Lifetime interactions of male hunter-gatherers

Human hunter-gatherers, despite living in small groups of 20-50 individuals, make social contacts with up to a thousand other individuals in across their lifetimes. That’s the conclusion of Kim Hill and colleagues, who incorporated long-term field observations from two hunter-gatherer populations (the Hadza of Tanzania and the Ache of Paraguay) to understand the extent of social contacts that adult males develop in the course of their lives.

This may seem obvious – after all, people talk to each other and readily tolerate the presence of hundreds of other people around them. But small human groups don’t give opportunities to chat with hundreds of people very often. Members of a small-scale band may only see people outside their immediate circle of 20-50 people every few months.

Chimpanzees and bonobos have similar community sizes, ranging up to 50 individuals or so. But chimpanzee groups are famously antagonistic, with many intergroup encounters resulting in violent aggression. Humans do have aggression between groups, even in small-scale societies, but not indiscriminately.

On balance, then, the greater interaction rate of human foragers should lead to more lifetime contacts. Hill and colleagues use their data to quantify the effect of this over an individual’s lifetime:

Among the Ache and Hadza, frequent visiting and long lifespans mean that adults typically interact with more than three hundred same-sex adults during their lifetimes. This implies a social universe of about a thousand individuals, when opposite-sex adults and children are included. Recent work on the San Bushmen also suggests a similarly high number of significant interactants [27]. Additionally, close companions often interact with a somewhat different set of individuals, so that the total number of indirect interactants that each individual hears about repeatedly in detailed stories, and could expect to possibly meet some time during their lifetime is clearly more than 1,000. This is a much higher number of individually known social interactants than reported for any other primate, and possibly more than any other species on earth. It is also much greater than the predicted 150 significant social interactants (known as "Dunbar's number") that was extrapolated from primate brain by social group size regressions [6], [7]. It should not surprise us that humans have more relationships than their brain size alone predicts, as humans alone use language and symbolic devices to store information about potential relationships. The main reason why humans interact with so many more individuals than other apes is because: 1) human lifespans are much longer, and 2) interaction between neighboring and distant residential social units is extensive.

It is notable that the authors approached this work by replicating the results in two very different groups. The replication adds strength to the conclusion about the extent of interaction in human societies compared to other primates.

This is a conceptually simple but very important piece of evidence about the value of human sociality. Our connections with other people give us the opportunity to learn things our intimate group members may not already know. Innovations can spread among groups, even some that are very difficult to produce, because the individuals within a group will have multiple contacts with those outside their own group. High-value inventions in this context will spread disproportionately faster.

The “lifetime interaction probability” is one quantitative focus of the paper:

The relationship between yearly probability of interaction and expected proportion of the population that will interact in a lifetime asymptotes quickly, and even low rates of interpersonal interaction among hunter-gatherers lead to lifetime interactions with most other adults (Figure 4). This is because the human adult lifespan is so much longer than that of chimpanzees. The average expected time that both members of an Ache or Hadza dyad who enter adulthood together will both still be alive is about 27 years while the average expected time that two male chimpanzees from the same community, entering adulthood together, are still both alive is only 6 years.

Lifetime interactions are not necessarily relevant to the issue of innovations spreading, at least not directly. The lifetime interaction probability between adults may be central to the maintenance of reputation. Reputation is one of the most effective controls on social behavior in small-scale human societies. Gossip has real bite, but even moreso when the hearers can be assumed to have interacted with the subject of the gossip at some point in their lives.


Hill KR, Wood BM, Baggio J, Hurtado AM, Boyd RT (2014) Hunter-Gatherer Inter-Band Interaction Rates: Implications for Cumulative Culture. PLoS ONE 9(7): e102806. doi:10.1371/journal.pone.0102806