ation of the assumptions underlying both research and current practices in science education.

In science education, there has been a frequent assumption that development is a kind of inevitable unfolding and that one must simply wait until a child is cognitively “ready” for more abstract or theory-based forms of content. In other words, through maturation with age, children will achieve certain cognitive milestones naturally, with little direct intervention from adults. Many science educators and policy makers have assumed that the power and limitations of children’s scientific reasoning at different grade levels could be derived from the stages delineated in the cognitive developmental literature. In this view, “developmentally appropriate” education would thus require keeping instruction within these bounds.

There are significant problems with this assumption. First, it assumes that the power and limitations of children’s scientific thinking within an age band can be described and predicted by stage-defining criteria, with limited variability or change therein. As we show in the chapters in Part II, the cognitive developmental literature simply does not support this assumption. In the words of John Flavell, a seminal cognitive developmentalist (1994), “Virtually all contemporary developmentalists agree that cognitive development is not as general stage-like or grand stage-like as Piaget and most of the rest of the field once thought” (p. 574). The foundation of research undermining a broad stage-like conception of cognitive development goes back at least three decades (e.g., Wollman, 1997a, 1997b).

In fact, variability in scientific reasoning within any age group is large, sometimes broader than the differences that separate contiguous age bands. In self-directed experimentation tasks, there are always some adults whose performance looks no better than that of the average child (Klahr, Fay, and Dunbar, 1993; Kuhn, Schauble, and Garcia-Mila, 1992; Kuhn et al., 1995; Schauble, 1996; see also the discussion in Chapter 5). Indeed, many adults never seem to master the heuristics for generating and interpreting evidence. Moreover, education, context, and domain expertise seem to play a strong role in whether and when these heuristics are appropriately used (Kuhn, 1991).

Stage-like conceptualizations of development also ignore the critical role of support and guidance by knowledgeable adults and peers. As noted in the National Research Council report How People Learn (1999), children need assistance to learn; building on their early capacities requires catalysts and mediation. Adults play a central role in “promoting children’s curiosity and persistence by directing their attention, structuring their experiences, supporting their learning attempts, and regulating the complexity and difficulty of levels of information for them” (p. 223). In the case of the science classroom, both teachers and peers can and must fill these critical roles. The power of schooling is its potential to make available other people, including

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