Infants laugh by 4 months, but whether they understand humour based on social or cognitive factors is unclear. We conducted two longitudinal studies of 4‐, 6‐, and 8‐month‐olds (N = 60), and 5‐, 6‐, and 7‐month‐olds (N = 53) to pinpoint the onset of independent humour perception and determine when social and cognitive factors are most salient. Infants were shown six events in randomized repeated‐measures designs: two ordinary events and two absurd iterations of those events, with parents’ affect manipulated (laugh or neutral) during the latter. Four‐month‐olds did not smile/laugh more at absurd events, but exhibited a significant heart rate deceleration. Five‐month‐olds independently appraised absurd events as humorous, smiling/laughing despite their parents’ neutrality. Parent laughter did not influence infants of any age to smile more, but captured 4‐month‐olds’ attention. Results suggest that 4‐month‐olds laugh in response to social cues, while 5‐month‐olds’ can laugh in response to cognitive features.