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Long before infants reach, crawl or walk, they explore the world by looking: they look to learn and to engage, giving preferential attention to social stimuli, including faces, face-like stimuli and biological motion. This capacity—social visual engagement—shapes typical infant development from birth and is pathognomonically impaired in children affected by autism. 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 information. 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 …