Abstract Shared book reading (SBR) between parents and children is often regarded as a significant mediator of social inequalities in early skill development processes. We argue that socially biased gaps between parents in the awareness of the benefits of this activity for school success contribute to inequalities between children in access to this activity and in their language development. We test this hypothesis with a large-scale field experiment assessing the causal impact of an intervention targeting parents of pre-schoolers on both the frequency of SBR and the receptive vocabulary of children. Results indicate that low-educated parents are more reactive to this intervention, with significant effects on the language development of their children. We conclude that cognitive barriers and information gaps on the potential of informal learning activities at home fuel social inequalities in early childhood. At the same time, light-touch interventions removing these barriers are a cost-effective way of reducing these inequalities.