Nov 17, 2019
[Thanks to Marco DG for proofreading and offering suggestions]
Several studies have shown a genetic link between autism and intelligence; genes that contribute to autism risk also contribute to high IQ. But studies show autistic people generally have lower intelligence than neurotypical controls, often much lower. What is going on?
First, the studies. This study from UK Biobank finds a genetic correlation between genetic risk for autism and educational attainment (r = 0.34), and between autism and verbal-numerical reasoning (r = 0.19). This study of three large birth cohorts finds a correlation between genetic risk for autism and cognitive ability (beta = 0.07). This study of 45,000 Danes finds that genetic risk for autism correlates at about 0.2 with both IQ and educational attainment. These are just three randomly-selected studies; there are too many to be worth listing.
The relatives of autistic people will usually have many of the genes for autism, but not be autistic themselves. If genes for autism (without autism itself) increase intelligence, we should expect these people to be unusually smart. This is what we find; see Table 4 here. Of 11 types of psychiatric condition, only autism was associated with increased intelligence among relatives. This intelligence is shifted towards technical subjects. About 13% of autistic children (in this sample from whatever social stratum they took their sample from) have fathers who are engineers, compared to only 5% of a group of (presumably well-matched?) control children (though see the discussion here) for some debate over how seriously to take this; I am less sure this is accurate than most of the other statistics mentioned here.