According to a recent study, people who have schizophrenia, autism, and anorexia nervosa have fewer children than the general population. This isn't a particularly happy topic or one that I enjoy dwelling on, but it is a rather important topic to think about when trying to understand whether autism is becoming more common.
One the one hand you have the idea that autism is strongly genetic and runs in families but on the other you have the inconvenient fact that having autism greatly reduces your chances of having children. You would think that those ideas aren't compatible, that the families who have a higher than normal number of members with autism would not keep going generation after generation as they would be "strongly selected against".
Of course the issue isn't really that simple as that. There are many other factors involved that might be contributing to these families staying around such as extremely mild symptoms of autism, recessive traits, the relative rarity of autism, and the like that could explain both facts.
But still, even if both ideas are true and autism is strongly genetic and "strongly selected against", you would, at best, expect it to stay about the same prevalence. It would look more like schizophrenia which is "genetic" (and environmental), runs in families, and is "strongly selected against" and still manages to stay at a relatively constant 1% of the population.
Yet with autism we have three ideas that really can't go together - that autism is strongly genetic, that autism makes you a lot less likely to have children, and that the number of cases of autism is growing fairly rapidly. Although to be fair, all three of these statements are gross oversimplifications of the complex realities of autism.
Just food for thought. The abstract of the study is below.
Fecundity of Patients With Schizophrenia, Autism, Bipolar Disorder, Depression, Anorexia Nervosa, or Substance Abuse vs Their Unaffected Siblings Context It is unknown how genetic variants conferring liability to psychiatric disorders survive in the population despite strong negative selection. However, this is key to understanding their etiology and designing studies to identify risk variants.
Objectives To examine the reproductive fitness of patients with schizophrenia and other psychiatric disorders vs their unaffected siblings and to evaluate the level of selection on causal genetic variants.
Design We measured the fecundity of patients with schizophrenia, autism, bipolar disorder, depression, anorexia nervosa, or substance abuse and their unaffected siblings compared with the general population.
Setting Population databases in Sweden, including the Multi-Generation Register and the Swedish Hospital Discharge Register.
Participants In total, 2.3 million individuals among the 1950 to 1970 birth cohort in Sweden.
Main Outcome Measures Fertility ratio (FR), reflecting the mean number of children compared with that of the general population, accounting for age, sex, family size, and affected status.
Results Except for women with depression, affected patients had significantly fewer children (FR range for those with psychiatric disorder, 0.23-0.93; P < 10-10). This reduction was consistently greater among men than women, suggesting that male fitness was particularly sensitive. Although sisters of patients with schizophrenia and bipolar disorder had increased fecundity (FR range, 1.02-1.03; P < .01), this was too small on its own to counterbalance the reduced fitness of affected patients. Brothers of patients with schizophrenia and autism showed reduced fecundity (FR range, 0.94-0.97; P < .001). Siblings of patients with depression and substance abuse had significantly increased fecundity (FR range, 1.01-1.05; P < 10-10). In the case of depression, this more than compensated for the lower fecundity of affected individuals.
Conclusions Our results suggest that strong selection exists against schizophrenia, autism, and anorexia nervosa and that these variants may be maintained by new mutations or an as-yet unknown mechanism. Bipolar disorder did not seem to be under strong negative selection. Vulnerability to depression, and perhaps substance abuse, may be preserved by balancing selection, suggesting the involvement of common genetic variants in ways that depend on other genes and on environment.
References Power RA, Kyaga S, Uher R, Maccabe JH, Långström N, Landen M, McGuffin P, Lewis CM, Lichtenstein P, Svensson AC. Fecundity of Patients With Schizophrenia, Autism, Bipolar Disorder, Depression, Anorexia Nervosa, or Substance Abuse vs Their Unaffected Siblings. Arch Gen Psychiatry. 2012 Nov 12:1-8. doi: 10.1001/jamapsychiatry.2013.268 . [Epub ahead of print] PubMed PMID: 23147713