Researchers at Autism BrainNet node UC Davis MIND Institute, led by Cynthia Schumann (picture is Thomas Avino, a lead author), found that while typically-developing children gain more neurons in a region of the brain that governs social and emotional behavior, the amygdala, as they become adults, people with ASD do not. The open access research published in PNAS studied 52 postmortem human brains, both neurotypical and ASD, ranging from 2 to 48 years of age. There were more neurons in young children with ASD, but as they got older, those numbers went down. The press release on this article can be read here.
Here is the link to the recording of the Q&A webinar with Dr. Thomas Avino.