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 capacityâsocial visual engagementâshapes typical infant development from birth5 and is pathognomonically impaired in children affected by autism6. Here we show that variation in viewing of social scenesâincluding levels of preferential attention and the timing, direction, and targeting of individual eye movementsâis 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 toddlersâincluding 166 epidemiologically-ascertained twins, 88 non-twins with autism, and 84 singleton controlsâwe find high monozygotic twin-twin concordance (0.91) and relatively low dizygotic concordance (0.35). Moreover, the measures that are most highly heritable, preferential attention to eye and mouth regions of the face, are also those that are differentially diminished in children with autism (Χ2=64.03, P<0.0001). These resultsâwhich implicate social visual engagement as a neurodevelopmental endophenotypeânot only for autism, but for population-wide variation in social-information-seeking8âreveal a means of human biological niche construction, with phenotypic differences emerging from the interaction of individual genotypes with early life experience7.