Decision-makers self-servingly navigate the equality-efficiency trade-off of free partner choice in social dilemmas among unequals

Cooperation is more likely upheld when individuals can choose their interaction partner. However, when individuals differ in their endowment or ability to cooperate, free partner choice can lead to segregation and increase inequality. To understand how decision-makers can decrease such inequality, we conducted an incentivized and preregistered experiment in which participants ( n =500) differed in their endowment and cooperation productivity. First, we investigated how these individual differences impacted cooperation and inequality under free partner choice in a public goods game. Next, we calculated if and how decision-makers should restrict partner choice if their goal is to decrease inequality. Finally, we studied whether decision-makers actually did decrease inequality when asked to allocate endowment and productivity factors between individuals, and combine individuals into pairs of interaction partners for a two-player public goods game. Our results show that without interventions, free partner choice, indeed, leads to segregation and increases inequality. To mitigate such inequality, decision-makers should curb free partner choice and force individuals who were assigned different endowments and productivities to form pairs with each other. However, this comes at the cost of lower overall cooperation and earnings, showing that the restriction of partner choice results in an equality-efficiency trade-off. Participants who acted as third-parties were actually more likely to prioritize inequality reduction over efficiency maximization, by forcing individuals with unequal endowment and productivity levels to form pairs with each other. However, decision-makers who had a ‘stake in the game’ self-servingly navigated the equality-efficiency trade-off by preferring partner choice interventions that benefited themselves. These preferences were partly explained by norms on public good cooperation and redistribution, and participants’ social preferences. Results reveal potential conflicts on how to govern free partner choice stemming from diverging preferences ‘among unequals’.