An article found here describes a study of middle-aged and older adults showing that men who have nightmares at least once a week are significantly more likely than other men to develop dementia. This correlation did not reach statistical significance for women. Speculating on the basis for this association, the researchers suggest that when dementia is in its undetectable early stages, it creates negative emotions that may be expressed as depression while awake and as nightmares while asleep. They hypothesize that medication that can reduce nightmares might also help to slow cognitive decline. Is the association between nightmares and development of dementia causal or correlational? If the latter, what third factor(s) might be responsible for it and why did it only appear in men? The study on which this article is based provides a good example of research methods whose interpretation depends on critical thinking about correlation and causation. It should provide a useful target for classroom discussion as well as a topic for student writing assignments aimed at promoting critical thinking.