Scientists and nonscientists alike construct theories to explain variations in the environment (Byrnes, 2001a). For example, zoologists devise theories to explain observable differences in the physical appearance of species, and developmental scientists create theories to explain changes in performance that occur between early childhood and adulthood. The authors in this book are chiefly concerned with variations in math performance that are evident when one compares boys with girls or men with women. Explaining these gender-based variations is not an easy task because the size and direction of differences change with age, content, measure, and context (Hyde, Fennema, & Lamon, 1990). The purpose of this chapter is to present a comprehensive account of gender differences that explains most of the variations that have been revealed to date. As I will argue later, the primary virtue of such an account is that it could form the basis of highly effective forms of intervention.

The rest of this chapter is organized as follows. In the first section, I provide a brief overview of the pattern of gender differences that have been reported in the literature. This pattern represents the phenomenon that needs to be explained by any theory of gender differences. In the second section, I summarize and critique existing explanations of these findings (including a Cognitive Process approach that my colleagues and I proposed in the mid-1990s). In the third section, I present a new explanatory model that was created to extend and integrate the existing explanations (called the Three Conditions model).