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.
(c) 2007-2011 John Elder Robison