I spent this Tuesday in Washington, reviewing autism research proposals. I really enjoy that work, because it puts me among some of the best minds in autism science. In the course of our discussions, an intriguing question arose.
We know autism is far more common in males, but the reason why remains elusive. It's one of those facts of autism that most people take for granted, and simply accept for what it is. In earlier essays on this blog I have considered possible explanations, from Simon Baron Cohen's theory that autism is "exaggerated maleness" to reasons why females might be undiagnosed and undetected.
All the explanations I have heard so far do not account for this interesting observation:
If the male/female ratio within a society is 50/50, any random group of families sould have a 50/50 distribution of sons and daughters. Some families would have one child, others would have three. Some would have all sons and others two daughters and a son. Taken together, we would expect the total of sons and daughters to be equal.
If we assemble a collection of families in which there is at least one autistic child, that distribution of sons and daughters is not 50/50. It favors the males. Any autism researcher who has worked with families knows that to be true, even in the absence of hard studies to quantify it. Why?
All of us know families that have all sons or all daughters. We don't make anything more of that that we do tossing a coin and having it come up heads three times in a row. Just chance, we say. But when you identify a group of families with a trait like autism, and they all have more sons than daughters . . . suddenly it stops looking random and starts to seem the result of something else.
If this were a roll of the dice, you'd start to think the dice were loaded.
One explanation is that some parents have a son with autism and stop having children. So the girls that might even the male/female ratio are never born. I think that explanation may be true today, but what about the ages before modern birth control?
Critics might say that we don't know how autism was distributed among the sexes a hundred years ago, and that's true. The autism diagnosis has only existed for sixty-some years. Yet we do have strong anecdotal evidence. Using that, some modern day people have "diagnosed" historical figures with autism based on what we know of them and their lives. How many of those individuals are female? Almost none.
Those "post-mortem diagnoses" are certainly subject to challenge and I'm sure some are even wrong. That said, they can't all be wrong and the male-female ratio in the known historical record of autism remains strikingly tilted toward the male side.
Geri Dawson suggested another possible explanation for the male-female imbalance. What if girl embryos are actually more susceptible to some factor implicated in autism, but in a different way? The factor that produces autistic baby boys might result in unsuccessful pregnancies when the fetus is female. The result - fewer baby girls with autism are born.
To the best of my knowledge, no one has studied pregnancies in families with autism. All that has been studied are the resultant children. We don't know how many miscarriages may have preceded or followed the birth of an autistic boy. The incidence of miscarriage in general has been studied and it would be interesting to know if families with autism deviate from the norms in that regard.
The son-daughter imbalance certainly ties in with the Baron-Cohen "maleness" theory. If autism indeed an expression of excessively male genetic material, that imbalance might result in more males being born in those families.
I spoke to several scientists and it became clear that this is one of those obvious questions that has never really been answered. There is the general belief that autism families contain more males, but we have no hard data to illustrate the difference. We also don't have any multi generational data, which could shed light on the heritability of the condition.
In my own family, I have one child, a son with Asperger's. My father had many Aspergian traits, but he died before anyone thought to explore that possibility. He had a brother, and no sisters. His father also had a brother and no sisters. His grandfather had three brothers and a sister. Is there a pattern there that relates to autism? I really don't know.
It would be very interesting to see a study that addressed this question. Perhaps a grad student somewhere will read this, and bring a research proposal to our next review meeting . . . .
Stranger things have happened.