Continuous and Discontinuous Cognitive Development²

Marjolein J. H. Rietveld¹, G. Caroline M. van Baal¹, Meike Bartels¹, and Dorret I. Boomsma<sup>1

1</sup> Department of Biological Psychology, Vrije Universiteit, Amsterdam, The Netherlands

In addition to continuous growth, cognitive abilities during childhood show clear discontinuous developmental patterns. An example of discontinuous cognitive development is the Piagetian conservation ability, which generally emerges in children around age 7 years. As part of a longitudinal twin study we collected data on both continuous and discontinuous cognitive development at four occasions during childhood. Continuous developmental data were collected by means of an intelligence test, administered to 209 twin pairs at ages 5, 7, 10 and 12 years. Discontinuous data were collected by means of a computerized version of Piaget's conservation ability task, administered to the same twins at age 7. Children were classified as conservers (54%) or non-conservers (46%). At each age, conservers achieved an intelligence score which was significant higher compared to the intelligence score achieved by non-conservers. This remained significant after correction for variation in age within each occasion of measurement. The significant effect appeared due to a large effect for nonverbal or performal cognitive ability. A trend was observed for verbal cognitive ability. Heritability is established for both conservation ability and intellectual development. We attempt to explore the underlying sources of the observed association between these two measures of cognitive development by using genetic structural modeling.