Introduction: When gender (see entries on “Gender, Overview” and “Sexual Identity”) and psychology are discussed together, so usually are differences between men and women, or girls and boys. Mass media, advice books, and popular psychology books frequently focus on how women and men differ; and daily life provides many instances where such differences can be observed. There is often disagreement about the existence of differences, about the size of existing differences, about their origin, and about what observed differences ultimately mean. For the last 100 years, psychologists have eagerly researched differences between men and women, with a view to reaching generalizable answers about how they differ and to using those answers in education and other policy fields. Consequently, research about differences between men and women, or girls and boys, has distinct potential practice relevance.