Touching heads
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Touching heads is an emotional expression.
It is absent in other apes, though they tend to use the same bodily expressions of emotions like humans. A 2012 study claimed that this behavior likely evolved in humans to share head lice among friends and relatives. Head lice infestations might serve as a protection against body lice by inducing cross-resistance. This can be adaptive because only the latter type of lice transmit potentially lethal human pathogens.