The Handbook of Sex Differences is a four-volume reference work assembled and written to assess sex differences in human traits (although findings regarding other species are also included). Based on the authors’ highly influential 2008 book Sex Differences, these volumes highlight important new research findings from the last decade and a half alongside earlier findings. Conclusions reached by meta-analyses are also included.In this, the work’s first volume, findings from thousands of studies are summarized regarding basic biology. Results having to do with sex ratios at birth and traits involving a wide range of bodily features are reported along with numerous complex aspects of biochemistry, neurology, and physical health. The eight chapters comprising Volume I are as follows: Reproduction, Development, and Morphology Anatomical and Physiological Factors Bodily Fluids, Biochemicals, and Biochemical Receptors The Brain: Structure and Functioning Physical Health and Illness Factors Responses to Physical and Chemical Environmental Factors Responses to Stress and to Pain Prenatal Factors The Handbook of Sex Differences is of significant importance for any researcher, student, or professional who requires a comprehensive resource on sex differences.