The aim of the proposed project is to shed light on early childhood gender-differentiated socialization and gender-specific susceptibility to parenting within families in relation to disruptive behaviour in boys and girls in the first four years of life. The popular saying boys will be boys refers to the observation that boys show more disruptive behaviours (e.g., noncompliance or aggression) than girls, a pattern that has been confirmed frequently in scientific research. There is also evidence that parents treat boys differently from girls in ways that are likely to foster boys disruptive behaviour, and that boys are more susceptible to problematic family functioning than girls. The crucial question is whether gender differences in socialization, susceptibility to socialization, and children s behavioural outcomes are also salient when the same parents are doing the parenting of both a boy and a girl. Within-family comparisons are necessary to account for structural differences between families. To this end, families with two children born 22-26 months apart will be recruited from the general population. To account for birth order and gender-combination effects, the sample includes four groups of 150 families each, with the following sibling combinations: girl-boy, boy-girl, girl-girl, and boy-boy. The study has a four-wave longitudinal design, based on the youngest sibling with assessments at ages 12, 24, 36, and 48 months, because gender differences in disruptive behaviour develop during the toddler years. Each assessment consists of two home visits: one with mother and one with father, including observations of both children and of the children separately. Parenting behaviours will be studied in reaction to specific child behaviours, including aggression, noncompliance, and prosocial behaviours.