Research carried out to date has yielded inconsistent results regarding the manner in which the severity and certainty of threatened penalties combine to affect deterrence. While some studies presented evidence showing an interactive combination, others yielded an additive pattern. The present study tested and supported the proposition that the process whereby the two variables combine depends on the type of penalty with which a person is threatened. In the case of penalties the severity of which can be psychologically weighed in interval values, the combined effect is multiplicative or interactive. On the other hand, when the severity of a penalty can be expressed only ordinally, the combined effect is largely additive.