Genetic influences on Piagetian conservation ability in seven year olds.

G. Caroline M. van Baal and Dorret I. Boomsma.

Department of Biological Psychology, Vrije Universiteit, Amsterdam, The Netherlands

In addition to continuous growth, cognitive abilities during childhood show clear discontinuous developmental patterns, as indicated by Piaget in the sixties. The transitions between the different developmental stages coincide with growth spurts in brain maturation (e.g., as indexed by electroencephalographic power and coherence). This may suggest that discontinuous cognitive development will be influenced more by genetic than environmental factors and thus will show a high heritability (i.e., the relative importance of genetic influences on interindividual differences). Surprisingly little research has been done to establish heritabilities of indices of discontinuous cognitive development like the Piagetian conservation ability, which generally emerges in children around age 7 years. We addressed this question using a twin design. In establishing and testing for the heritability of conservation ability, we specifically included the possiblity of sex differences. Around their 7^th birthday, 192 twin pairs performed a computerized version of Piaget’s conservation ability task, which tests volume conservation ability. Children were either classified as conservers or non-conservers. Tetrachoric twin correlations were monozygotic (MZ) males = .95 (N=37); dizygotic (DZ) males = .44  (N=41); MZ females =  .59 (N=42); DZ females =  .40 (N=34); DZ male-female pairs =  .51 (N=38). Using threshold models in structural equation modelling, we found heritabilities of 94% (males) and  62% (females)  (c² =12.83, df=12, p=.381). Sex differences were significant. (Dc² = 4.04, df=1, p<.05), possibly reflecting maturational differences. These results support Piaget's assertions regarding the biological origins of conservation ability.