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. Statement of contribution What is already known on this subject? By 6 months, infants can independently appraise absurd events as humorous, but it is not known whether younger infants can. What does this study add? This study replicated the finding on younger infants, showing that 5-month-olds are similarly capable of independent humour appraisal. These studies also found that although 4-month-olds do not respond to absurd events with positive affect, they do exhibit a heart rate decrease that is unrelated to looking. These studies help delineate when social and cognitive factors contribute to infant humour perception.