This discussion is one of the main reasons why we need the neurologists on board with this. As we gain more convincing evidence that there are hallmark brain anatomical and physiological markers, genetic markers, and even immunological markers associated with autism, it puts autism into a category of a "medical condition" that has secondary psychological and psychiatric components.
The problem that I am seeing is that despite having the Diagnostic Statistical Manual available to psychologists and psychiatrists, there is a huge discrepancy in testing and diagnostic methodology, leaving it open to more subjectivity and bias. Having actual medical criteria may help solidify and validate the diagnosis.
Is autism being over or under diagnosed? Probably both depending upon one psychologist's interpretation versus another. Is the community, at large, more aware of autism? Yes, and may account for more people being diagnosed, as well as, more people seeking a diagnosis. Are there more people being born with autism? Likely, and for a few reasons. One, our culture has transitioned quite a bit over the past 50 years. With the advent of birth control pills for women, combined with more women seeking higher education and careers, women are choosing to have their children later in life, often in their 30's, even 40's, as opposed to their late teens and 20's. These same women are also getting married later, meaning older male partners. Age of the parents is highly associated with autism. The genetic data from the Autism Genome Project has made this very clear. Clearly there is "genetic autism", associated with families (grandparents, cousins, fathers, siblings, etc), however, old sperm and eggs are highly associated with autism, as well as, a long, long list of other chromosomal defects and birth defects, mild and severe. Sorry, it just is a fact. It's literally my job to deal with the consequences in the neonatal unit that I have worked in for over 35 years. Combine that data with the fact that the intrauterine environment also plays a huge part, and for context, this means the hormonal milieu, the build up of toxins in the mother's body, exposures to infectious agents, and inflammatory mediators. All of which occur in the older mother. People gain weight as they enter their 30's and 40's, more people are obese, have diabetes and hypothyroidism, estrogen, progesterone, and testosterone imbalances, most often resulting in elevated estrogens. We tend to eat a lot of foods that are packaged in boxes, bags, and cans, full of artificial this and that, preservatives, etc, creating stress on the body in the form of toxins and inflammatory mediators. Most people don't have a lot of whole foods in their diets anymore. All of which have associations with autism.
This is our world right now, so expect more autism.