The neuroethology of friendship

Abstract

Friendship pervades the human social landscape. These bonds are so important that disrupting them leads to health problems, and difficulties forming or maintaining friendships attend neuropsychiatric disorders like autism and depression. Other animals also have friends, suggesting that friendship is not solely a human invention but is instead an evolved trait. A neuroethological approach applies behavioral, neurobiological, and molecular techniques to explain friendship with reference to its underlying mechanisms, development, evolutionary origins, and biological function. Recent studies implicate a shared suite of neural circuits and neuromodulatory pathways in the formation, maintenance, and manipulation of friendships across humans and other animals. Health consequences and reproductive advantages in mammals additionally suggest that friendship has adaptive benefits. We argue that understanding the neuroethology of friendship in humans and other animals brings us closer to knowing fully what it means to be human.

Keywords: cognition; ethology; evolution; friendship; social networks.

Figure 1

In highly social animals like rhesus macaques (Macaca mulatta), (A) friends groom each other and (B) provide each other with support in agonistic encounters against other group mates. (C) Affiliative behaviors positively predict reproductive output in this species, suggesting that social bonds are adaptive. These bonds may function to mediate the costs of competition that arise from living in stable social groups. Friendship is underpinned by numerous neural and physiological mechanisms, and may require specific cognitive abilities, such as (D) gaze following, that allow individuals to successfully coordinate their actions with others and navigate a complex social world. Photo credits: Lauren J.N. Brent

Figure 2

Social networks in three primate species. Networks are based on spatial proximity in (A) female rhesus macaques (n = 21), (B) coalitionary support in male chimpanzees (n = 10), and (C) named friendships in humans (n = 57). Nodes represent individuals; lines represent interactions between pairs of individuals. The thickness of the lines in (A) increase with the frequency of interaction. Arrows in (C) indicate whether named friendships were reciprocal. Individuals toward the center are more embedded in their social networks than those toward the periphery. Ties between closely related female rhesus macaques are highlighted in pink and demonstrate maternal kin bias (A). Social network position is heritable in humans and rhesus macaques, and has been associated with reproductive success in rhesus macaques and chimpanzees. Figure (A) was generated from the authors’ unpublished data, figures (B) and (C) were reproduced with permission from Refs. and , respectively.