Trust as social investment: A life-history model of environmental effects on ingroup and outgroup trust

Trust among closely-related individuals (ingroup) and trust among non-related individuals (outgroup) can be seen as different social investment that involves different life-history tradeoffs. We tested this life-history model using the World Values Survey and the World Health Organization datasets and examined how ingroup and outgroup trust are related to sex, individual-level resource availability, and society-level environmental threats. Results show that, at the individual level, financially disadvantaged people trusted ingroups less. At the societal level, violent-conflict threats were associated with lower ingroup and outgroup trust. Furthermore, higher disease-caused mortality was associated with lower ingroup trust but not lower outgroup trust. Moreover, fertility was associated with lower outgroup trust but not lower ingroup trust. We also found that the sex effect (men trusted others more than women did) was more prominent in societies with greater violent-conflict threats and higher fertility, but less prominent in societies with lower mortality from communicable diseases. These findings are explained within the life-history framework.