A sex difference is a distinction of biological and/or
physiological characteristics typically associated with either males or
females of a species in general. This article focuses on quantitative
differences which are based on a gradient and involve different
averages. For example, males are taller than females on average,
but an individual female may be taller than an individual male.
Although sex is believed by many to be binary, with "male" and "female"
representing opposite and complementary sex categories for the purpose
of reproduction, there are many individuals whose anatomy does not
conform to either male or female standards. Such individuals, called intersexuals , are sometimes infertile but are often capable of reproducing.