How much of autism and ADHD is genetic? What do twin studies show?

Twin studies suggest that genes play a large part in both autism and ADHD. Studies of twins and families have estimated the heritability of autism at around 80 to 90 percent, and meta-analyses have shown ADHD to be primarily genetic with a heritability of 70 to 80 percent. But "heritability" is easy to misread. It does not mean that autism is solely determined by genetics, since environmental factors also contribute. And it is a statistic about differences across a population, not a measure of how much of any one person's traits comes from their genes.

Here is how a twin study works. Identical twins share identical DNA, while fraternal twins share about half of theirs. If a trait is caused purely by environment, both kinds of pair should resemble each other equally. If it were completely genetic, identical pairs would in theory match 100 percent of the time and fraternal pairs usually much less. A 1977 study of 11 identical and 10 fraternal pairs, the first of its kind for autism, found that both twins were autistic in 36 percent of identical pairs and none of the fraternal pairs.

Later studies gave different numbers, and the range was wide: identical-twin studies put autism's heritability anywhere between 36 and 95.7 percent. A 2015 meta-analysis of twin studies put the contribution of genetics between 64 and 91 percent. A critic argued that the pre-2006 studies were too small.

Genetics here is not one gene. Autism typically cannot be traced to a single-gene mutation, and mutations in many genes are typically implicated in each autistic person. In ADHD too, a number of genes are involved.