Feature Review
The Anatomy of Friendship

https://doi.org/10.1016/j.tics.2017.10.004Get rights and content

Trends

Having friends has dramatic effects on our happiness, mental well-being and longevity.

There is a limit to the number of friends we can manage at any one time, sometimes known as ‘Dunbar’s number’.

This limit is imposed by a combination of the time and the cognitive demands (the latter a function of prefrontal cortex volume) of maintaining relationships.

There are striking gender differences in how relationships are maintained.

The Internet has not (yet) changed any of this.

Friendship is the single most important factor influencing our health, well-being, and happiness. Creating and maintaining friendships is, however, extremely costly, in terms of both the time that has to be invested and the cognitive mechanisms that underpin them. Nonetheless, personal social networks exhibit many constancies, notably in their size and their hierarchical structuring. Understanding the processes that give rise to these patterns and their evolutionary origins requires a multidisciplinary approach that combines social and neuropsychology as well as evolutionary biology.

Section snippets

The Meaning of Friendship

Over the past two decades, considerable evidence has emerged to suggest that the most important factor influencing our happiness, mental well-being, physical health, and even mortality risk, not to mention the morbidity and mortality of our children, is the size and quality of our friendship circles – something that also turns out to be true for anthropoid primates (Box 1). Friends provide moral and emotional support, as well as protection from external threats and the stresses of living in

The Limits to Friendship

Defining friendships in this wide sense, how many friends do we typically have? The average size of personal social networks seems to be about 150, whether these are determined from face-to-face contacts 18, 19, telephone call databases [20], or postings in online environments 21, 22, 23. This includes all extended family relationships as well as friends in the more conventional sense. More surprisingly, this turns out to be a common size for human organisations, including community size in

The Circles of Friendship

In contemporary societies, most personal social networks consist of extended family (including in-laws, or affines) and friends in about equal proportions [19]. These form two separate, but interlocked, subnetworks (family vs. friends) that typically interact only to a limited extent. Notably, we seem to treat close in-laws as though they were genetic family, and for the good biological reason that they share with us a genetic interest in our offspring [1]. People who come from large extended

A Two-Process Model of Social Bonding

In anthropoid primates, close friendships act as coalitions, one of whose functions is to buffer the individual, and particularly females, against the stresses that arise from living in close spatial proximity. Primates (and humans) live in groups mainly to minimise external ecological threats such as predation risk, raiding by neighbours, or environmental risk 50, 54, 55. In effect, primate groups are implicit social contracts: the relationships on which they are built are promissory notes

How Time Limits Friendship Networks

Time is a limited resource for all animals [77] including humans 78, 79, and if the quality (and hence functionality) of a relationship depends on the time invested in it [10], each individual has to decide how to distribute his/her available social effort, or capital, across his/her network. The network layers of Figure 2 appear to be associated with very specific contact values (Figure 3) 10, 19, 20. If someone is contacted less often than the defining rate (once a week for the 5-layer, once

A Crucial Role for Cognition

On the cognitive side, some form of cost accounting (a totting up of favours owed and promises broken) must be important [10]. A survey of the causes of relationship breakdown, for example, has identified lack of caring, poor communication, jealousy, and alcohol/drugs as the main causes (accounting for approximately 57% of all breakdowns) [122], all of which suggest that some kind of tally is being kept. However, there have been no studies that have explored the cognitive bases of this

The Seven Pillars of Friendship

One of the most striking things to emerge out of the friendship literature in the past decade or so has been the homophily effect: friends tend to be similar to each other on many dimensions (though personality is not often one of these) [155]. Personal social networks are commonly homophilous for gender, for example: men’s networks have significantly more men in them and women’s networks have significantly more women 26, 40. In part, this seems to reflect the fact that men and women have

Has the Internet Changed Our Social World?

A natural question to ask is whether the advent of the digital world, and social networking sites in particular, has changed any of these patterns. Relationships require a significant time investment (Figure 3) and there is a very strict upper limit of four on the number of people we can engage in conversation at any one time 133, 134, 135, 136, 137, 138, making it difficult, given the limit on the time available each day for social activities (approximately 20% of our day, based on activity

Concluding Remarks

Friendships (including family) are the single most important factor affecting our health and well-being. However, friendships are costly to maintain, both cognitively and in terms of the time that needs to be invested in them. These limit the number of friends we can have to around 150, and obliges us to distribute our social time/capital unevenly among them as a function of the benefits they provide us with. The endorphin system seems to play a crucial role in the maintenance of friendships,

Acknowledgements

Much of the research on which this article is based was funded by the British Academy Centenary Research Project, the UK EPSRC TESS project, the EU FP7 SOCIALNETS project and a European Research Council Advanced Investigator grant (No. 295663). I am grateful for the very helpful suggestions of the three referees.

References (222)

  • J. Lehmann

    Group size, grooming and social cohesion in primates

    Anim. Behav.

    (2007)
  • G. Miritello

    Time as a limited resource: communication strategy in mobile phone networks

    Soc. Netw.

    (2013)
  • S.B.G. Roberts et al.

    The costs of family and friends: an 18-month longitudinal study of relationship maintenance and decay

    Evol. Hum. Behav.

    (2011)
  • C.A. Salmon et al.

    On the importance of kin relations to Canadian women and men

    Ethol. Sociobiol.

    (1996)
  • R.I.M. Dunbar

    The social role of touch in humans and primates: behavioural function and neurobiological mechanisms

    Neurosci. Biobehav. Rev.

    (2010)
  • L.J. Young

    Oxytocin and vasopressin receptors and species-typical social behaviors

    Horm. Behav.

    (1999)
  • T.R. Insel et al.

    Neuropeptides and the evolution of social behavior

    Curr. Opin. Neurobiol.

    (2000)
  • M. Burton-Chellew et al.

    Are affines treated as biological kin? A test of Hughes’ hypothesis

    Curr. Anthropol.

    (2011)
  • O. Curry

    Altruism in social networks: evidence for a “kinship” premium

    Br. J. Psychol.

    (2013)
  • R. O’Gorman et al.

    Distinguishing family from friends

    Hum. Nat.

    (2017)
  • R.B. Cialdini

    Reinterpreting the empathy–altruism relationship: when one into one equals oneness

    J. Personal. Soc. Psychol.

    (1997)
  • E. Berscheid

    The Relationship Closeness Inventory: assessing the closeness of interpersonal relationships

    J. Personal. Soc. Psychol.

    (1989)
  • D.J. Hruschka

    Friendship: Development, Ecology, and Evolution of a Relationship

    (2010)
  • J.D. Korchmaros et al.

    Emotional closeness as a mediator of the effect of genetic relatedness on altruism

    Psychol. Sci.

    (2001)
  • J.D. Korchmaros et al.

    An evolutionary and close-relationship model of helping

    J. Soc. Pers. Relatsh.

    (2006)
  • A.J. Sutcliffe

    Relationships and the social brain: integrating psychological and evolutionary perspectives

    Br. J. Psychol.

    (2012)
  • S.L. Brown et al.

    Selective investment theory: recasting the functional significance of close relationships

    Psychol. Inq.

    (2006)
  • J.B. Silk

    Using the ‘F’-word in primatology

    Behaviour

    (2002)
  • R.I.M. Dunbar et al.

    Bondedness and sociality

    Behaviour

    (2010)
  • J.J.M. Massen

    Close social associations in animals and humans: functions and mechanisms of friendship

    Behaviour

    (2010)
  • R.I.M. Dunbar

    Bridging the bonding gap: the transition from primates to humans

    Philos. Trans. R. Soc. Lond. B Biol. Sci.

    (2012)
  • M. Argyle et al.

    The rules of friendship

    J. Soc. Pers. Relatsh.

    (1984)
  • J. Dunn

    Children’s Friendships

    (2004)
  • R.A. Hill et al.

    Social network size in humans

    Hum. Nat.

    (2003)
  • J.O. Haerter

    Communication dynamics in finite capacity social networks

    Phys. Rev. Lett.

    (2012)
  • B. Gonçalves

    Modeling users’ activity on Twitter networks: validation of Dunbar’s Number

    PLoS One

    (2011)
  • R.I.M. Dunbar

    Do online social media cut through the constraints that limit the size of offline social networks?

    R Soc. Open Sci.

    (2016)
  • R.I.M. Dunbar

    Coevolution of neocortex size, group size and language in humans

    Behav. Brain Sci.

    (1993)
  • S.B.G. Roberts et al.

    Communication in social networks: effects of kinship, network size and emotional closeness

    Pers. Relatsh.

    (2011)
  • T.V. Pollet

    Extraverts have larger social network layers but do not feel emotionally closer to individuals at any layer

    J. Individ. Differ.

    (2011)
  • K. Bhattacharya

    Sex differences in social focus across the life cycle in humans

    R. Soc. Open Sci.

    (2016)
  • S. Woldram

    Data Science of the Facebook World

    (2013)
  • J. Altmann

    Baboon Mothers and Infants

    (1980)
  • H.E. Fung

    Age-related patterns in social networks among European Americans and African American: implications for socioemotional selectivity across the lifespan

    Int. J. Aging Hum. Dev.

    (2001)
  • L.J.N. Brent

    Family network size and survival across the lifespan of female macaques

    Proc. Biol. Sci.

    (2017)
  • L.L. Carstensen

    Selectivity theory: social activity in life-span context

    Annu. Rev. Gerontol. Geriatr.

    (1991)
  • R.I.M. Dunbar

    Mind the gap: or why humans aren’t just great apes

  • A. Barnard

    Universal systems of kin categorization

    Afr. Stud.

    (1978)
  • R.I.M. Dunbar et al.

    Social networks, support cliques and kinship

    Hum. Nat.

    (1995)
  • V. Arnaboldi

    Analysis of co-authorship ego networks

  • Cited by (193)

    • Friendship and partner choice in rural Colombia

      2023, Evolution and Human Behavior
    View all citing articles on Scopus
    View full text