In addition to explaining normative patterns of development, researchers are also interested in providing a causal account for why a target phenomena should take any of the variety of different pathways it has been observed to follow. Sometimes this task is relatively straightforward—especially when pathways differ only in mean level or age of onset. Then it can be the case that the same factors that explain normative change and stability also can account for different pathways. Pathways are traversed at earlier ages or at higher mean levels because some individuals have more of the factors that promote the phenomena and less of the factors that undermine it, whereas pathways are traversed at later ages or at lower mean levels because some individuals have less of the factors that promote the phenomena and more of the factors that undermine it. In the case of motivation, this kind of “explanatory continuity” has been found for some of the differences between girls and boys in motivational development. In general, many of the same factors that predict and explain engagement in girls (who start and remain higher in motivation) also predict and explain engagement in boys; they just operate on a lower plane for boys.
The task of differential explanation is made more challenging when the causal factors that produce normative development are not the same ones as those that generate the different pathways. Such “explanatory discontinuity” seems to be the case for differences in motivation, engagement, and achievement for students from different socio-economic classes and ethnic groups. Explanatory models for white middle class students simply did not include factors like affordable medical care (e.g., to treat ear infections which otherwise produce high rates of school absence), dangerous neighborhoods (e.g., which can interfere with getting to school and completion of homework), and discrimination from teachers and peers.