  An individually costly act that benefits all group members is a public good.
Natural selection favors individual contribution to public goods only when some
benefit to the individual offsets the cost of contribution. Problems of sex
ratio, parasite virulence, microbial metabolism, punishment of noncooperators,
and nearly all aspects of sociality have been analyzed as public goods shaped
by kin and group selection. Here, I develop two general aspects of the public
goods problem that have received relatively little attention. First, variation
in individual resources favors selfish individuals to vary their allocation to
public goods. Those individuals better endowed contribute their excess
resources to public benefit, whereas those individuals with fewer resources
contribute less to the public good. Thus, purely selfish behavior causes
individuals to stratify into upper classes that contribute greatly to public
benefit and social cohesion and to lower classes that contribute little to the
public good. Second, if group success absolutely requires production of the
public good, then the pressure favoring production is relatively high. By
contrast, if group success depends weakly on the public good, then the pressure
favoring production is relatively weak. Stated in this way, it is obvious that
the role of baseline success is important. However, discussions of public goods
problems sometimes fail to emphasize this point sufficiently. The models here
suggest simple tests for the roles of resource variation and baseline success.
Given the widespread importance of public goods, better models and tests would
greatly deepen our understanding of many processes in biology and sociality.
