Comparing Apples and Oranges: Are Children and Adults Two Different Species?

With this article, we address some of the theoretical and methodological issues faced when attempting to take a developmental approach to understand a psychological phenomenon that encompasses the entire lifespan, that is, from birth to old age. Most prominent among these issues is the challenge of defining and operationalizing a psychological construct valid for the entire lifespan. This entails both the questions of measurement equivalence and continuity and change in the theoretical meaning of a construct. We discuss six different psychological constructs from this perspective. The question that we ask throughout this endeavor is (with a twinkle in our eye) whether adults and young children are members of two different species. From a biological perspective, they are, of course, members of the same species, homo sapiens sapiens. However, the answer might need to be clarified from a cognitive developmental perspective. First, it is difficult to define a construct continuously across the entire lifespan. Hence, the question remains whether constructs such as depression or the self are similar or even the same in early childhood and old age. Second, it is impossible to apply the same measures to assess the constructs across the lifespan. Third, competencies, knowledge, and processing strategies change substantially, particularly from early childhood to later ages. Consequently, it appears that members of the “extreme ends” of the life span, infants and the elderly, seem to be members of “two different species.” However, once we have a theoretical understanding of which less specific measurement is equivalent, we can start to link the data on the construct development. Thus, we are not comparing apples and oranges because we build into the analyses the theoretically justified assumption that a limited set of lifespan developmental principles must and can explain how apples turned into oranges.