Investigated the extent to which the existence of interpersonal risk may disrupt cooperative behavior in a situation where cooperation is the most effective behavior for the achievement of a desired goal. 8 experiments with a total of 106 pairs of undergraduates indicated that: (a) cooperation was disrupted by the introduction of interpersonal risk, substantially when the risk was small and almost totally when the risk was large; (b) the effect of large risk on cooperation did not change substantially with time; (c) increasing the difference in rewards for cooperating and working individually did not substantially reduce the effect of large risk; (d) large risk did not appreciably disrupt cooperation when taking was not reinforcing to the taker; (e) when the presence of large risk was not related to task choice, the effect of risk on cooperation was reduced but not eliminated; (f) the ability to communicate when large risk was introduced very substantially reduced the effect of risk on cooperation; and (g) if the ability to communicate was permitted following disruption of cooperation by risk, it eventually led to the emergence of cooperation for a majority of groups. However, cooperation emerged more slowly than when communication was available prior to disruption by large risk. (29 ref.) (PsycINFO Database Record (c) 2016 APA, all rights reserved)