OBJECTIVE:This study examined use of strategies by twins during cognitive tasks to determine the effects of strategy-use on estimates of heritability.BACKGROUND:Performance on many cognitive tasks has been found to be more similar for monozygotic (MZ) than dizygotic (DZ) twins. The cognitive mechanisms mediating these similarities are largely unknown.METHODS:Think-aloud protocol analysis was used during 3 cognitive tasks typically considered to have high heritability and susceptibility to strategy-use. In addition, a battery of traditional paper tests was administered to examine potential effects of cognitive abilities.RESULTS:Performance on 3 cognitive tasks showed effects of strategies, and performance on 2 of the tasks showed a genetic influence. On 1 of these tasks differences in strategies explained a significant portion of the genetic influences. Measures of cognitive ability and metacognitive knowledge could not explain individual differences in strategy use.CONCLUSIONS:This is the first demonstration that the estimated heritability of performance on cognitive tasks is mediated, at least in part, by the use of specific cognitive strategies. Future studies using similar techniques will permit a description of the development of cognitive mechanisms mediating heritable cognitive abilities, and a deeper understanding of the integration of genetic and environmental factors at the level of cognitive strategies and processes.