What about the Girls? Examination of Gender Differences in a University Wide ASD Sample

Background:  ASDs have long been thought to be more prevalent in males (Fombonne, 1999; Lingam et al., 2003), and the CDC currently estimates that males are five times more likely to have an ASD than females (CDC 2012).  Attempts to describe sex differences within the disorder generally identify IQ as the main differentiating factor between the sexes, reporting that females were more likely to have intellectual disabilities than males with ASD (Wing, 1981; Tsai & Beisler, 1983; Lord & Schopler, 1987; Volkmar, Sparrow and Szatmari, 1993). More recent work with young children with ASD (Carter et al. 2007; Hartley & Sikora, 2009) suggests significant differences exist between the sexes in cognitive profiles but not in overall cognitive level. Ongoing changes to diagnostic labels and criteria, rising population prevalence estimates, and increased public awareness of ASD necessitate further clarification of potential sex differences in prevalence, cognitive and adaptive skills, and autism symptoms.

Objectives:  To assess for sex differences in a sample of 1018 males (M = 6.24 ± 4.35, range = 1.20-36.41) and 196 females (M= 6.18 ± 5.03, range = 1.32-36.41), drawn from a university based clinical research database.

Methods: We examined possible differences between the sexes with regards to cognitive level, adaptive behavior (Vineland-II; Sparrow et al., 2005) and autism symptoms (Calibrated Severity Scores [CSS]; Gotham et al., 2009), collectively and after controlling for age, cognitive level and ADOS module (Lord et al., 2000).  

Results:  When the entire sample was analyzed, no significant sex differences were found in cognitive level (t = -1.53; p = .127), adaptive behavior (t = -1.19; p = .234), or autism severity (t = -.851; p = .395).  When the sample was divided by age range, no significant differences emerged in males or females in the 0-2:11 or 3-5:11 age range. CSS scores were significantly higher for males in the 6-12:11 range (p = .021) and the 13-17:11 range (p = .037). No significant differences emerged in cognitive or adaptive level. When divided by cognitive level, no significant differences emerged in CSS or adaptive level. When divided by ADOS module, a significant difference emerged in CSS for Module 3 (p= .047), with males scoring significantly higher than females.

Conclusions:  Contrary to the majority of research in the area, no significant differences in cognitive skills were found between the sexes.  Based on differences in CSS, males presented with significantly higher autism symptomatology in the school age range and when assessed with a Module 3 ADOS.  This raises the question of whether the difference in overall prevalence rates between males and females is related to true sex differences or if gender differences (i.e., culturally masculine vs. feminine traits) play a more significant role. Perhaps prevalence rates for school-aged females are artificially deflated, due to a milder symptom presentation or socio-cultural biases in assessment of autism symptomatology in females.