Facial Expression of Emotional State

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Human Uniqueness Compared to "Great Apes": 
Speculative Difference
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Facial expressions are a primary means of visual communication among mammals. They are controlled through the contraction of facial muscles, called mimetic muscles, which are largely conserved across primate phylogeny, although monkeys and apes display a greater number of facial expressions than other mammals. Historically, researchers have debated whether facial expressions are used voluntarily to manipulate the social environment, or whether they represent more involuntary emotional signals. Likewise, there has been debate over whether facial expressions among animals, particularly nonhuman primates, are evolutionarily homologous to human expressions. By comparing the action of specific facial muscles, researchers have shown that some chimpanzee facial expressions are homologous to humans and there is little debate that these convey information about underlying emotional state.

The Human Difference: 

Chimpanzees have a heavy brow ridge which constrains the visibility of upper facial movements and, as a result, chimpanzee facial expressions involve mostly lower facial muscles. Homology in facial expression between humans and chimpanzees is thus constrained to lower facial movements.

Universality in Human Populations: 

Universal except in cases of facial paralysis


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