99% Genetic? Individual Differences in Executive Function Are Almost Perfectly Heritable

Your ability to control thought and behavior relative to your peers - a set of capacities known as "executive functions" - is almost entirely genetic in origin, according to a newly in-press paper from Friedman et al. Over 560 twins completed tests to measure fundamental components of these executive functions, and the results were analyzed in terms of how similar identical twins performed to one another relative to fraternal twins (all twins in the study were reared together). Astonishingly, the results show that the variance common to all executive functions is correlated roughly twice as much between identical twins as between fraternal twins, and that individual variance in executive function falls directly in line with what would be expected from a perfectly heritable trait.

The components of executive function (as determined through previous latent variable analyses) can be loosely described as inhibition (the ability to resist habit), updating (the ability to quickly change the focus of attention or the contents of working memory), and shifting (the ability to quickly change goals and respond appropriately). Previous work demonstrated distinct patterns of individual performance on the tasks thought to measure each of these capacities, suggesting that they are partially dissociable but also that they contain substantial overlap with one another - thus there is both "unity" and "diversity" to the construct of executive function. This work requires the use of a statistical technique known as latent variable analysis, in which consistent covariation between various measures is extracted into a "latent" or "unobserved" variable which theoretically represents a purer measure of cognitive constructs that is relatively uncontaminated by measurement error. The same tasks and statistical techniques were used in the current study, in addition to a structural equation model known as "ACE" for partitioning variance into genetic, shared environment, and non-shared environmental influences.

This combination of statistical tools is powerful, because it allows the authors to fit the coefficients of a particular equation (the structural model) to predict the performance of individual twins on these tasks. For example, for identical twins, the "A" component refers to genetic influences and must be perfectly correlated between these twins (and thus is set to "1"), whereas among fraternal twins it's set to .5 (since they share only half their genes). The "C" component refers to shared environmental influences, which is also set to "1" for both kinds of twins, since they were all reared in the same household. Finally, the "e" component refers to unshared environmental factors, which by definition are uncorrelated between twins.

Once these structural models are in place, we can examine the extent to which inter-twin correlations differ between identical and fraternal twins. For example, if primarily environmental factors are involved, then correlations may be similar between identical and fraternal twins, and the "C" model parameter will be highly significant in the structural model. If genetic factors are mostly involved, then scores among fraternal twins should be correlated no more than half as strongly as correlations among identical twins, and the "A" parameter will be highly significant in the structural model.

The results from this approach are jaw-dropping: variance shared among each variety of executive function (inhibition, updating, and shifting) is nearly perfectly heritable: the contribution of the "A" component to those correlations is 99%. This heritable variance in the common executive function predicts nearly all of the genetic variance in the inhibition factor, consistent with the idea that those constructs are isomorphic from a heritability standpoint. Second, genetic influences on updating and shifting were roughly half due to the common executive function (43% and 44%, respectively) and half due to unique genetic influences (56% and 42%, respectively). Thus, the overall picture is that executive functions, in both their unity and diversity, are somewhere between 86 to 100% heritable.

Furthermore, Friedman et al. integrated measures of general intelligence ("g", estimated through the WAIS IQ test) and perceptual speed (essentially the speed with which subjects can complete very simple tasks) to show that the genetic contribution to executive function is not completely explained by genetic contributions to those more commonly-studied abilities. This is consistent with previous work showing that IQ is only moderately heritable (with 50-70% of variance explained due to genetic factors, far short of the 99% explained here).

The authors conclude that environmental influences can not be "substantial contributors to the unity and diversity of executive functions" (i.e., the apparent structure of executive functions is due to different patterns of genetic contribution). In fact, this study shows that executive functions are among the most heritable cognitive traits ever discovered - nearly completely genetic in origin.

Yet Friedman et al are careful to point out that these results do not mean that environment simply doesn't matter - environmental influences can change the mean value of any pair of twin's score, but the extent to which two twins share that deviation from the mean is almost entirely attributable to genetic influences.

Of course, there are some limitations to the study:

- Gene-environment interactions are not accounted for here; in other words, genes may predispose individuals to seek out certain environments, which would be lumped into estimates of genetic variance in the current study

- Although this was a community-wide sample of twins, environmental influences might appear artificially small in samples which come from the same SES. So these estimates might differ if conducted in third-world countries, where environmental influences might curtail the full expression of genetic variance in executive functions

- Although it's possible that identical and fraternal twins do differ slightly in the degree to which they share environments (e.g., if identical twins have their similarity exaggerated by dressing similarly, and if fraternal twins seek to differentiate each other), scientific estimates of these factors seem to have minimal influence on heritability estimates

One important future direction for this work is the determination of the specific genes which contribute to executive function and their mechanism of action. Such research could pave the way directly towards more accurate computational modeling and perhaps even direct pharmacological manipulation of executive functions.

Friedman, N. P., Miyake, A., Young, S. E., DeFries, J. C., Corley, R. P., & Hewitt, J. K. (in press). Individual differences in executive functions are almost entirely genetic in origin. Journal of Experimental Psychology: General.