May 31, 2008

New twin study finding on non-IQ cognition

From Science Blogs:

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

[ BPR , Cognitive Neuroscience ]
Posted on: May 13, 2008 10:54 AM, by Chris Chatham

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). …

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). [More]

My published articles are archived at -- Steve Sailer


Anonymous said...

"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)."

50-70% heritability for IQ may be too low.

If environmental factors expained, say, 35% of IQ variance, would not black children adopted by intelligent white couples have higher adult IQs than blacks raised by blacks?

Anonymous said...

Steve, doesn't the massive heritability of executive function show how large the influence of sexual selection is, driven by culture, in human evolution?

And what are the implications for a massive cultural shift in the West across all races for less executive function and more social dominance among men by women?

Anonymous said...

I have a hard time believing that differences in executive function are 99% heritable; the 86% figure is probably closer to the mark. For emample, what if what one identical twin gets exposure to meth or coke and the other doesn't (yes addiction seems to be substantially heritable, but not anywhere near 99%). Also, the methodology of extropolating directly from fraternal twins to identical is problematic; for example, identical twins will share 100% of epistatic and dominant variance as well as 100% of additive variance; fraternal twins share 50% of additive variance but considerbly less than 50% of epistatic/dominance variance (possibly zero).

On a different note, executive function seems like something that is highly important to survival and success in life. My guess is that many people who are diagnosed as bipolar, ADHD, schizoaffective, and others have executive function problems. Unfortunately there is no psychiatric diagnosis for "poor executive function" per se. Many of the traits connected to executive function seem to be connected to "character" which is an essentially useless (or worse) term in chemistry, biology, psychology, and sociology. In fact one goal of the natural and social sciences should be to abolish the idea of character, replace it with science and ultimately, practical solutions. The homeless and mentally ill (for example) will ultimately benefit more from advancements in biotechnology than attacks on either their character or the character of "the rich," "the overprivileged" etc.

Anonymous said...

Wha? Mental anything is heritable? That doesn't make me feel good. It makes me feel like calling you names.

- Prototypical response of the deeply-American do-goodin' PC better-than-thou-in-every-way dolt

Brett said...

"If environmental factors expained, say, 35% of IQ variance, would not black children adopted by intelligent white couples have higher adult IQs than blacks raised by blacks?"

Not if the most important environmental factors were prenatal.

Anonymous said...

At first glance, doesn't this lend weight to my mantra that IQ is probably a smaller piece of the "black failure" pie than many race-realists would like to admit?

Anonymous said...

Does executive function correlate with extraversion? It seems like a very good trait for being an "alpha male", but possibly harmful for someone lower down the hierarchy.

But the 99% heritability seems almost impossible.

Anonymous said...

I want to see the eventual reactions of people like James Heckman if these findings hold up (even "mostly hold up"). For years Heckman has been waving away correlations between IQ and economic/social performance/status, preferring to discuss the influence of "executive function" which he thinks is very malleable and pretty much reflects youthful environment:

"Enriched early intervention programs targeted to disadvantaged children have had their biggest effect on noncognitive skills: motivation, self-control and time preference. We know that there's a scientific basis for this finding. The prefrontal cortex, which is a center of these noncognitive skills, matures late. The executive function, the very definition of ourselves as people, the way we motivate ourselves, these things are malleable until quite late stages—into the 20s, according to research by neuroscientists. This means that in principle we can modify these behaviors. Noncognitive skills are powerfully predictive of a number of socioeconomic measures (crime, teenage pregnancy, education and the like) as I show in a recent paper with Jora Stixrud and Sergio Urzua."

If executive-function, or some clear maximum- potential- for- executive- function, is strongly genetic and if there are large ethnically-correlated variations to be found, then that will give Heckman and friends something to think about.

The tactic of responding to IQ-related studies which provoke questions of inherited ability by changing the subject to "executive function" won't be so popular if executive function appears to be as much or more of an inherited ability as IQ.

Anonymous said...

Oh dear, oh dear. I know I'm a machine, but must I be reminded of it? I try my best to forget, but the scientists, especially the Darwiniacs, always insist. My fate is my DNA. (Then these same scientists go on and use purposive language and such-like, and I simply cannot fathom the origin of this purposiveness, for they are but machines.)

Anonymous said...


I had a long response that got lost in cyberspace. Damn computer, damn internet, damn blog!

Anyway, I think this is an overextension of the meaning of executive function, perhaps demonstrating the inability of researchers to generate a new term something along the lines emotional maturity that describes the effects of self-control vs impulsivity on task completion. I don't like extrapolations based on meanings of executive function that combine the fields of cognitive psychology, neuropsych and emotion-motivation.

When discussing executive system in the context of working memory and ability to focus on a task, I think a study could easily demonstrate the genetic basis but this study may mingle too many issues that should be kept separate. Since the twins in the study were reared together, there's no control for the effects of upbringing on what I see as emotional maturity - at least in the definition of executive function from Wikipedia.

Of course if we knew exactly what the twins were doing when they inhibited, updated, changed focus or shifted it might help determine how the researchers defined executive function.

With nothing else to go on, however, I'm put in mind of another study in which a "scientist" determined that liberals were smarter than conservatives because they supposedly responded faster to a change in pattern of letters presented on a computer monitor than did their dinosaur like conservative peers.

Anonymous said...

anon. said

I know I'm a machine, but must I be reminded of it?

You aren't a machine, but you aren't an infinitely malleable spirit soaring limitlessly in the universe, either.

You have a body. It doesn't stop at the neck. Sorry. Don't fight it, work with it.