A theory is proposed that predicts the existence of numerous gender differences in cognition and behavior. The basis for these expectations is the single assumption that females have evolved tendencies to form long term sociosexual alliances with a competent resource provisioner. This assumption is teamed with evidence that males are actually a variant on the female sex with brains masculinized in ways that help them respond to female mating preferences. To orchestrate male responses to female biases in mates, the theory asserts that androgens (male sex hormones) have two main effects on human brain functioning. One is a diminished sensitivity to most environmental stimuli. The other involves shifting cognitive functioning away from the left hemisphere toward a more even and task-specialized hemispheric distribution. Many of the cognitive and behavioral differences between males and females predicted by the theory are described.