city like Cambridge there are about five new babies born a day. For some reason babies tend to be born in the middle of the night, about two or three o'clock in the morning. You have to have a very dedicated research team who are willing to wait. In Cambridge, mothers only stay in hospital for one day. Maybe one night. Then they are sent home, in order to vacate the bed for another expectant mother. In terms of your window of opportunity for testing babies, you therefore have to be there at the right time. We had two very hard-working master's students who approached mothers to ask for parental consent — maybe that was easier in a city like Cambridge, because parents know that in a university town, research is going on.

The test is not invasive — the baby just has to lie on their back and look up. They were presented with each object for only one minute, because babies tend to get very restless very quickly. It's a difficult experiment to conduct, because babies spend most of their time sleeping, or feeding, or crying. You have to wait until they're not doing any of those three things. When they're awake and calm, you have a couple of minutes to present the stimuli.

The camera is well-hidden off to one side. Babies can't see very far — the depth of vision of a newborn baby is only between 15 and 20 centimeters. So it is unlikely that the presence of the camera itself affected how the baby responded.

I was expecting the experiment to be received more controversially, because as far as I know it is one of the first demonstrations of a sex difference in the mind at birth. In fact it was published without any fuss. It may be simply that the climate has now changed, and that people are much willing to accept that there are sex differences in the mind, and that these might even be partly biological. If that's true, then this is good news for scientists who are interested in how the mind works.

My thesis with regard to sex differences is quite moderate, in that I do not discount environmental factors; I'm just saying, don't forget about biology. To me that sounds very moderate. But for some people in the field of gender studies, even that is too extreme. They want it to be all environment and no biology. You can understand that politically that was an important position in the 1960s, in an effort to try to change society. But is it a true description, scientifically, of what goes on? It's time to distinguish politics and science, and just look at the evidence.