Several studies have shown that parents with joint physical custody have higher income and educational levels than those who are granted sole custody. It has been asserted that the specific characteristics of joint physical custody families may, at least partially, explain why many studies report a positive association between joint physical custody and child adjustment. Past research has barely explored the evolution of the socio-demographic characteristics of joint custody families. Using the Health Behaviour in School-aged Children data from 2002 to 2014, we aim at analyzing to what extent there is an evolution towards less selectivity of joint physical custody families in Spain. In addition, we test to what extent joint physical custody is beneficial for children’s well-being. In doing so we examine whether the positive effects of joint custody on children’s well-being differ by socio-economic background. Preliminary findings show that for children whose parents have a higher socio-economic background, joint physical custody is beneficial while we see no difference between children in joint physical custody compared to children in sole mother custody. Such findings suggest that population composition matters and that the current increase in the socio-economic heterogeneity of families with joint custody might result in an overall reduction of its beneficial effects.