In the following paragraph, researchers Sherlock Campbell and James Pennebaker describe a remarkable statistical relationship.

Multiple laboratories have demonstrated that people who are asked to write about traumatic experiences subsequently exhibit better physical health than people who are asked to write about superficial topics. In these studies, individuals are randomly assigned to write about either emotional or nonemotional topics for 15 to 20 min per day for 3 to 5 consecutive days. In the past 15 years, dozens of replications have demonstrated that emotional writing can influence frequency of physician visits, immune function, stress hormones, blood pressure, and a host of social, academic, and cognitive variables. These effects hold up across cultures, ages, and diverse samples (Campbell & Pennebaker, 2003, p. 60)^[1].

In other words, researchers have answered the interesting and important question of whether engaging in what has come to be called “expressive writing” improves people’s health. It does. But there is a second question that is equally interesting and important: Why? What psychological and biological variables, structures, and processes are involved, and how do they connect the act of expressive writing to improved health? Several ideas have been proposed. For example, people who write about traumatic experiences might habituate to them. That is, the more they think about them, the less negatively they react both psychologically and physiologically—leading to improvements in mental and physical health (Lepore, Greenberg, Bruno, & Smyth, 2002)^[2].

This example illustrates that, like all scientists, researchers in psychology distinguish between two sorts of knowledge: their systematic observations and their explanations or interpretations of those observations. Typically, the former are called phenomena and the latter are called theories. Up to this point in the book, we have focused on phenomena. In this chapter, however, we focus on the equally important role of theories. We begin by exploring the distinction between phenomena and theories in more detail. We then look at the wide variety of theories that researchers in psychology construct. Finally, we consider how researchers use theories, and we present some strategies for incorporating theory into your own research.