  Explaining why humans cooperate in anonymous contexts is a major goal of
human behavioral ecology, cultural evolution, and related fields. What predicts
cooperation in anonymous contexts is inconsistent across populations, levels of
analysis, and games. For instance, market integration is a key predictor across
ethnolinguistic groups but has inconsistent predictive power at the individual
level. We adapt an idea from 19th-century sociology: people in societies with
greater overlap in ties across domains among community members (Durkheim's
"mechanical" solidarity) will cooperate more with their network partners and
less in anonymous contexts than people in societies with less overlap
("organic" solidarity). This hypothesis, which can be tested at the individual
and community level, assumes that these two types of societies differ in the
importance of keeping existing relationships as opposed to recruiting new
partners. Using multiplex networks, we test this idea by comparing cooperative
tendencies in both anonymous experimental games and real-life communal labor
tasks across 9 Makushi villages in Guyana that vary in the degree of
within-village overlap. Average overlap in a village predicts both real-world
cooperative and anonymous interactions in the predicted direction; individual
overlap also has effects in the expected direction. These results reveal a
consistent patterning of cooperative tendencies at both individual and local
levels and contribute to the debate over the emergence of norms for cooperation
among humans. Multiplex overlap can help us understand inconsistencies in
previous studies of cooperation in anonymous contexts and is an unexplored
dimension with explanatory power at multiple levels of analysis.
