Research on sex differences reveals a mix of clear biological patterns and substantial overlap between males and females.

Genetically, males and females differ in their sex chromosomes, which strongly influence reproductive anatomy and hormone profiles. However, many traits people associate with “male” or “female” are shaped by many genes of small effect plus environment, not by a simple genetic switch.

Physically, average differences are well documented. Males tend to be taller, have more muscle mass, higher hemoglobin, and different fat distribution. Females typically have higher body fat percentage, different pelvic anatomy, and distinct patterns of immune response, including greater susceptibility to some autoimmune diseases and different reactions to infections and vaccines.

In the brain, sex hormones influence development and function, but structural and functional differences are mostly small and highly overlapping. No robust evidence supports categorizing brains into two distinct “male” and “female” types. Many cognitive differences reported in earlier work shrink or disappear when studies use larger samples and better methods.

Behaviorally and psychologically, sex differences are usually modest at the population level. On average, males show higher physical aggression and risk taking, while females often score higher on measures of empathy and some social skills. Yet distributions are broad, with many individuals of one sex scoring above the average of the other.

Overall, biology clearly contributes to sex differences, particularly in reproduction, some aspects of physiology, and health. At the same time, context, culture, and individual experience play major roles, and individual variation within each sex is large.