Touching heads is a uniquely

human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

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 clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister g ...

, 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 and mannerisms made by individuals, organisms, systems or artificial entities in some environment. These systems can include other systems or organisms as wel ...

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 feeding exclusively on human blood. Humans are the only known hosts of thi ...

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'') is a hematophagic ectoparasite louse that infests humans. It is one of three lice which infest humans, the other two being the head louse, and the cra ...

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 {{social-psych-stub