Commons dilemmas present individuals with a choice between self and co
llective interest and they provide a useful paradigm for considering m
any environmental problems. We study them because individuals' harvest
choices can be self-interested or cooperative, and too many self-inte
rested choices literally could end the world. This paper first reviews
the main theories about and influences on cooperation in the commons.
Second, it suggests two new directions by (a) clarifying the nature o
f cooperation both conceptually and mathematically, and (b) outlining
a new theoretical perspective. Measures of cooperation must include fo
ur elements: the number of harvesters, the amount bf the resource, the
rate of resource regeneration, and the size of harvests. Our theoreti
cal suggestion is to place greater emphasis on the rise and fall of va
rious influences on harvester behaviour over the course of a commons d
ilemma, an emphasis we call the sequential-influence approach.