Autism in on the increase. Or is it? The diagnostic criteria are pretty vague, and they keep changing. And to ignite this explosive mix, a diagnosis of autism now secures a hefty range of services and benefits.
This paper from California is quite tricky to follow. The basic idea is simple, which is to count how many individuals had a diagnosis of autism added to an existing diagnosis of mental retardation. This is called 'accretion' of an extra diagnosis. Autism and mental retardation (MR) are related because two key features of autism - poor social skills and little speech - would of course also be features of learning disability. Hence in young children they are difficult to tell apart. In California, most people receiving services for autism or MR are tested each year; so the database used has individual information plus individual test scores for the period 1992 to 2005.
The complication arises because in earlier years an IQ test would be done and MR diagnosed, with the extra diagnosis of autism following later; in later years children were more likely given a diagnosis of autism and then not tested for IQ. So a statistical model is needed to estimate how much accretion would have happened if people weren't in later years jumping straight to a diagnosis of autism without any IQ testing.
Another paper, from Australia , is simpler - no modelling involved! The key graph shows prevalence each year from 1983 to 1999 and marks on the graph which years saw the major changes in diagnostic criteria. Note that this paper focusses on autism spectrum disorder - a wider category than the Californian study.
Elsewhere in the same issue of IJE is a paper from Nottingham looking at trends in schizophrenia and other psychotic disorders, based on evidence from three big studies which happen to have been done there around 1978, 1993 and 1997. The incidence of schizophrenia seems to have gone down but other psychoses - especially drug related - have gone up. Overall the proportion of of people using illicit drugs hasn't changed much in this time (up from 22% to 33% in young men from 1994 to 1998) so the authors speculate that illict drugs are stronger than they used to be, or used more heavily.