Both interpretations hold validity to some extent. Individuals differ in altruistic personality (Bierhoff, Klein, & Kramp, 1991) and in their need for approval (Crowne & Marlowe, 1964). Because helping is a social norm, it is likely that individual differences in helping reflect individual differences in both personality characteristics. Furthermore, altruistic personality and need for approval may not be the only determinants of helping (Montada & Bierhoff, 1990). Most psychological phenomena studied in the history of psychology were found to be multidetermined. Helping is not an exception to this rule.
The multidetermination of human behavior has extremely important implications for research designs in general (Shadish, Cook, & Campbell, 2002) and for multimethod assessment in particular (Wiggins, 1973). This is true because the explanations of behavior and its measurement are two sides of the same coin. Our example again serves to illustrate this important fact. Consider the three methods for assessing help previously suggested: I could obtain self-reports from my neighbors (Method 1), I could observe their behavior secretly (Method 2), and 1 could interview their wives (Method 3). The result of each method will be multidetermined. To keep things simple, consider two causes for each method. Individual differences in helping behavior according to Method 1 might be caused by individ ual differences in altruistic personality and by individual differences in need for approval. Individual differences obtained with Method 2 might be caused by individual differences in altruistic personality and by my sympathy for the neighbors. Liking versus disliking may create a perceptual bias, leading me to overestimate or underestimate the help. Individual differences obtained with Method 3 might be caused by individual differences in altruistic personality and by individual differences in marital satisfaction. In a happy relationship, a neighbor's spouse might overestimate her husband's help, whereas in an unhappy relationship, she might underestimate that help.
Several important conclusions can be drawn from this analysis: First, a method usually measures more than one cause or factor. Second, the results obtained with different methods will converge to the extent that they share causes or factors. In our example, the common factor was altruistic personality In addition to this common factor, each method measured a unique or specific factor. The unique factors of Methods 1, 2, and 3 were needed for approval, sympathy, and marital satisfaction, respectively. Third, the extent of convergence among different methods depends on the relative weight of their common and unique factors. If altruistic personality has strong effects on behavior in comparison to need for approval, sympathy, and marital satisfaction, consistency across methods is increased. By contrast, if the specific factors had large effects on behavior compared to altruistic personality, convergence among the methods is decreased. Fourth, the example shows choice of methods as a matter of theory. The more we know about the causes of behavior, the better can we measure behavior and the more likely we can develop methods that measure predominantly what we want to measure. Regarding our example, if we wanted to measure altruistic personality, we would select, on the basis of theory, methods that were affected as much as possible by altruistic personality and as little as possible by diagnostically irrelevant factors.
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