r.i.p. blank slate (and what does “the environment” mean anyway?)

so the mother of all meta-analyses of twin studies and the heritability of human traits was published the other day: “Meta-analysis of the heritability of human traits based on fifty years of twin studies” [pdf].

the authors looked at “17,804 traits from 2,748 publications including 14,558,903 partly dependent twin pairs, virtually all published twin studies of complex traits.” 14.5+ MILLION twin pairs! as james thompson said, this study pretty much represents “the mother of ‘F*** Off’ samples.” (~_^) in future, if someone says to you that twin studies were debunked a long time ago, blah, blah, blah, just point them to this paper.

and the upshot is: we are not blank slates. we never were.

from the paper, “[A]cross all traits the reported heritability is 49%.” in other words, these researchers found that pretty much half of the variance in all sorts of physical and behavioral traits in humans — the differences that we see between people — can be accounted for by genetics.

here’s a key table from the paper. i took the liberty of jiggling it around a bit so it would fit better on the blog (h2 is what you should be looking at here — that’s narrow sense heritabilty):

twin studies meta-analysis

the press has picked this up as there being an even split between nature and nurture, genes versus “the environment.” here, for example, from the huffington post*:

“Nature Or Nurture? The Long-Running Debate May Finally Be Settled”

“It’s an age-old debate: do our genes make us who we are, or is it the environment in which we were raised?

“There’s long been agreement that both ‘nature’ and ‘nurture’ play some role in determining many aspects of our physical and mental selves, from our height and weight to our intelligence and disposition. But as to which plays the bigger role in shaping us, scientists have never seemed to agree.

“That debate may now be over, thanks to a sweeping analysis of studies conducted around the world for more than five decades. The analysis — involving more than 14.5 million twin pairs from 39 countries — indicates that nature and nurture are virtually tied.

“Across all of our traits, in other words, genes and environment exert equal influence.”

yeeeessss…but what is “the environment”? on hearing that most people will think of things like reading bedtime stories to kids or playing mozart to your unborn fetus. but those sorts of things are decidedly not what the environment is in this context. from kevin mitchell of wiring the brain:

i’ve blogged this before, but i think we all need to read it again — from steven pinker on “the environment”:

“Even the technical sense of ‘environment’ used in quantitative behavioral genetics is perversely confusing. Now, there is nothing wrong with partitioning phenotypic variance into components that correlate with genetic variation (heritability) and with variation among families (‘shared environment’). The problem comes from the so-called ‘nonshared’ or ‘unique environmental influences.’ This consists of all the variance that is attributable neither to genetic nor familiar variation. In most studies, it’s calculated as 1 – (heritability + shared environment). Practically, you can think of it as the differences between identical twins who grow up in the same home. They share their genes, parents, older and younger siblings, home, school, peers, and neighborhood. So what could make them different? Under the assumption that behavior is a product of genes plus environment, it must be something in the environment of one that is not in the environment of the other.

But this category really should be called ‘miscellaneous/unknown,’ because it has nothing necessarily to do with any measurable aspect of the environment, such as one sibling getting the top bunk bed and the other the bottom, or a parent unpredictably favoring one child, or one sibling getting chased by a dog, coming down with a virus, or being favored by a teacher. These influences are purely conjectural, and studies looking for them have failed to find them. The alternative is that this component actually consists of the effects of chance – new mutations, quirky prenatal effects, noise in brain development, and events in life with unpredictable effects.

“Stochastic effects in development are increasingly being recognized by epidemiologists, frustrated by such recalcitrant phenomena such as nonagenarian pack-a-day smokers and identical twins discordant for schizophrenia, homosexuality, and disease outcomes. They are increasingly forced to acknowledge that God plays dice with our traits. Developmental biologists have come to similar conclusions. The bad habit of assuming that anything not classically genetic must be ‘environmental’ has blinkered behavioral geneticists (and those who interpret their findings) into the fool’s errand of looking for environmental effects for what may be randomness in developmental processes.”

and more from kevin mitchell:

“Just because some trait is not genetic does not mean it is not innate. If we are talking about how the brain gets wired, any number of prenatal environmental factors are known to have large effects. More interestingly, however, and probably a greater source of variance across the population, is intrinsic developmental variation. Wiring the brain is a highly complex procedure, reliant on cellular processes that are, in engineering terms, inherently ‘noisy’. Running the programme from the same starting point (a specific genotype) does not generate exactly the same output (the phenotype) every time. The effects of this noise are readily apparent at the anatomical level, when examining the impact of specific mutations, for example. In many cases, the phenotypic consequences are quite variable between genetically identical organisms, or even on two sides of the same brain. (If you want to see direct evidence of such developmental variation, take a directly face-on photograph of yourself, cut it in half and make mirror-image copies of the left and right sides. You will be amazed how different the two resultant faces are).

If the way the brain is wired is determined, not just by the starting genotype, but, to a large extent by chance events during development, then it is reasonable to expect this variation to be manifest in many psychological traits. Such traits may thus be far more innate than behavioural genetics studies alone would suggest.

in other words, it’s NOT genes + environment (or nature + nurture) — not as most people would think of it anyway. it’s genes + shared environment (which, since it’s shared, i.e. the same for the individuals in question, oughtn’t to make a difference, right?) + nonshared environment (which can include de novo mutations and development noise, which also may be heritable! iow, variation itself might be a genetic trait.). not much room for the effects of nurture here.

so, when you see a figure like 51% for “environmental” causes behind the differences we see in traits between people, remember that that very much includes biological causes like new mutations that are particular to individuals and developmental “noise,” which again may ultimately be regulated by genes.

(and, no, it’s not epigenetics either! see here and here and here.)

h/t once again to jayman for cluing me in on this in the first place! (^_^)

*to give credit where credit is due, the huff post journalist did mention that part of what’s included in “the environment” is measurement error. that is correct. edit: see comment below about measurement error. so the 49% heritabilty figure should be considered a very conservative figure.

p.s. – there’s even a dedicated website where you can have a look at all the heritability numbers for yourself. enjoy!

see also: Gone with the Wind from dr. james thompson, Nature, nurture and noise from kevin mitchell, and About Developmental Noise and Environmental Hereditarianism from jayman.

previously: it’s not nature and nurture…

(note: comments do not require an email. the blank slate.)


  1. @jayman – “I left a few words there on this.”

    nice one!

    i’m going to steal that quote of staffan’s and put it here, since it’s so important!:

    “So, what does the ‘new’ research from the 1980s, that is now finally beginning to reach public awareness, tell us about human nature? The most obvious part is that nature is a major factor. This is typically summed up in textbooks in the 50/50 rule, claiming that genes and environment can explain about half of the variance each of things like intelligence, personality, psychopathology etc. Which is easy to remember – but also incorrect. This is due to the fact that there is something called measurement error. Most studies are done in a way that doesn’t distinguish this error from the environmental factor. So it’s 50 percent nature and 50 percent environment plus measurement error. Studies that have managed to minimize measurement error typically yield heritabilities for personality traits and similar characteristics around 70 percent. You also have the fact that some of the traits linked to the most important life outcomes, like intelligence and impulsiveness, have even higher heritabilities, around 0.75-0.80.”

    everybody go read staffan’s post!: The Nurture Enigma – How Does the Environment Influence Human Nature?


  2. Whatever the meta-ratios are for these sources of variation, evolution brought us to this point and it is what it is because it has worked. Evolution does not respond to social advocacy.


  3. Indeed this is a key point. It’s not like in the bad old days of The Bell Curve when Murray said that as environmental differences were eliminated genetical would assume more and more importance. The thing about fate and fortune is that neither one of them is under our control.


  4. “Running the programme from the same starting point (a specific genotype) does not generate exactly the same output (the phenotype) every time.” It seems only yesterday that it was fashionable to refer to that phenomenon as “Chaos Theory”. In fact, it was only yesterday. Butterflies and hurricanes: remember them?


  5. Actually, when I was young we said “parametric sensitivity”, which has its merits: or, may I suggest “fundamental irreproducibility”?


  6. Stray shot: have the blank slateists ever explained why the horse-racing people are so obsessed by stud books, or why farmers will pay such a fortune for bull sperm? Or do they somehow think that we humans are above such animal considerations?


  7. Dog breeding is also widely practiced and frequently regarded as praiseworthy. It is also big business and serves as a surrogate form of offspring eugenics for many in the middle class gentry. In the arena of memetics, organized religion and public education have a similar modality; i.e. desired psychological traits are reinforced and nonconforming traits are discouraged.


  8. @dearieme – “Or do they somehow think that we humans are above such animal considerations?”

    yes. i think for a long time (several decades) the idea has been that the normal rules of evolution did not apply to us (humans). we were above all that. we’re special. we’d developed culture, you see, so natural selection just ceased working on us 50,000 years ago (or whatever it was supposed to be). which really makes no sense at all if you think about it for half a sec.

    and as cochran & harpending and others have pointed out, with the increasing numbers of individuals since the agricultural revolution — and, therefore, the increasing number of mutations — human evolution has actually sped up in the last 10,000 years! (>.<)


  9. we’re special. we’d developed culture

    and as cochran & harpending and others have pointed out, with the increasing numbers of individuals since the agricultural revolution — and, therefore, the increasing number of mutations — human evolution has actually sped up in the last 10,000 years!

    But will we like where the culture and sped up takes us?


  10. The Blank Slate is the greatest scientific debacle whose history will never be written, or at least not accurately. It would simply be too embarrassing for the “men of science” to admit the true magnitude of the disaster, so the details and the identities of the key players on both sides of the debate are gradually being dropped down the memory hole. For all practical purposes, the one man who was more effective and influential than any other in smashing the Blank Slate orthodoxy may as well have never existed. One sees his key hypotheses confirmed in the literature on almost a daily basis without attribution, but it doesn’t count as “heroic vindication” for him. You see, he was a “mere playwright,” who dared to shame the “men of science” with the truth. Their amour propre couldn’t bear the insult. As a result, he is dismissed as “totally and utterly wrong” in a single paragraph of Pinker’s bowdlerized “history” of the Blank Slate for not “thinking right” about, of all things, group selection!

    Want to learn the real history? Then you’ll have to get used to sifting through the source material. It’s out there. It’s just a question of piecing it together. Here’s something for starters. Thumb through your collection of old magazines until you find the February 1972 issue of “Encounter” magazine. It includes an article by Ardrey entitled “4-Dimensional Man.” The article is interesting in its own right, but he mentions in passing a collection of essays on human and animal behavior entitled “Behavior and Evolution,” edited by Roe and Simpson, noting that it is “fundamental reading for any layman reviewing the sudden sweep of the New Biology.” In it one will find the term “evolutionary psychology,” along with an account of what was known at the time about the development of innate behavior, starting with the earliest life forms. It was published in 1958! There is no indication that the authors were worried about bullying or vilification by the contemporary high priests of the Blank Slate. Apparently that only really took off in the 60’s, after the “received wisdom” had been challenged by outsiders like Ardrey whose books and articles were too popular to ignore. Keep digging and you’ll find a lot more that can’t quite be reconciled with the current version of the “history” of the Blank Slate.


  11. The supposed uniqueness of humankind explain why most people continue eat meat and treat it as something natural. Anthropocentrism.

    I already explain why environmental factors really are. ENVIRONMENT. We are genetic accumulations who are Interacting inside a scenario, create by others and maintained by us. Environmental factors are derived by this scenario, the inpact of it among us, in our minds from nano ( impact of a only one individual) to macro-scale( mass immigration, decisions of a tiny hierarchically superior individuals among us). Unique environment factors correlate with real free will combined with dynamics of structure or geopolitic events, basically chaos theory or better, domino theory. Hypothetical example, if US no have a immigration politics ( North Murrica Union, ;-)) i couldn’t immigrate.
    Meritocratic model also have huge impact among us because while some undesirable people ( to the system) will des-selected other people will be selected, with positive potential for their existencial and or reproductif fitness.


  12. @dearieme – “Anyone who thinks humans aren’t really animals hasn’t seen enough newborns.”

    exactly! i think that part of the reason the blank slate has been so widely and *readily* believed in recent years is because too many people have been having just one or two kids. innate natures *much* more apparent when you have three or four or (lord help us!) ten children.


  13. @john – “could someone please comment on the contribution of copy number variation in such heritability”

    well, there you’re back to de novo mutations, no? which would fall under “the environment” — but obviously would be a biological cause of any differences.


  14. @cracker – “But will we like where the culture and sped up takes us?”

    that’s a different sort of question altogether. some might say yes, some will say no.


  15. For anyone reading who doesn’t know how the Blank Slate con trick came about it was invented as a political weapon in the debate over US immigration policy in the 1920s.

    Such a small thing but responsible for so much.


  16. @Wanderer

    I don’t think it was quite that simple. A good article on what the advent of the Blank Slate looked like to an intelligent observer in the 20’s appeared in the February 1924 issue of H. L. Mencken’s “American Mercury:”


    The author, H. M. Parshley, was a Mencken regular who later translated Simone de Beauvoir’s “The Second Sex” into English. Search his name along with that of Clarence Darrow of “Inherit the Wind” fame, and you’ll find they carried on an interesting debate on eugenics in the Mercury at about the same time. Mencken was a superb editor who also published rejoinders to Parshley by Boas and others. Two excellent books on the early years are “in Search of Human Nature,” by Carl Degler, who understood what the Blank Slate really was, and “The Triumph of Evolution,” by Hamilton Cravens, who didn’t. Cravens was a very talented and thorough historian who had managed to bamboozle himself into believing that the darkest days of the Blank Slate orthodoxy were “The Triumph of Evolution.” In spite of that, he managed to dig up a great deal of very interesting source material about the early years.

    If you want to study the history in real depth, I recommend this (with apologies for the advertising):



  17. What are the environmental factors of fish **

    The water temperature, the density of predators, etc.


  18. @Santoculto

    Eating meat IS natural. Enough with the pathologically altruist vegan pseudoscience. Humans are omnivores like chimpanzees, if we weren’t we wouldn’t have any instinctive hunger for meat. Human vegetarianism has no basis in biology, it is purely a moralistic stance.


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