Though it is largely admitted that non-cognitive skills matter for adult outcomes, little is known about how the family environment affects their formation. In this paper, we use a cohort study of children born in 2000-2001 in the U.K. (Millennium Cohort Study) to estimate the effect of family size on socio-emotional skills, measured by the Strengths and Difficulties Questionnaire. To account for the endogeneity of fertility decisions, we use a well-known instrumental approach that exploits parents' preference for children's gender diversity. We show that an increase in family size negatively affects the socioemotional skills of the two first children in a persistent manner. However, we show that this negative effect is entirely driven by girls. We provide evidence that this gender effect is partly driven by an unequal response of parents' time investment in favor of boys and, to a lesser extent, to an unequal demand for household chores.JEL classification: I20, J13, J16