"For hyperkinetic disorder, an increase in cumulative incidence was observed across all cohorts such that each successive birth cohort had a significantly higher cumulative incidence than the previous cohort (P<0.001)," the authors wrote. "For example, at 5 years of age the cumulative incidences of hyperkinetic disorder for the 1994-1995 cohort, 1996-1997 cohort, and 1998-1999 cohort were 26%, 100%, and 200% higher, respectively, than the cumulative incidence for the 1992-1993 cohort."
Similarly, the 1994-1995 birth cohort had a significantly higher cumulative incidence of Tourette syndrome than either the 1990-1991 cohort (P=0.005) or the 1992-1993 cohort (P=0.006), although there was no significant change from children born in 1990 through 1993 in cumulative incidence of Tourette's.
Looking at autism spectrum disorders overall, they found that the 1998-1999 birth cohort had significantly higher cumulative incidence proportions than the 1994-1995 birth cohort (P=0.004), but there were no other significant differences among the various birth cohorts.
Looking only at those with a diagnosis of childhood autism (38% of all children with autism spectrum disorders), the investigators found that the cumulative incidence proportion was significantly higher for children in the 1998-1999 birth cohort than those born either from 1994-1995 (P<0.001) or 1996-1997 (P=0.02).
There was no significant change in autism incidence, however, between the 1994-1995 and 1996-1997 birth cohorts.
The authors did not find any significant difference across the various birth cohorts in the cumulative incidence proportions for obsessive-compulsive disorder.
"It is difficult to explain why obsessive-compulsive disorder was the only disorder displaying another pattern; the reason may be etiologic, due to non-etiologic diagnostic differences, or due to the relatively short follow-up," they wrote.