Psychologists study how humans make decisions by giving people “toy” problems. In one study, for example, my colleagues and I described to subjects a hypothetical disease with two strains. Then we asked, “Which would you rather have? A vaccine that completely protects you against one strain or a vaccine that gives you 50 percent protection against both strains?” Most people chose the first vaccine. We inferred that they were swayed by the phrase about complete protection, even though both shots gave the same overall chance of getting sick.