Democracies go to war with each other less frequently than dictatorships do with each other. This is an established empirical regularity. However, it is not clear whether there is a causal link between democracy and peace. We use laboratory experiments to study whether there is a causal impact. We study the bellicosity of democracies compared with two types of dictatorships, inclusive and exclusive, where each society is composed of three members. We also analyze how bellicosity depends on the presence of the possibility of deliberation among the members of a society. Neither the ‘voting’ nor ‘inclusion’ aspect of democracy nor ‘deliberation’ in isolation has a positive causal impact on peace. However, when all three are combined, there is evidence that their combination produces less bellicosity than some kinds of dictatorship. It is the availability of deliberation that makes the crucial distinguishing difference for democracy in our experiment. We observe democratic peace only in the presence of deliberation.