No Sex Differences Found in Cognitive Ability in Children with Autism Spectrum Disorder

Background:  The notably discrepant sex ratio in autism spectrum disorder (ASD) is a longstanding phenomenon.  Previous research examining sex differences in cognitive abilities has varied over time, with fewer recent studies demonstrating significant sex differences.

Objectives:  To evaluate possible sex differences in cognitive functioning in a large clinical sample of children referred diagnosed with ASD.

Methods:  Data was gathered through 18 Autism Treatment Network (ATN) sites from 2007 to 2013. We included 2 samples in this analysis, which were derived from the assessment performed at the time of diagnosis:  Sample 1) Mullen Scales of Early Learning (MSEL: 1,346 children, 1,117 boys (83%); and 229 girls (17%)), and; Sample 2) Stanford-Binet Intelligence Scales, 5^th Edition (SB-5: 2,717 children, 2,306 boys (84.9%) and 411 girls (15.1%)).

Results:  In Sample 1, the mean age at assessment was 3.46 years (SD=0.99) with no difference between sexes. There was a slight overrepresentation of girls among families who identified as Black or "Other" race (p=.03), but no differences for ethnicity (Hispanic or not) or parental education were found.  No significant difference was found between sexes for MSEL composite total scores (59.4 vs 58.5; NS) nor for any of the 5 MSEL subscales (Gross Motor, Visual Reception, Fine Motor, Expressive Language, and Receptive Language, all p>.15).  In Sample 2, the mean age at assessment was 7.26 years (SD=3.34) with girls being significantly younger than boys (6.9 vs 7.3; p=.025). No difference for race, ethnicity or parental education was observed. All children had an SB-5 Abbreviated Battery IQ (ABIQ), with no difference between sexes (83.2 vs 81.2; p=.10). For the subsample (N=753; 80.4% boys) with full SB-5 scores, no difference was found between boys and girls for Non Verbal IQ (NVIQ, 80.4 vs 80.0), for Verbal IQ (VIQ, 75.8 vs 77.4) or for Full Scale IQ (FSIQ, 77.2 vs 77.7). Similarly, no difference was found for any of the factors (Fluid Reasoning: 80.8 vs 82.9; Knowledge: 77.4 vs 79.6; Quantitative Reasoning: 83.8 vs 83.2; Visual-Spatial Processing: 82.9 vs 80.5; Working Memory: 78.8 vs 78.9; all NS).  We performed further analyses on cognitive scores grouped in 5 levels of IQ: <50, 50 to 69, 70 to 84, 85 to 99 and 100 and over. In sample 1, the percent distribution for these 5 bands of MSEL scores was: 43.5, 37.5, 10.5, 5.1 and 3.3, with no sex difference (chi²=5.3, 4 df; p=.26). In Sample 2, the percent distribution for the 5 IQ levels was respectively: 9.4, 20.8, 18.4, 23.9 and 27.5, with no sex difference (chi²=3.9, 4 df; p=.42). Similar, non significant results were obtained for the subsample of 753 subjects with full SB-5 scores.

Conclusions:  In this large sample of children with ASD in the United States, no sex differences were found in overall cognitive ability or any of the SB-5 or MSEL subscales; this held true even after grouping by cognitive ranges. These findings are consistent with more current research, which may suggest that differences noted in the earlier research may be related to evolving diagnostic criteria over time.