McCann and Holmes (1984) tested the hypothesis that exercise reduces depression. They randomly assigned depressed undergraduate women either to an exercise condition (attending an aerobics class a couple of times a week for ten weeks) or to a relaxation training condition (the individuals relaxed at home by watching a videotape over the same period of time). Although the results showed that the exercise group reported less depression at the end of the ten-week period than did the relaxation group, why can't the researchers conclude that exercise reduces depression?
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