What separates the science of psychology from intuition, opinion, and common sense? Like most of us, you likely have your own ideas about why people behave the way they do, and many are likely correct. For example, women cry more often than men, children are infl uenced by their parents, and people often want what they cannot have. These observations are of little value precisely because they are so common or obvious. There are many more observations about people’s behavior that require more information, or evidence, to confi rm. Do people marry others like themselves, or do “opposites attract?” Are we more motivated by money or by meaning in our lives? What is the best way to make a desired change in our behavior?

To answer these questions, psychologists use the scientific method . This method tests hypotheses by gathering measurable evidence to determine whether they are true. This often requires designing experiments that capture the behavior of interest and then systematically observing the resulting data. For example, one study had college-age male skateboarders attempt a series of tricks. Half of them did so while an attractive young female experimenter looked on. The skateboarders who saw the woman watching them took more risks with their tricks and had a higher level of testosterone in their saliva.

Because the experiment kept the situation the same except for the presence of the woman, we can conclude that it was her presence alone that aroused the men and led them to take more risks. Empirical studies gain information by carefully collecting observations, or data. Data can include quantitative observations, such as the number of minutes a task takes or a rating on a scale for a survey question. Data can also be qualitative when the “qualities” of data are systematically described rather than counted. For example, interviews collect people’s own expressions of their experiences as a source of data. Empirical studies lie at the heart of psychology because they allow us to draw conclusions about human behavior and thinking.

Psychologists want to uncover the “what” of a given phenomenon, such as “intelligence”: what is the nature of intelligence? What are its features? What consequences does it have for a person? The research design, or overall structure of a study, depends on the specific questions of interest. And, because the field of psychology is so diverse in its questions, its methods are also highly varied. Psychologists seek universal truths about all people, as well as an understanding of the important differences among them, such as gender, culture, ethnicity, and personality, that result in differing experiences in life.

In addition, we want to know the “why” of a phenomenon; that is, an explanation of its causes and the multiple factors that influence it. What does it mean to say that someone is “intelligent,” and what causes some people to be judged as more intelligent than others? Using the scientific method, psychologists can add new knowledge about psychology.