Sixty university students solved four intuitive probability problems in which two base rates (the probabilities that people in a country are genetically predisposed to a disease) and two likelihoods (the probabilities that people who are genetically predisposed to the disease will actually develop it even if they are vaccinated) were provided. Participants made judgments on whether they would get vaccinated on the basis of the information. The number of participants who responded that they would get vaccinated was significantly different across problems. More participants responded that they would get vaccinated when the base rate was 800 out of 1000 than when it was 1 out of 1000. If the base rates were the same, more participants responded that they would get vaccinated when the likelihood was 15% than when it was 75%. These results suggest that people make intuitive probabilistic judgments on the basis of both base rates and likelihoods.