I think the main reason is that there is a lot more knowledge and awareness about autism now than before.Autism is a rather young diagnosis, not because autism didn't exist before or was less common, but because it wasn't recognized and diagnosed the same way it is today.People with all kinds of developmental disorders and various mental disabilities used to be lumped together into one group and thought of as imbeciles or thought to be possessed by demons. They were hid away somewhere in institutions and were not visible in society. Nowadays there is a lot more knowledge about those disorders, so they are being diagnosed more accurately as separate disorders and they are being treated and accommodated. People who have them are being included in general society more than before and have therefore become more visible. A lot has changed in just a few decades.There is also a lot more public awareness of autism now than before, so people are more likely to know what autism is, recognize some of the signs and seek diagnosis and help if they suspect that they or their children may have it. It is also not a taboo anymore or something that people try to hide in shame.A lot of people who are adults today are undiagnosed or have only recently been diagnosed, because autism was little known when they were growing up and they were therefore never diagnosed or they were misdiagnosed with something else.I doubt that autism is really on the rise as much as people think it is. I think the increase seen in the statistics can mostly be explained by increased knowledge and awareness and more accurate diagnosis.I'm 28 years old and have a mild form of autism myself. I wasn't diagnosed with it until I was in my twenties, but the symptoms were there my whole life, just unrecognized due to limited knowledge/awareness about autism spectrum disorders.