 Moving further away from pure voluntarism, a prominent approach distinguishes between deserved and undeserved shares of cooperative surplus. Nozick is sometimes criticized for assuming that persons are entitled to that portion of the market price for their goods and services characterizable as “scarcity rent” (B. Fried 1995: 230). Rent is defined as a gain that exceeds what is necessary to incentivize cooperative exchange. Excess gains in market exchange are often attributable to natural scarcity, including scarcity of natural talents. Whether persons deserve such windfalls flowing from brute luck in the birth lottery remains a deeply contested moral question. Some find a desert-basis in effort and other forms of personal sacrifice, including delayed gratification (as in investment contracts). On this view, parties are owed at least as much of the surplus from a cooperative scheme as would constitute fair compensation for their sacrifices (Reiman 1983; Sadurski 1985; Milne 1986). Others finds a desert-basis in productive contribution. Parties to contractual exchange deserve fair remuneration for their marginal product (D. Miller 1976; Miller 1999; J. Riley 1989). Marginal product is defined as a factor’s distinctive contribution to raising the value of goods and services, holding other factors constant. The central moral intuition, here, is that people ought to receive benefits from exchange in proportion to how much they improve the wellbeing of others (Lamont 1994). The productive contribution theory is sometimes motivated on general consequentialist grounds (rather than notions of moral desert), with rents explained as the premium necessary for incentivizing parties to put their talents to productive and wellbeing-maximizing use (Arnold 1987).