We here use data from the British Cohort Study (BCS) to link family size to age-16 children's contribution to household chores and the adult housework gender gap. Assuming that home production is an increasing function of family size and using an instrument to account for the endogeneity of fertility, we show that larger families have a different effect on boys and girls at age 16: girls in large families are significantly more likely to contribute to housework, with no effect for boys. We then show that childhood family size affects the housework gender gap between the cohort members and their partners at age 34. Women who grew up in larger families are more likely to carry out a greater share of household tasks in adulthood, as compared to women from smaller families. In addition, growing up in a large family makes cohort members more likely to sort into households with a wider housework gender gap as adults. We show that the persistent effect of family size is due to the adoption of behaviours in line with traditional gender roles: a lower likelihood of employment and shorter commutes for women, along with a higher employment probability for their partners.