Reducing inequalities in child mental health is a public health priority, yet the pathways that link social conditions to mental health outcomes in the early years are unclear. Few studies have compared the social distribution and prevalence of mental health problems across countries, or have compared pathways to any inequalities. Understanding these pathways is critical in order to guide public policy to improve child health and reduce inequalities. In Life Cycle cohort families we aim to assess how early years risk factors mediate the relationship between childhood SECs and subsequent inequalities in child mental health and cognitive development.