The precautionary principle has emerged as a leading guide to public decision-making about environmental risks under irreversibility and uncertainty. In this paper, we adopt a two-period model with irreversibility and agents differentiated by their degree of optimism to character-ize the conditions under which the precautionary principle applies. We show in particular that it is more often applied if the decision-maker has an intermediate optimism index, if scientific research is more effective at reducing uncertainty, and if the same decision-maker makes the decisions for both periods. Moreover, we show that socially optimal decisions lead to apply the precautionary principle more often than an elected decision-maker does.