Concordance in Symptom Severity and Face Processing Among Twins with and without Autism

Background:   Autism spectrum disorder (ASD) stems in part from heritable and genetic factors, as evidenced by higher rates of ASD concordance among monozygotic (MZ) twins relative to dizygotic (DZ) twins. Although the precise mechanisms underlying these factors are not fully understood, behavioral and electrophysiological markers of social functioning and social cognition provide avenues through which to explore patterns of genetic influence. Within social cognition, the area of face processing is of particular relevance to ASD. Not only are face-processing impairments well documented in ASD, but recent evidence supports a genetic contribution to face-processing among typically developing individuals.

Objectives:   Our goals in this study were to examine concordance within twin pairs on measures of (1) diagnostic severity as indices of social functioning; (2) memory for faces as an index of social cognition; and (3) electrophysiological response to faces as an index of brain response to social stimuli. Stronger correspondence between twins within MZ pairs relative to DZ pairs would provide support for genetic contributions to these domains.

Methods:   Within a larger sample, we identified 27 MZ and 19 DZ twin pairs across two diagnostic groups: concordant for the presence of an ASD (n=19) and concordant for the absence of an ASD (n=27). Twins ranged in age from 5 years to 22 years (mean = 11.1 years) and included both male (n=59) and female participants (n=33). Over the course of multiple visits, twins completed measures of diagnostic status (ADOS, ADI-R), intellectual functioning (WASI), social cognition (memory for human faces), and electrophysiological response to faces.

Results: Behavioral findings suggest intriguing patterns of correspondence within twin pairs across these measures. Severity of ASD symptoms as assessed with the ADOS calibrated severity score was correlated within MZ twin pairs but not within DZ pairs. However, this pattern was reversed for symptom severity assessed via parent report on the ADI-R, for which DZ pairs were more strongly correlated than MZ pairs. With regard to social cognition, DZ twins showed more evidence of correspondence within pairs than did MZ twins, although correspondence in both groups was enhanced when controlling for intellectual functioning. Analysis of associated electrophysiological markers is currently underway. We anticipate that markers relevant to the processing of human faces (e.g., P100, N170) will be more highly correlated within MZ twin pairs than within DZ pairs. We also predict that correlations between behavioral and electrophysiological measures will be stronger among participants without ASD than among those with ASD.

Conclusions: These findings highlight the potential contribution of genetic factors to social functioning and social cognition among individuals with ASD, but imply that these influences are not straightforward. Whereas MZ twin pairs showed greater similarity than DZ twins on some measures, other measures revealed greater similarity within DZ twins. This pattern suggests nuanced and complex genetic mechanisms for social and cognitive development.