New data shed light on the recent mysterious rise in the number of kids being diagnosed with autism.
The new research doesn't explain the increase, but it tells us more about it. It shows that the rise in Californian autism diagnoses (reported to the state DDS) over the period 1996 to 2005 was a cohort effect, meaning that the rates of diagnosis have got higher, the later a child was born.
A child who's 10 today (born 2002) has double of the chance of having a recorded diagnosis compared to a 14-year-old born just four years earlier, in 1998.
"That doesn't tell us anything new!" you might object (I did at first). "All that means is that rates have risen, and we knew that already". But actually it does tell us something important. Because the data could have turned out differently; rates could have risen without a cohort effect, if, in recent years, lots of diagnoses were being handed to children regardless of their age.
That didn't happen. Almost all children in California who get a diagnosis, get it at age 3 or 4. In more recent years, the average age at diagnosis actually fell slightly. The peak used to be age 4, it's now 3.
So it's not that children in general have been getting diagnosed with autism more. It's that young children are getting diagnosed more; children aren't being diagnosed "retrospectively", as it were.
Another interesting finding is that the rise in rates of 'high-functioning' autism has been much bigger than the rise in low-functioning autism (i.e. autism alongside intellectual disability), although that has risen as well.
Edit: but note that their defintion of 'functioning' is rather unique; see the comments.
So what does this mean?
These data are consistent with various interpretations. It could be that rates of autism have really risen in California over this time period. But it could also be that people are getting more likely to detect and diagnose it - in young children.
Keyes, K., Susser, E., Cheslack-Postava, K., Fountain, C., Liu, K., and Bearman, P. (2011). Cohort effects explain the increase in autism diagnosis among children born from 1992 to 2003 in California International Journal of Epidemiology DOI: 10.1093/ije/dyr193