This study makes a significant contribution to investigations of household behavior by testing for a willingness to cooperate and share income by men and women who are either in couple with each other or complete strangers. We present results from an economic experiment conducted with 100 co-habiting heterosexual couples. We compare defection behavior in the prisoner’s dilemma within real couples to pairs of strangers. One out of four participants chose not to cooperatewith their spouse. To understand why spouses might prefer defection, we use a novel allocation task to elicit the individual’s trade-off between efficiency and equality within a couple. We further investigate the impact of socio-demographic and psychological characteristics of the couples. We find in particular that lack of preferences for joint income maximization, having children and being married lead to higher defection rates in the social dilemma.