Touching heads is a uniquely
human
Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...
emotional expression that does not occur in nonhuman primates. All races, age groups and sexes of humankind interpret this behavior as an expression of positive emotions, such as love—including brotherly love, friendship etc.
It is absent in other
apes
Apes (collectively Hominoidea ) are a superfamily of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and Europe in prehistory, and counting humans are found glo ...
, though they tend to use the same bodily expressions of emotions like humans. A 2012 study claimed that this
behavior
Behavior (American English) or behaviour (British English) is the range of actions of Individual, individuals, organisms, systems or Artificial intelligence, artificial entities in some environment. These systems can include other systems or or ...
likely evolved in humans to share
head lice
The head louse (''Pediculus humanus capitis'') is an obligate ectoparasite of humans. Head lice are wingless insects that spend their entire lives on the human scalp and feed exclusively on human blood. Humans are the only known hosts of this s ...
among friends and relatives. Head lice infestations might serve as a protection against
body lice
The body louse (''Pediculus humanus humanus'', also known as ''Pediculus humanus corporis'') or the cootie is a hematophagic ectoparasite louse that infests humans. It is one of three lice which infest humans, the other two being the head lou ...
by inducing cross-resistance. This can be adaptive because only the latter type of lice transmit potentially lethal human pathogens.
References
Gestures
Human communication
Human head and neck
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