Background/purpose: The association between sex and diagnostic behavior of autism spectrum disorder (ASD), and the effects of comorbid mental retardation (MR) and attention-deficit hyperactivity disorder (ADHD), were explored. Methods: Based on the Taiwan Longitudinal Health Insurance Database (LHID)-2000 and data from 1996 through 2008, the cumulative incidence of ASD over time was compared between the sexes (both cohorts n = 38,117) using the log-rank test. The effects of comorbid MR and ADHD on the incidence of ASD were evaluated using Cox proportional hazard regression analysis. The age at first diagnosis of ASD in the two sexes was compared using the independent-sample t-test. Results: The incidence was higher in males than in females (0.0007 vs. 0.0002) across ages. Comorbid MR or ADHD increased the incidence of ASD in both sexes; comorbid MR or ADHD also decreased the male to female hazard ratio of ASD, with no significant differences in the incidence density of ASD between sexes. ADHD delayed diagnosis in both sexes (males: 6.61 vs 5.10, p < 0.0001; females: 6.83 vs 4.69, p = 0.0037). Conclusions: The general concept of a higher incidence of ASD among males was noted in this study of a Taiwanese population, but disappeared in those with comorbid MR or ADHD, indicating unique vulnerabilities to MR/ADHD or under-identification of high-functioning females with ASD in childhood. Increasing the diagnostic sensitivity of ASD in those with comorbid ADHD is important due to a delayed diagnostic age in this group.
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