The plasticity of intellectual development: insights from preventive intervention.

Debates regarding the plasticity of intelligence are often fired by a confusion between 2 distinct realms of development, that is, between developmental functions (e.g., a group's average IQ over time) and individual differences (e.g., the relative rank ordering of individual IQs within a group). Questions concerning the stability of these 2 realms are statistically independent. Thus there are 2 kinds of intellectual plasticity, and there may be no developmental convergences between them. In the present study, data from an early intervention program were used to investigate the 2 kinds of plasticity separately and to examine certain possible convergences between them. The program involved children at risk for developmental retardation who were randomly assigned at birth to 2 rearing conditions (i.e., educational daycare vs. no educational intervention) and whose intellectual development was then studied longitudinally to 4 years of age. Our findings indicate that developmental functions are moderately alterable through systemic early education, particularly after infancy, whereas individual differences are moderately stable, again particularly after infancy. They also indicate that the 2 kinds of plasticity are independent; the alteration of developmental functions through daycare affects neither the stability nor the determinants of individual differences. We discuss the implications that these findings have for current models of mental development, for the nature-nurture debate, and for arguments concerning the efficacy of early intervention programs.