Research on individual differences is focused on the variability in performance that exists across the members of any group. Within a group defined by one or more dimensions (e.g., all first-grade boys), not everyone performs identically. Furthermore, if more than one skill is relevant for a learning task, the profiles of skills may vary from child to child. One boy might have relatively poor mental rotation skills but an excellent sense of his body in physical space, whereas another boy might have the reverse profile.

THE CONCEPT OF GROUP DIFFERENCES

Given the variability among individuals within any kind of group (such as those defined by the learner’s biological sex or age), why should one bother with identifying differences at the level of groups? There are several reasons why it is useful to identify patterns of skills and behaviors across groups, even though knowledge of these group differences can provide only probabilistic information about the characteristics of any given learner.

The first reason is practical and stems from the way educational systems are structured. In particular, the two dimensions along which spatial skills and abilities are most commonly examined in psychological research—chronological age and gender—are also the dimensions along which many educational opportunities are differentiated.

Of the two, chronological age is more pervasive in the American education system. With few exceptions (e.g., in very small school districts or home schooling), most education is delivered in age-segregated schools and classrooms. Educational distinctions also occur with respect to gender despite the fact that there are no longer divisions between required curricula for boys and girls as was once the case (girls were required to take home economics, boys were required to take shop, and neither group was allowed into the other’s classes). Nevertheless, proportions of males and females enrolled in elective subjects (e.g., computer science) continue to differ. Furthermore, private and public schools may offer intentional opportunities for single-sex classes, and even in classrooms containing students of both sexes, educational experiences may differ if not by design then by practice (as when disproportionate numbers of boys are found at classroom computers).

Although group membership will not ensure that every member of the group has a particular set of characteristics, knowledge of group membership is useful, given that it is—for practical reasons—impossible to measure individuals on every relevant dimension. In the absence of individually administered assessments of relevant variables, knowledge of group characteristics can provide hints about what kinds of experiences and skills learners bring to a new educational situation. However, the predictive power of gender, for example, is insufficient to use gender alone as a basis for recommending differential educational opportunities (see Box C.1).

A second reason to identify group differences is that they may suggest factors that account for individuals’ development of different or better spatial skills and strategies. This knowledge could, in turn, provide educational guidance as to learning strategies that might be easier and/or more effective. Thus, the finding that spatial skills or strategies are not randomly distributed across groups may be useful in identifying what kinds of factors account for differential outcomes. For example, if boys as a group do better than girls as a group in learning to assemble car motors, we can look for differences between boys and girls that are potentially relevant. We might find that boys, on average, have been given 10 times more model cars to put together during childhood than have girls. We might hypothesize that something that helps people figure out how pieces of motors fit together is “model experience.” To test this hypothesis, we could examine whether those girls who were given model cars to assemble during childhood do better at assembling car motors than do girls who were not given model cars. Of course, an affirmative answer would not prove the causal connection because it could be that children with better spatial skills begged for, and



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