The objective of this article is to investigate the impact of agent heterogeneity (as regards their attitude towards cooperation) and payoff structure on cooperative behaviour, using an experimental setting with incomplete information. A game of chicken is played considering two types of agents: ‘unconditional cooperators’, who always cooperate, and ‘strategic cooperators’, who do not cooperate unless it is in their interest to do so. Overall, our data show a much higher propensity to cooperate than predicted by theory. They also suggest (...) that agent heterogeneity matters: the higher the proportion of ‘strategic cooperators’ in the population, the higher their probability to cooperate. Finally, our data confirm that higher rewards to cooperation (embedded in the payoff structure) tend to lower defection. Taken together, our results suggest that the subjects might be non-expected utility maximizers, dealing with both outcomes and probabilities in a non-linear manner. (shrink)