The environment is more important than genetics in setting the risk for autism, according to a new study that's got the media in a tizzy.
The paper, which is free, is here: Genetic Heritability and Shared Environmental Factors Among Twin Pairs With Autism
It's a twin study, and like all such research, it aims to estimate heritability, the proportion of the variability in autism risk caused by straightforward genetic effects. A heritability of 0% means no genetics and 100% means purely genetic. Note, however, that complex interactions between genes, epigenetics, and gene-environment interactions would throw the whole thing off.
Twin studies rely on the fact that there are two kinds of twins. Identical, or monozygotic (MZ), pairs have identical DNA, while dizygotic (DZ) twins are no more alike than any other brothers or sisters, genetically. So MZ twins ought to be more alike than DZ twins (have a higher "concordance"), and the size of the MZ-DZ difference is a measure of heritability.
There have been several previous twin studies of autism, and they've tended to find a heritability of around 90%, with high MZ concordance and very low DZ. However, these tended to be small and used outdated methods of diagnosis.
The new study used California records to find all twin pairs, born in the state between 1987 and 2004, where at least one of the twins had a diagnosis of autism on the DDS register of people receiving state services for developmental disorders.
They found 1156 twin pairs. Of these, they managed to recruit and get full data from 202 pairs. They gave all these 404 kids full autism diagnostic assessments. This is not a great response rate. Parents of responders tended to be slightly better educated and more likely to be white than the non-responders.
Here's the key data: concordance was higher in MZ twins, but not by nearly as much as previous studies would predict. Putting these data into a statistical model, assuming a baseline rate of autism of 1% in boys and 0.3% in girls, found that the most likely explanation was a heritability of about 35-40% and an effect of "shared environment", i.e. family factors, of 55-60%.
So. Autism's not very genetic?
Maybe. This is certainly a major study and all autism researchers need to take note. But there's some caveats.
My major concern is that the DZ concordance might be too high, because a kid might be more likely to get diagnosed with autism if their twin already had a diagnosis. Suppose you're a parent and one of your twins is diagnosed - of course you're going to worry about the other one, and start thinking, are they really so different?
Although all the people in this study were (re)assessed for study purposes, the diagnostic instruments are hardly immune to the effects of prior diagnosis. The ADI interview is based on parental report of early childhood behaviour. Parents know whether the other twin has autism. The other interview, the ADOS, is based on direct observation of the patient, so it might avoid this - but you have to score on the ADI to get a diagnosis.
This, by itself, wouldn't explain the discrepency between these data and older twin studies. But we also know that diagnoses of autism in general has skyrocketed recently. People seem to be becoming more willing to accept that diagnosis, and more aware of the symptoms. So it's quite possible that some of the "unaffected" twins from older studies would get a diagnosis today if they were to have the kind of modern, formal assessment done in this study.
This doesn't mean that the new study is wrong. If this explanation is true, then the study is quite right - there is a strong shared environmental influence on autism diagnosis. But not necessarily on autism.
One reason to suspect that this is going on - and this is purely a hunch - is that the estimates of shared environmental influence, i.e family environment, was 55%. This is exceptionally high, because almost every other human disorder or trait for which twin studies have been done, have reported low shared environmental effects, and high individual environmental effects (smoking, alcoholism, anxiety, depression). In fact people have written books about this.
Maybe autism's different. Yet I'm more willing to accept that autism diagnosis is different.
A related, but seperate, point: it's very likely that some autism is more genetic than others. In particular we know that some cases are caused by single genetic variants, and these tend to be severe with associated low IQ and sometimes other abnormalities; this is sometimes called "syndromic" autism.
It's always easier to spot a severe case than a mild one. So it's quite possible that older studies had a higher proportion of these cases, because the diagnostic system was only able to pick up those ones. Maybe in more recent times, as diagnosis has expanded, "autism" is coming to cover a "less genetic" set of things.
The good thing about these data is that they span births from 1987 to 2004. So it would be possible to check this theory by looking to see whether the early data i.e. the older twins, have a higher heritability.
Finally, Michelle Dawson pointed out on Twitter that there's another large twin study from Wisconsin, as yet unpublished but presented at a conference. They found broadly comparable results.
Joachim Hallmayer, et al. (2011). Genetic Heritability and Shared Environmental Factors Among Twin Pairs With Autism Archives of General Psychiatry
