Longitudinal Changes in Cognitive Ability in Male and Female Children with ASD

Background: Early studies suggested that female children with autism spectrum disorders (ASD) exhibit lower overall cognitive functioning then male children with ASD.  This hypothesis was challenged in a recent study, which found no overall difference in cognitive level between boys and girls in toddlers with ASD (Carter et. al., 2007).  To explain this discrepancy, it is possible that previous studies missed a group of higher functioning girls with ASD, or that boys with ASD are more likely to show improvements in cognitive ability over time.  Although longitudinal examination is warranted to determine the cause of this discrepancy in research findings, no study thus far has examined longitudinal changes in the expression of ASD symptoms or associated cognitive deficits, between boys and girls.
Objectives: The current study examined changes in cognitive ability from 2 to 4 years of age in a group of children diagnosed with ASD.  
Methods: The participants in this study included young children who were evaluated twice after screening positive on the Modified Checklist for Autism in Toddlers (M-CHAT).  Diagnostic stability was examined in 139 boys and 29 girls who received a diagnosis of ASD at one or both evaluations, at mean ages of 27 and 53 months.  Further comparisons of cognitive ability were completed on 76 boys and 17 girls who maintained an ASD diagnosis and completed the Mullen Scales of Early Learning at both evaluations.
Results: Broad diagnostic stability was similar for boys (84% maintained diagnosis) and girls (79% maintained diagnosis).  Within the spectrum, female children were equally likely to be diagnosed with PDD NOS or Autistic Disorder (AD), while male children were more likely to receive an AD diagnosis at both evaluations.  Repeated measures ANOVAs were completed on four of the Mullen subtests (Visual Reception, Fine Motor, Receptive Language, and Expressive Language) with test administration as a within-subjects factor and sex as a between-subjects factor.  There were no significant interaction effects for any of the subtests.  The Visual Reception, Receptive Language, and Expressive Language all showed a main effect for administration, such that both male and female children received higher t-scores at the second evaluation.  These three subtests showed no main effect for sex.  The Fine Motor subtest did not exhibit a main effect for administration, but revealed a trend (p=.07), suggesting that female children scored lower than male children.   Follow-up chi square analyses indicated that there were no significant differences between male and female children, for any Mullen subtest, on the percent of children who stayed the same across the time points, decreased by more than 1 SD over time, or increased by more than 1 SD.
Conclusions: These data indicate that female children with ASD exhibit a similar profile of change in cognitive abilities as male children with ASD.  This finding supports the first theory indicated above and suggests that increased recognition of the symptoms of ASD has allowed higher functioning girls to be correctly diagnosed more often.  These data are still preliminary and ongoing efforts are aimed at recruiting additional girls with ASD.