Why is a diagnosis of autism more common these days? Well, I do agree in part with the comment that "more people are looking" which, in my opinion is good. The educators and doctors have become more aware of, and open to, considering this, and it has much less stigma now than it did years ago. I was born in the 1950s and autism was essentially unknown and thought to be only in non-verbal, non-functioning people. Me? I was just considered plain old "weird" by most, in spite of high intelligence, rapid learning, etc. I'm not sure today's Aspies get called "weird" any less than I did (nor, is it any less painful, I suspect) but at least the teachers/principals/parents are not joining in that chorus. Although I do not have enough background to judge the suggestions that todays trends to wait to start a family until the parents are in their 30s, I do suspect there may be much truth in that. I did not receive a formal diagnosis until I was in my mid 50s and by then it did not really matter much. I'd already long ago decided that I am not "broken" nor do I need "fixing"....I just accept that I see the world differently than others, and I'm ok with that. I have certain strengths, which I like, and I know being a bit out of the mainstream is the price I pay for those strengths and gifts. To me, it is a good trade. And, as a side observation, I have not been called "weird" to my face for several years now, I suspect that as the people I associate with age (along, of course, with me growing older) they become more accepting of the idea that my experiences, my thoughts are just as valid and not in any way less valuable than their own. No one my age cares much about what others think anymore, about being in the "IN" crowd, or worries about being accepted. So, time does heal most wounds it seems.
This topic has been covered before, but to summarize, you are correct, it is being diagnosed more often. Several reasons for this:
1. As you suggested, increased awareness, as well as many being diagnosed as adults.
2. The ASD-1/Asperger's variants are now included, as are other variants, under the umbrella of the autism spectrum.
3. Basically, "old eggs and sperm". Society now-a-days are more likely to begin families in their 30's and 40's, not in their late teens and early 20's, as was the norm some 50-60 years ago and earlier. From an obstetrical perspective, a pregnant woman who is 35 is considered and literally labeled as "geriatric". (I know, because this is part of my profession). Once you are in your early to mid 30's, the incidence of Down's Syndrome, autism, and a long, long list of congenital anomalies rise up significantly.
4. The rise in obesity has led to hormonal alterations in insulin, thyroid hormones, testosterone, and estrogen. All of which, independently, are associated with an increased risk of autism, but combined creates added risk.
5. The rise in environmental toxins in our water, air, and food are independent risk factors.
6. Inflammation from auto-immune diseases and viral infections during pregnancy.
7. Some geographical areas associated with the technology industries (California's Silicon Valley, for example) tend to attract autistic males and females. Their children will be at a higher genetic risk of having an autism condition.
8. Premature birth at very early gestation (prior to 28 weeks) is associated with an elevated risk of autism because the developing brain is being robbed of all the maternal hormones and growth factors associated with the normal development of the brain, as well as the fact that the extrauterine environment is absolutely obnoxious and overstimulating to the premature brain, resulting in abnormal neural growth patterns. This would be considered one of the "injury models" of autism. 30-40 years ago, most of these babies would have not survived to have autism.
These are just a handful of potential reasons for increased diagnosis of autism.
The older the parents, the more likely the time that #3, 4, 5, and 6 will have compounding effects.
Conversely, what I know now, after some 40 years in the field of neonatology, human beings
should be having children in their late teens and early 20's from a physiologic perspective. The less likely to have age-related DNA strand breaks, obesity-related hormonal perturbations, time of contact with environmental toxins, and premature birth due to inflammatory processes and incompetent cervix. This societal idea that (1) I need to complete my education (2) establish my career and (3) then maybe consider a family is causing not only a rapid decline in the overall birthrate, but also significantly increasing the numbers of babies being born premature with and without autism and congenital anomalies.