Using a public goods experiment with heterogeneous income and marginal-per-capita-returns (MPCR), this paper investigates the main drivers of behavior for high- and low-income individuals playing separately and in mixed groups. A finite mixture model was used to split each income type into two categories. While a third of low-income individuals were “free-riders”, the dominant portion was classified as “opportunists”, who try to benefit from the presence of high-income individuals by encouraging contributions through cooperation. “Free-riders” were far less among high-income individuals, where the overruling majority were “selfists” who, due to self-interest and caution, contribute less when low-income individuals are present.