Back in Psychology: Evolution of a Science, you read about B. F. Skinner, who studied the principle of reinforcement, which states that the consequences of a behaviour determine whether it will be more or less likely to occur in the future. So, for example, a rat’s rate of lever pressing will increase if it receives food reinforcement after each lever press. When you tell a classmate about this principle, she only shrugs. “That’s obvious. Anyone who’s ever owned a dog knows how to train animals. If you ask me, psychology is just common sense. You don’t have to conduct scientific experiments to test things that everyone already knows are true.” How would you explain the value of studying something that seems like “common sense”?

You are watching television with a friend when a news program reports that a research study has found that people in Europe who work longer hours are less happy than those who work shorter hours, but in the United States it is the other way around: Americans who work long hours are happier (Okulicz-Kozaryn, 2011). “That’s an interesting experiment,” he says. You point out that the news only said it was a research study, not an experiment. What would have to be true for it to be an experiment? Why are all research studies not experiments? What can you not learn from this study that you could learn from an experiment?

After the first test, your professor says she has noticed a strong positive correlation between the location of students’ seats and their test scores: “The closer students sit to the front of the room, the higher their scores on the test,” she says. After class, your friend suggests that the two of you should sit up front for the rest of the semester to improve your grades. Having read about correlation and causation, should you be skeptical? What are some possible reasons for the correlation between seating position and good grades? Could you design an experiment to test whether sitting up front actually causes good grades?

A classmate in your criminal law class suggests that mental illness is a major cause of violent crimes in Canada. As evidence, he mentions a highly publicized murder trial in which the defendant was diagnosed with schizophrenia. What scientific evidence would he need to support this claim?

You ask your friend if he wants to go to the gym with you. “No,” he says, “I never exercise.” You tell him that regular exercise has all kinds of health benefits, including greatly reducing the risk of heart disease. “I don’t believe that,” he replies, “I had an uncle who got up at 6 a.m. every day of his life to go jogging, and he still died of a heart attack at age 53.” What would you tell your friend? Does his uncle’s case prove that exercise really does not protect against heart disease after all?