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218 PART III: Experimental MethodsKey ConceptIt is important to differentiate experiments involving independent variableswhose levels are selected from those involving independent variables whoselevels are manipulated. Experiments involving independent variables whose levelsare selected—like individual differences variables—are called natural groupsdesigns. The natural groups design is frequently used in situations in which ethicaland practical constraints prevent us from directly manipulating independentvariables. For example, no matter how interested we might be in the effects ofmajor surgery on subsequent depression, we could not ethically perform majorsurgery on a randomly assigned group of introductory psychology students andthen compare their depression symptoms with those of another group who didnot receive surgery! Similarly, if we were interested in the relationship betweendivorce and emotional disorders, we could not randomly assign some peopleto get divorced. By using the natural groups design, however, we can comparepeople who have had surgery with those who have not. Similarly, people whohave chosen to divorce can be compared with those who have chosen to staymarried.Researchers use natural groups designs to meet the first two objectives of thescientific method: description and prediction. For example, studies have shownthat people who are separated or divorced are much more likely to receivepsychiatric care than are those who are married, widowed, or have remainedsingle. On the basis of studies like these, we can describe divorced and marriedindividuals in terms of emotional disorders, and we can predict which group ismore likely to experience emotional disorders.Serious problems can arise, though, when the results of natural groupsdesigns are used to make causal statements. For instance, the finding thatdivorced persons are more likely than married persons to receive psychiatriccare shows that these two factors covary. This finding could be taken to meanthat divorce causes emotional disorders. But, before we conclude that divorcecauses emotional disorders, we must assure ourselves that the time-order conditionfor a causal inference has been met. Does divorce precede the emotionaldisorder, or does the emotional disorder precede the divorce? A natural groupsdesign does not tell us.The natural groups design also poses problems when we try to satisfy thethird condition for demonstrating causality, eliminating plausible alternativecauses. The individual differences studied in the natural groups designare usually confounded—groups of individuals are likely to differ in manyways in addition to the variable used to classify them. For example, individualswho divorce and individuals who stay married may differ with respectto a number of characteristics other than their marital status, for example,their religious practices or financial circumstances. Any differences observedbetween divorced and married individuals may be due to these other characteristics,not to divorce. The manipulation done by “nature” is rarely thecontrolled type we have come to expect in establishing the internal validity ofan experiment.There are approaches for drawing causal inferences in the natural groupsdesign. One effective approach requires that individual differences be studiedin combination with independent variables that can be manipulated. This