There are large differences in earnings between men and women. Recent work highlights the importance of parenthood for the existence of gender inequality in the labour market. Estimates of the long-run ‘child penalty’, i.e. the impact of having children on women’s relative to men’s earnings, are large and vary substantially across countries. Neither the existence of child penalties nor the striking cross-country variation in child penalties is well understood. BELIEFS will collect a representative dataset of 80,000 individuals in the 28 EU Member States to study the role of several factors in explaining the cross-country differences in child penalties. It will examine the role of (i) beliefs about the benefits/costs to fertility and labour supply decisions, (ii) preferences for having children and for work/leisure, (iii) constraints, and (iv) social norms. BELIEFS will explore different dimensions of heterogeneity and study the individual-level (gender, age etc.) and country-level (labour regulations, family policies etc.) determinants of these factors. It will study whether there are misperceptions of norms and identify whether informing individuals of prevalent social norms shifts their beliefs about the benefits/costs to men/women working and their support for public policies. BELIEFS examines educational, fertility and labour supply decisions in a dynamic life-cycle framework and explores the role of beliefs, preferences, constraints and norms in those decisions. The dynamic framework will also be used to study the role of perceived child penalties in explaining fertility and educational choices. The project is highly ambitious in its scope and it is highly innovative in its combination of research methods. Ultimately, this research agenda will shed light on what drives gender gaps in labour market outcomes as well as which policies may be effective in narrowing these gaps.