Monozygotic twins show high concordance in eye- and mouth-looking, and this behaviour is markedly reduced in toddlers with autism spectrum disorder.
Long before infants reach, crawl or walk, they explore the world by looking: they look to learn and to engage1, giving preferential attention to social stimuli, including faces2, face-like stimuli3 and biological motion4. This capacity—social visual engagement—shapes typical infant development from birth5 and is pathognomonically impaired in children affected by autism6. Here we show that variation in viewing of social scenes, including levels of preferential attention and the timing, direction and targeting of individual eye movements, is strongly influenced by genetic factors, with effects directly traceable to the active seeking of social information7. In a series of eye-tracking experiments conducted with 338 toddlers, including 166 epidemiologically ascertained twins (enrolled by representative sampling from the general population), 88 non-twins with autism and 84 singleton controls, we find high monozygotic twin-twin concordance (0.91) and relatively low dizygotic concordance (0.35). Moreover, the characteristics that are the most highly heritable, preferential attention to eye and mouth regions of the face, are also those that are differentially decreased in children with autism (χ2 = 64.03, P < 0.0001). These results implicate social visual engagement as a neurodevelopmental endophenotype not only for autism, but also for population-wide variation in social-information seeking8. In addition, these results reveal a means of human biological niche construction, with phenotypic differences emerging from the interaction of individual genotypes with early life experience7.