The Scarr-Rowe Interaction in Complete Seven-Year WISC Data from the Louisville Twin Study: Preliminary Report

Abstract

We examine updated Wechsler IQ data in 7-year old twins from the Louisville Twin Study for evidence of an interaction between the heritability of IQ and socioeconomic status. Data records that had never been entered were recovered, allowing us to increase previously reported sample sizes by more than 20%. Twin families were assigned socioeconomic status scores using a Hollingshead index based on parental education and occupation. A structural equation model in which genetic and environmental variances were modeled as squared linear functions of SES provided ambiguous replication of earlier findings from the National Collaborative Perinatal Project: relations between SES and heritability for performance and full scale IQ were in the same direction as the previous report, but at p < 0.07. As was the case in Turkheimer et al. (Psychol Sci 14(6):623-628, 2003), no interaction was found for VIQ. These results cannot yet be taken as a definitive replication of Turkheimer et al. (Psychol Sci 14(6):623-628, 2003). Many more measurement occasions, subtests and environmental moderators remain to be analyzed.

Keywords: GE interaction; Socioeconomic status; Wechsler intelligence scale for children.

Figure 1

Histograms of VIQ, PIQ, FSIQ and socioeconomic status

Figure 2

Scatter plot of socioeconomic status and seven-year WISC scores

Figure 3

Stacked variance plots showing A, C and E variances as an exponential-linear function of socioeconomic status for VIQ, PIQ and FSIQ