The book “Neurotribes” goes into this in great detail. The number of diagnoses have increased, primarily due to a widening of the criteria used to diagnose autism as the DSM [1] has gone from version 3 to 5. In the early days of autism research, it used to be necessary to meet multiple criteria (i.e. profound social withdrawal, language peculiarities such as echolalia, resistance to change, repetitive behaviors, etc., all occurring in very early childhood). Today, it’s possible to receive a diagnosis in adulthood, and to do so while meeting criteria across fewer of the above domains.

This widening has multiple causes, not the least of which was an attempt to help parents of autistic children access parenting resources they otherwise couldn’t without a diagnosis in-hand.

1. https://en.m.wikipedia.org/wiki/Diagnostic_and_Statistical_M...