Mirror neurons, which fire when someone either performs an action or observes it, are not defective in people with autism, scientists report today in Neuron. The findings dispute the theory that flaws in the mirror neuron system give rise to the disorder.

Researchers have found a higher density of several types of interneurons — nerve cells that connect sensory and motor neurons in the brain— in postmortem brain tissue from individuals with autism, compared with healthy controls. The findings appear in the February issue of Acta Neurologica Scandinavica.

Genetic variations that tweak the brain’s release of oxytocin — a hormone involved in social bonding and establishing trust — may increase the risk of developing autism or traits of the disorder, according to three new studies published in the past few months.

Donald T. was not like other 5-year-old boys. Leo Kanner knew that the moment he read the 33-page letter from Donaldʼs father that described the boy in obsessive detail as “happiest when he was alone… drawing into a shell and living within himself… oblivious to everything around him.”