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186 PART III: Experimental MethodsPennebaker and his colleagues conducted many experiments in which theyassigned one group of participants to write about personal emotional events andanother group to write about superficial topics. Consistent with the hypothesesderived from the inhibition theory, participants who wrote about emotionaltopics had better health outcomes than participants who wrote about superficialtopics. Not all the results, however, were consistent with the inhibitiontheory. For example, students asked to dance expressively about an emotionalexperience did not experience the same health benefits as students who dancedand wrote about their experience. Pennebaker and Francis (1996) did a furthertest of the theory and demonstrated that cognitive changes that occur throughwriting about emotional experiences were critical in accounting for the positivehealth outcomes.Our brief description of testing the inhibition theory illustrates the generalprocess involved when psychologists do experiments to test a hypothesis derivedfrom a theory. If the results of the experiment are consistent with whatis predicted by the hypothesis, then the theory receives support. On the otherhand, if the results differ from what was expected, then the theory may need tobe modified and a new hypothesis developed and tested in another experiment.Testing hypotheses and revising theories based on the outcomes of experimentscan sometimes be a long and painstaking process, much like combining thepieces to a puzzle to form a complete picture. The self-correcting interplaybetween experiments and proposed explanations is a fundamental tool psychologistsuse to understand the causes of the ways we think, feel, and behave.Well-conducted experiments also help to solve society’s problems by providingvital information about the effectiveness of treatments in a wide varietyof areas. This role of experiments has a long history in the field of medicine(Thomas, 1992). For example, near the beginning of the 19th century, typhoidfever and delirium tremens were often fatal. The standard medical practice atthat time was to treat these two conditions by bleeding, purging, and other similar“therapies.” In an experiment to test the effectiveness of these treatments,researchers randomly assigned one group to receive the standard treatment(bleeding, purging, etc.) and a second group to receive nothing but bed rest,good nutrition, and close observation. Thomas (1992) describes the results ofthis experiment as “unequivocal and appalling” (p. 9): The group given thestandard medical treatment of the time did worse than the group left untreated.Treating such conditions using early-19th-century practices was worse thannot treating them at all! Experiments such as these contributed to the insightthat many medical conditions are self-limited: The illness runs its course, andpatients recover on their own.LOGIC OF EXPERIMENTAL RESEARCH• Researchers manipulate an independent variable in an experiment toobserve the effect on behavior, as assessed by the dependent variable.• Experimental control allows researchers to make the causal inference thatthe independent variable caused the observed changes in the dependentvariable.