In almost all European Union countries, the gender wage gap is increasing across the wage distribution. In this 2008 presidential lecture I briefly survey some recent studies aiming to explain why apparently identical women and men receive such different returns and focus especially on those incorporating psychological factors as an explanation of the gender gap. Research areas with high potential returns to further analysis are identified. Several examples from my own recent experimental work with Patrick Nolen are also presented. These try to distinguish between the role of nature and nurture in affecting behavioural differences between men and women that might lead to gender wage gaps.