Conformity to descriptive and injunctive norms has been explained by informational and normative social influence. We argue that in addition to these two types of social influence, outcome expectancy can mediate descriptive norms’ impact on people’s intentions in the case of collective behaviours such as hand washing to prevent virus spread. Two studies manipulate norm type (descriptive vs injunctive) and norm level (low vs high) and show their effects on intention to perform the behaviour. In Study 1 (N = 216), outcome expectancy was positively influenced by descriptive norm and was associated with intention. In Study 2 (N = 731), outcome expectancy was influenced by descriptive but also by injunctive norm. Similar to Study 1, outcome expectancy was significantly associated with intention. Our data support the idea outcome expectancy is an important antecedent of intention and an additional mechanism underlying the effects of descriptive norms and, in some instances, injunctive norms.