Look at this chart comparing the synaptic density of autistic and non-autistic young people.
Although the sample size is small (which is understandable given how few people not only die in childhood but also have their brains donated to science and also have an autism diagnosis on file) there is virtually no OVERLAP. Every single autistic person is above the regression line for neurotypicals and every single neurotypical is below the regression line for autistic people. What makes this so striking is you’d expect just from misdiagnosis, some people diagnosed with autism would actually be neurotypical and yet despite such error, we still get virtually no overlap, suggesting age controlled synaptic density is virtually a perfect predictor of autism.
So perhaps autism should simply be defined as which regression line you’re closer to in the above chart. The one for autistic people, or the one for neurotypicals. Just as Downs syndrome is diagnosed by an extra 21st chromosome, without need for questionnaires, autism too could have a nice neat medical definition.
On the other hand, I’ve long argued that brain size is a cause of intelligence and that intelligence is the ability to adapt, so how can I associate synaptic density (a proxy for brain size) with a maladaptive condition like autism?
My guess is there are two kinds of big brained people. Those who have big brains because they have a lot of neurons, and those who have big brains because they have a lot of redundant connections between those neurons. The former will tend to be gifted while the latter will tend to have autism, though many will tend to have both. If we limited our samples to nerotypicals, the correlation between brain size and IQ might jump from perhaps 0.4 (the current estimate) to 0.5 or even 0.6.
How exactly extra connections cause autism is not entirely clear. One idea I heard an Asian researcher suggest (sorry I forget his name) is that connections compete to form in a Darwinian struggle, so if you don’t prune the weak ones, you drag down the fitness of the brain.
But why is social intelligence especially impaired in autism? My guess is that since most brain pruning occurs in late childhood and adolescence, the very years where we acquire the most social skills, it is the social brain that especially fails to prune.
This content was originally published here.