Apicella et al. (2012) Social networks and cooperation in hunter-gatherers

Apicella CL, Marlowe, FW, Fowler JH, & Christakis NA (2012) Social networks and cooperation in hunter-gatherers. Nature 481, 497-502.

From the abstract:

• We characterize the social networks of the Hadza, a population of hunter-gatherers in Tanzania.
• We show that Hadza networks have important properties also seen in modernized social networks, including a skewed degree distribution, degree assortativity, trans- itivity, reciprocity, geographic decay and homophily.
• We demonstrate that Hadza camps exhibit high between-group and low within-group variation in public goods game donations.
• Network ties are also more likely between people who give the same amount, and the similarity in cooperative behaviour extends up to two degrees of separation.
• Social distance appears to be as important as genetic relatedness and physical proximity in explaining assortativity in cooperation.
• Our results suggest that certain elements of social network structure may have been present at an early point in human history.
• Also, early humans may have formed ties with both kin and non-kin, based in part on their tendency to cooperate. Social networks may thus have contributed to the emergence of cooperation. 

Here’s why it matters:

• Technological advances (such as in communication, transportation and agricultural systems), demographic changes (such as in population density, inter-group marriage and dispersal), and social innovations (such as in formal institutions) have all changed the social landscape of humans from that in which they evolved. This raises the question of whether features observed in modernized social networks are ancient or contemporary in origin.
• However, evidence suggests that natural selection may have played a part in the formation of human networks (NOTE: this seems obvious to me, but possibly it’s not to others).
• Evolutionary theories of cooperation rely on explicit or implicit assumptions regarding social structure. Cooperation can evolve if individuals tend to interact with others of the same type (positive assortment).
• If real-world interactions do not exhibit such assortativity, then none of these theories can explain the widespread cooperation in humans that we observe today.
• No prior study has attempted to map the complete social network of a foraging population to study its dyadic building blocks (ties between pairs of people) and macroscopic structure, as well as the role of cooperation.

Methods

We performed a comprehensive, socio-centric network study of the Hadza hunter-gatherers of Tanzania.

Campmate network: ‘Who would you like to live in the next camp?’

Gift network: ‘To whom would you give a (real) gift of honey?’

205 subjects, with 1,263 campmate ties and 426 gift ties.

Compared with random networks – do they differ in the same way as modern social networks?

Results

­Network structure

Cumulative distributions of in-degree (the number of times an individual is nominated). As is typical of networks, the degree distributions have significantly fatter tails than a similarly sized group composed of individuals randomly forming the same number of social ties. Degree distributions for the male and female campmate networks did not differ.

As in modernized societies, the probability of a social tie decreases with increased geographic distance.

The probability that two individuals are connected increases as the genetic relatedness between the pair increases, in both the campmate and the gift networks.

The big take home is that Hadza social networks look a lot like social networks in modern societies.

Cooperation

There is also homophily in cooperative behavior in the public goods capes. Cooperators tend to be connected to other cooperators, and non-cooperators to non-cooperators.

A key prediction of some evolutionary models is thus that there should be relatively more variance in cooperative behaviour between groups as compared to within groups.

Fig. 2a compares the observed variance in donations to the public good to the variance obtained when we keep the population structure fixed and randomly reshuffle the observed distribution of donations across all individuals. Compared to chance, there is significantly more between-camp variation (P = 0.01) and significantly less within-camp variation (P = 0.01) in cooperative behaviour.

Homphily in gift network extends two degrees of separation (Fig 2b). At 3 degrees of separation, there is significant anti-correlation in the campmate networks, suggesting that cooperative and non-cooperative clusters tend to be polarized (though this might also reflect a finite size effect, given the small size of Hadza society).

Dyadic analyses of social ties show that people who donate more do not have higher out-degree or in-degree (Fig. 2c). This suggests that we can reject the hypothesis that hunter-gatherers unconditionally prefer to form ties with cooperators.