An assumption that has guided the work reported here is that more customary accounts of structural development have tended to ignore a number of potential sources of variation and individual differences in the process of cognitive development. In the language employed throughout this chapter, these differences are said to represent both the effects of intrinsic task constraints, such as those imposed by the particular stimulus materials, procedural strategies and response modalities employed, and the effects of external constraints or influences on children's actual developmental rate, such as those introduced by variations in the social life worlds of the subjects tested, or other unspecified psychological characteristics that might influence their performance, or more generally, their cognitive developmental status. Not accounting for intrinsic
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constraints has led researchers systematically to overestimate children's test abilities . Similarly, failure to pay adequate attention to the external constraints has led other researchers to observe especially advantaged subpopulations and thus to conclude that various cognitive competencies are consolidated at a much earlier age than is the case in the population as a whole.