Sexual dimorphism, sometimes known as severe gonochorism, is the distinct differences between the sexes within a species. A species with high sexual dimorphism has very noticeable difference between e.g. males and females. For example, the irish elk had such high sexual dimorphism that it was maladaptive, possibly being a major contributing factor in the extinction of the species.
Sexual dimorphism has been observed in several other animal groups including humans. For instance, males have larger brains than females. In humans, there is significant sexual dimorphism in the facial features, and these features vary from person to person. There is also much more variance in the upper body and hands than in the lower body and legs. For instance, women are shorter than men, and have larger breasts and narrower shoulders. These traits may have been selected for during the course of evolution in response to differences in reproductive opportunities. These differences in body features may explain why people have such a wide range of gender identities and sexual orientations, and why so many people find their sexual identity to be at odds with their natal sex.
Otto Weininger argued that in humans, the less sexually dimorphic the genders, the less of a problem female sexual liberation becomes. Whereas, if there is high sexual dimorphism in humans, female sexual liberation leads to agitation, domestic abuse against husbands, and corruption of the arts and science by feminine women only expressing vanity and seeking male attention.
The famous MRA Warren Farrell argued that in humans sexual dimorphism could lead to a fisherian runaway and extinction of the species if females continue to select for, "killer-hunter", type men with the advent of nuclear technology, though some think this is hysterical and overblown.
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