Recent psychological and neuropsychological research suggests that executive functions -the cognitive control processes that regulate thought and action -are multifaceted and that different types of executive functions are correlated but separable. The present multivariate twin study of three executive functions (inhibiting dominant responses, updating working memory representations, and shifting between task sets), measured as latent variables, examined why people vary in these executive control abilities and why these abilities are correlated but separable from a behavioral genetic perspective. Results indicated that executive functions are correlated because they are influenced by a highly heritable (99%) common factor that goes beyond general intelligence or perceptual speed, and they are separable because of additional genetic influences unique to particular executive functions. This combination of general and specific genetic influences places executive functions among the most heritable psychological traits. These results highlight the potential of genetic approaches for uncovering the biological underpinnings of executive functions and suggest a need for examining multiple types of executive functions to distinguish different levels of genetic influences. Keywords HERITABILITY; EXECUTIVE CONTROL; INHIBITION; UPDATING; TASK SWITCHING; FRONTAL LOBE TASKSIndividuals vary widely in their abilities to control their own thoughts and actions. Some people seem ruled by impulses, while others manage successfully to regulate their behaviors. From the perspective of cognitive psychology, such variation reflects individual differences in executive functions, a collection of correlated but separable control processes that regulate lower-level cognitive processes to shape complex performance. Although there is no clear consensus yet on how best to define or conceptualize executive functions, they are considered key mechanisms in many models of normal and abnormal cognition, such as cognitive development (e.g., Lyon & Krasnegor, 1996;Zelazo, Carter, Reznick, & Frye, 1997), agerelated decline in cognitive abilities (e.g., Hasher, Zacks, & May, 1999;Lowe & Rabbitt, 1997), and disorders such as ADHD (Barkley, 1997;Nigg, 2006), autism (Russell, 1997), schizophrenia (Frith, 1992), and substance use problems (Garavan & Stout, 2005).Address correspondence to: Naomi Friedman, Institute for Behavioral Genetics, 447 UCB, University of Colorado, Boulder, CO 80309, Email: naomi.friedman@colorado.edu, NIH Public Access NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptDespite the centrality of executive functions in current psychological research, little is known about the sources of normal individual differences in executive functions. Why do people vary in their executive control abilities? We approached this question from a behavioral genetic perspective, which elucidates the etiology of individual differences by providing estimates of the extent to which they are due to genetic and environmental...

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Accumulating evidence suggests that executive functions (EFs) are related to intelligence, despite neuropsychological results initially considered evidence of no such relation. However, findings that EFs are not unitary raise the issue of how intelligence relates to different EFs. This study examined the relations of fluid and crystallized intelligence and Wechsler Adult Intelligence Scale IQ to three separable EFs--inhibiting prepotent responses (inhibiting), shifting mental sets (shifting), and updating working memory (updating)--in young adults. Updating was highly correlated with the intelligence measures, but inhibiting and shifting were not. Furthermore, in structural equation models controlling for the inter-EF correlations, updating remained strongly related to intelligence, but the relations of inhibiting and shifting to intelligence were small and not significant. The results indicate that intelligence measures differentially relate to these three EFs, suggesting that current intelligence measures do not equally assess a wide range of executive control abilities likely required for many "intelligent" behaviors.

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Conflict of interest statement: No conflicts declared.Freely available online through the PNAS open access option.

Although common sense suggests that environmental influences increasingly account for individual differences in behavior as experiences accumulate during the course of life, this hypothesis has not previously been tested, in part because of the large sample sizes needed for an adequately powered analysis. Here we show for general cognitive ability that, to the contrary, genetic influence increases with age. The heritability of general cognitive ability increases significantly and linearly from 41% in childhood (9 years) to 55% in adolescence (12 years) and to 66% in young adulthood (17 years) in a sample of 11 000 pairs of twins from four countries, a larger sample than all previous studies combined. In addition to its far-reaching implications for neuroscience and molecular genetics, this finding suggests new ways of thinking about the interface between nature and nurture during the school years. Why, despite life's 'slings and arrows of outrageous fortune', do genetically driven differences increasingly account for differences in general cognitive ability? We suggest that the answer lies with genotype-environment correlation: as children grow up, they increasingly select, modify and even create their own experiences in part based on their genetic propensities.

In the context of current concerns about replication in psychological science, we describe 10 findings from behavioral genetic research that have replicated robustly. These are "big" findings, both in terms of effect size and potential impact on psychological science, such as linearly increasing heritability of intelligence from infancy (20%) through adulthood (60%). Four of our top 10 findings involve the environment, discoveries that could have been found only with genetically sensitive research designs. We also consider reasons specific to behavioral genetics that might explain why these findings replicate.