Intellectual and Social Cognitive Functioning in Monozygotic and Dizygotic Twins with ASD

Background: Autism is a highly heritable neurodevelopmental disorder. Previous studies have found sub-threshold social impairments in siblings of children with ASD, as well as in unaffected twins.

Objectives: The primary aim of this investigation was to examine the intellectual and social cognitive functioning of monozygotic and dizygotic twins with ASD, with at least one affected twin. This was part of a larger study examining the behavioral and neural correlates of twins with ASD, their siblings, and neurotypical twin pairs.

Methods: Monozygotic and dizygotic same-sex twins with ASD, and age-, gender- matched neurotypical twins between the ages of 6 and 14 years were included. Intellectual functioning was assessed using the Stanford-Binet Intelligence Scales (SB5). Social abilities were assessed using the Social Responsiveness Scale (SRS), the Theory of Mind (ToM) and Affect Recognition (AR) Social Perception subtests of the NEPSY-II, as well as a reading the mind in the eyes (Eyes) task, and a Theory of Mind (Smarties) task.

Results: A total of 92 participants were included in this investigation: 24 monozygotic with ASD, 40 dizygotic, and 28 neurotypical twins (12 MZ, and 16 DZ). There was no significant difference in age or gender between groups. Difference scores (Δ= Most severe – least affected) on intellectual and social cognitive measures were compared between MZ and DZ neurotypical pairs.  As expected, a significant difference was found on the Δ score between monozygotic and dizygotic ASD twin pairs on the SRS total T-score with a trend toward significance on FSIQ score. When comparing the most severe proband between monozygotic and dizygotic twins, there were no significant differences on intellectual abilities or social cognitive functioning measures. The least severely affected MZ probands performed significantly better than the most severely affected DZ probands on both affect recognition tasks (AR and Eyes).

Conclusions: Findings from this study suggest that the most severe probands of MZ and DZ twin pairs do not differ significantly from one another on several clinical measures. Interestingly, the least severely affected MZ probands perform better on measures of affect recognition than the most severely affected DZ probands, raising the question of whether aspects of affect recognition are less heritable.  This observation is intriguing, but final conclusions cannot be made in light of the small sample size. These preliminary findings warrant additional investigation to comprehensively examine all aspects of intellectual and social cognition in monozygotic and dizygotic twins with autism of all ages.