This chapter explores the ways in which the sexes are similar and different in their cognitive abilities. It presents a balanced overview of the current findings in the research literature on sex differences in intelligence. The chapter discusses sex differences with regard to evolutionary perspectives, biological perspectives and sociocultural perspectives. A biopsychosocial model based on the inextricable links between the biological bases of intelligence and environmental events is an alternative to the nature-nurture dichotomy. The truth about sex differences in intelligence depends on the nature of the cognitive task being assessed, the range of ability that is tested, the age and education of the participants, and numerous other modifying variables. Data showing differences between men and women in intelligence do not support the notion of a smarter sex, nor do they imply that the differences are immutable.