title: Reciprocal Exchange Patterned by Market Forces Helps Explain Cooperation in a Small-Scale Society.
abstract: Social organisms sometimes depend on help from reciprocating partners to solve adaptive problems [1], and individual cooperation strategies should aim to offer high supply commodities at low cost to the donor in exchange for high-demand commodities with large return benefits [2, 3]. Although such market dynamics have been documented in some animals [4-7], naturalistic studies of human cooperation are often limited by focusing on single commodities [8]. We analyzed cooperation in five domains (meat sharing, produce sharing, field labor, childcare, and sick care) among 2,161 household dyads of Tsimane' horticulturalists, using Bayesian multilevel models and information-theoretic model comparison. Across domains, the best-fit models included kinship and residential proximity, exchanges in kind and across domains, measures of supply and demand and their interactions with exchange, and household-specific exchange slopes. In these best models, giving, receiving, and reciprocating were to some extent shaped by market forces, and reciprocal exchange across domains had a strong partial effect on cooperation independent of more exogenous factors like kinship and proximity. Our results support the view that reciprocal exchange can provide a reliable solution to adaptive problems [8-11]. Although individual strategies patterned by market forces may generate gains from trade in any species [3], humans' slow life history and skill-intensive foraging niche favor specialization and create interdependence [12, 13], thus stabilizing cooperation and fostering divisions of labor even in informal economies [14, 15].
authors.affiliation: Department of Anthropology, Emory University, 1557 Dickey Drive, Atlanta, GA 30322, USA; Department of Anthropology, University of California, Santa Barbara, Santa Barbara, CA 93106, USA. Electronic address: adrian.jaeggi@emory.edu.
authors.forename: Adrian V
authors.initials: AV
authors.lastname: Jaeggi
authors.affiliation: Department of Anthropology, Emory University, 1557 Dickey Drive, Atlanta, GA 30322, USA.
authors.forename: Paul L
authors.initials: PL
authors.lastname: Hooper
authors.affiliation: Department of Anthropology, University of New Mexico, Albuquerque, NM 87131, USA.
authors.forename: Bret A
authors.initials: BA
authors.lastname: Beheim
authors.affiliation: Department of Anthropology, University of New Mexico, Albuquerque, NM 87131, USA.
authors.forename: Hillard
authors.initials: H
authors.lastname: Kaplan
authors.affiliation: Department of Anthropology, University of California, Santa Barbara, Santa Barbara, CA 93106, USA. Electronic address: gurven@anth.ucsb.edu.
authors.forename: Michael
authors.initials: M
authors.lastname: Gurven
mesh_terms: D001499:Bayes Theorem; D001838:Bolivia; D003299:Cooperative Behavior; D000067565:Farmers / Q000523:psychology*; D006801:Humans; D007199:Indians, South American / Q000523:psychology*; D007398:Interpersonal Relations; D008962:Models, Theoretical
keywords: Tsimane’; biological market theory; division of labor; kin selection; reciprocity; trade