Touching heads

A bodily expression of positive emotions source ↗

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.¹ 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.²

References

Darwin Charles (1998) The Expression of the Emotions in Man and Animals, 3rd edn, London: Harper Collins.
Rozsa L, Apari P (2012). "Why infest the loved ones – inherent human behaviour indicates former mutualism with head lice" (PDF). Parasitology. 139: 696–700. doi:10.1017/s0031182012000017. PMID 22309598.

[1] Darwin Charles (1998) The Expression of the Emotions in Man and Animals, 3rd edn, London: Harper Collins.

[2] Rozsa L, Apari P (2012). "Why infest the loved ones – inherent human behaviour indicates former mutualism with head lice" (PDF). Parasitology. 139: 696–700. doi:10.1017/s0031182012000017. PMID 22309598.

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