Table 1.

Operationally defined “types” of sex differences

Category	Description	Examples
Type I—sexual dimorphism	Endpoint consists of two forms, one more prevalent in males and the other more prevalent in females. Endpoint may be present in one sex and absent in the other.	Copulatory behavior, bird song, nurturing, postpartum aggression, courtship displays
Type II—sex differences	Endpoint exists on a continuum and average is different between males and females.	Pain thresholds, food preferences and intake, odor detection, fear, anxiety, learning, memory, stress responding, sensory processing
Type III—sex convergence and divergence	Endpoint is the same in males and females but neural underpinnings are different. Alternatively, a sex difference may appear only in response to a challenge such as injury or stress.	Parental behavior, problem solving strategies, response to stress

Prior to embarking on the study of a particular sex difference it is useful to consider the nature of the difference. The distinctions presented in this table provide basic guidelines but are not exclusive and in many cases are species specific. Some sex differences may not fit neatly into any of these categories and certainly there are many that have yet to be discovered.