In 1961, the late psychiatrist Daniel Freedman made what would become one of the most replicated — and most mysterious — discoveries in the history of autism research. Comparing blood levels of the neurotransmitter serotonin in 4 non-autistic and 23 autistic children, he found significantly higher levels among the latter group. Since then, researchers have repeatedly identified this trait, called hyperserotonemia, in about a third of autistic people tested.
It’s not difficult to theorize how hyperserotonemia might be linked to a range of autism traits. Neurons that release serotonin extend into practically every part of the brain, where they modulate signals sent among other neurons. Selective serotonin reuptake inhibitors (SSRIs), drugs that raise levels of serotonin in the brain’s synapses, treat psychiatric conditions, such as anxiety and obsessive-compulsive disorder, that can co-occur with autism. And serotonin prompts the gut to contract and facilitate digestion, which is often impaired in autistic people.