When genes matter for intelligence

By Razib Khan | January 12, 2011 3:01 am

Image credit: Aleksandra Pospiech

One of the interesting and robust nuggets from behavior genetics is that heritability of psychological traits increases as one ages. Imagine for example you have a cohort of individuals you follow over their lives. At the age of 1 the heritability of I.Q. may be ~20%. This means that ~20% of the variation in the population of I.Q. explained by variation in the genes of the population. More concretely, you would only expect a weak parent-offspring correlation in I.Q. in this sample. At the age of 10 the heritability of I.Q. in the same sample may be ~40%, and in mature adulthood it may rise to ~80% (those are real numbers which I’ve borrowed from Robert Plomin). Many people find this result rather counterintuitive. How can a trait like intelligence become “more genetic”?

Remember that I’m talking about heritability here, not an ineffable “more” or “less” quantum of “genetic” aspect of a trait. In other words: does variation in genes due to different parental backgrounds matter for a trait? Second, the nature of psychological traits is somewhat slippery and plastic. As I’ve noted before the correlation between a score on a 10-world vocabulary test and general intelligence is pretty good. You can expect people with high scores on the vocabulary test to have higher I.Q.’s than those who have low scores. But if you take an individual and lock them in a room without human contact for their first 15 years, they are unlikely to exhibit any such correspondence. You don’t have to be a rocket scientist to understand why. Quantitative behavior genetic traits are complex and are subject to a host of background conditions, and express themselves in an environmental context.

So why can you explain more of the variance of a psychological trait like I.Q. at age 40 than at age 5 with genes? It has to do with environment. Specifically, intelligence isn’t something you’re born with, it’s something that you develop over time, through a complex confluence between biology and environment. The developmental process exhibits a level of contingency as well. Decision A redounds to the choice between B and C, which redounds between a further set of choices. Small initial differences in disposition and talent can compound over time through positive feedback loops. Practice may make perfect, but perfection may be a goal to which you aspire only if you have initial talent or inclination.

In other words, your genetic disposition can shape the environment you select, which can then serve to express your genetic potential in a specific manner. Children have less power in selection of their environment than adults. Over time the model is that environmental variables which differentiate children diminish in importance as they select contexts and situations which express their own preference sets as adults. This dynamic can be illustrated with a rather strange example. Consider two siblings who are pressured to be academic by their parents. One has a natural disposition toward scholarly activities, while the other does not. Their realized performance difference in youth may be small. People can respond to incentives! But at 18 the two siblings become adults, and begin to make their own decisions. At 25 one sibling may be a university drop out, and the other a graduate student. The modest differences in adolescence may start amplifying due to the positive feedback loops which consist of a set of choices which exhibit dependencies. Of course siblings would tend to be more similar than two random individuals off the street. But even within families there is genetic variance and so innate differences of disposition (the average difference in I.Q. between siblings is about the same as the average difference in I.Q. between two random people off the street, one standard deviation, or 15 points).

ResearchBlogging.orgModeling behavior genetic phenomena in a rough & ready fashion is then a matter of keeping dynamic networks of parameters in your head. Traits aren’t constructed about of static blocks; they’re the outcomes of a set of parameters at a given moment, as well as a developmental arc shaped by a previous set of parameters (some of them the same, some of them new). Thinking like this gives you a method by which to analyze phenomena, it does not tell you in a clear and general manner how a whole range of phenomena emerged down to the last detail.

The analysis doesn’t just apply to populations over time. You can also look to different groups which are contemporary. In 2003 a paper was published, Socioeconomic Status Modifies Heritability of IQ in Young Children. The major findings are illustrated by this figure (I’ve added some clarifying labels):

On the x-axis you see socioeconomic status (SES). This variable is a compound of traits which reflect’s one’s position in the social status hierarchy. Income and wealth are clearly important, but a salesman for a fertilizer company could presumably be more economically well off than a physics professor. So other variables such as education also matter. It is clear then that as SES increases genetic variation explains much more of the variation in I.Q., while environment explains less and less. The shared environment is rather straightforward: your family. The non-shared environment is more vague, and to some extent is just the remainder from the model which predicts I.Q. In The Nurture Assumption Judith Rich Harris posited that non-shared environment was mostly peer group effects. Interestingly, by adulthood non-shared environment tends to be a more important variable than shared environment for most psychological traits.

Any guess for why genetic variance is more efficacious in prediction of I.Q. among the high status than the low status? Here’s a clue: heritability of height is much higher in developed nations than in developing nations. In other words, environment explains more of the variance in height in developing nations, while it explains almost none of the height in developed nations. There’s only so much you can eat, and there are diminishing returns on nutritional inputs. In developed nations most of the environmental variance has been removed due to adequate nutrition. When you remove the environmental variance, the genetic variance remains. Heritability is roughly the ratio of the additive genetic variance over the total variance, so its value gets larger.

The analogy to I.Q. should be relatively easy. Don’t tell Amy Chua, but there are probably diminishing marginal returns on “nurturing” environments for a child when it comes to their intellectual development. You have only a maximum of 24 hours in the day you can study and drill, and a personal library of 10,000 is probably not very different from 1,000, if all the books fall within the purview of your interest. Even in well off suburban communities there are differences of wealth and income, but on the margin vast increases in wealth and income do not allow one’s child to develop their mental faculties proportionality greater. What there remains in well off suburban communities are differences of genetic disposition and aptitude. Bill Gates’ children are probably good candidates for the Ivy League. Not because he is worth billions of dollars in relation to a professional whose net assets barely break a million. Gates got into Harvard, and reputedly did well before dropping out to pursue his business. His wife is also an overachiever.

This is I believe a fascinating topic, and needs to be explored in more detail. Some members of the same group now have a study out which shows that differences in socioeconomic status matter differently for infants at 10 months and tots are 2 years. Emergence of a Gene × Socioeconomic Status Interaction on Infant Mental Ability Between 10 Months and 2 Years:

Recent research in behavioral genetics has found evidence for a Gene × Environment interaction on cognitive ability: Individual differences in cognitive ability among children raised in socioeconomically advantaged homes are primarily due to genes, whereas environmental factors are more influential for children from disadvantaged homes. We investigated the developmental origins of this interaction in a sample of 750 pairs of twins measured on the Bayley Short Form test of infant mental ability, once at age 10 months and again at age 2 years. A Gene × Environment interaction was evident on the longitudinal change in mental ability over the study period. At age 10 months, genes accounted for negligible variation in mental ability across all levels of socioeconomic status (SES). However, genetic influences emerged over the course of development, with larger genetic influences emerging for infants raised in higher-SES homes. At age 2 years, genes accounted for nearly 50% of the variation in mental ability of children raised in high-SES homes, but genes continued to account for negligible variation in mental ability of children raised in low-SES homes.

They used a standard SEM model. I’m not going to go over that in detail, but suffice it to say that they related a set of variables to the outputs which they wanted to predict, performance on I.Q. tests for very young children. If you are curious, the demographic sample was rather diverse, and controlling for race did not impact their outcomes. So let’s outline what’s going on here.

First, predicted:

– Performance at 10 months
– Performance at 2 years

Second, putative predictors:

– Genes (A). Specifically, additive genetic variance
– Shared environment (C)
– Non-shared environment (E)

I’ve reedited some of the main results. On the Y axis you see the % of variance explainable by A, C, and E. The variance components are broken down into two levels: SES, and age. 2 SD means 2 standard deviations. In a normal distribution that’s the ~2% tail at the ends.

What you see are two trends with age and SES:

– For infants at the age of 10 months parents matter. Genes do not. SES is not a major issue.

– For tots at the age of 2 years, SES matters quite a bit. You see a recapitulation with the earlier data, where higher SES parents seem to be providing environments which probably exhibit diminishing marginal returns (environmental variance doesn’t have much of an effect on the margin), so that genetic variance is much more important by default. The trend is clear as you move in a step-wise fashio up the class ladder. Though I have to say, the top ~2% in SES is an elite group already, so I wonder what sort of environmental variance could be found there.

The figure to the left shows the same outcome out of their model, only now the curves illustartes the variation of the effects as you modify SES in a continuous fashion. These are estimates generated out of their model, so that probably explains the > 100% values you see on the margins. The key is to focus on the broad qualitative trends. Even at 2 years of age genes start to trump shared environment ~1 standard deviation above the norm (though not aggregate “environment”). If the earlier data is correct, the heritability will continue to increase over time for higher SES individuals, as their affluent backgrounds will give them perfect freedom to take them where their dispositions lead them.

Why does all this matter? There are practical outcomes to this sort of research. I’ll quite from the paper:

These findings build on a growing body of literature that highlights the importance of early life experiences for cognitive development…Current evidence suggests that, although children maintain a great deal of neurobiological and behavioral plasticity well past infancy…the predictive validity of infant mental ability for later cognitive ability is moderate…We agree with Bornstein and Sigman…who have strongly argued against the perspective “that infancy might play little or no role in determining the eventual cognitive performance of the child and, therefore, that individuals could sustain neglect in infancy if remediation were later made available”…Heckman…has recently taken an economic perspective on this topic. He argued that prophylactic interventions for disadvantaged younger children produce much higher rates of return on what he termed “human skill formation” than later remedial interventions for older children and adults do. On the basis of this perspective, Heckman concluded that “at current levels of funding, we overinvest in most schooling and post-schooling programs and underinvest in preschool programs for disadvantaged persons”….

My understanding is that the long-term effectiveness of even Head Start is non-existent, so I don’t know what proposals could be made based on this. Preschool for 1-2 years? I find it broadly plausible that high SES parents do provide more enriching environments, but I don’t see the detailed understanding necessary for genuinely effective prescriptions. Rather, we’re doing conventional trial & error when it comes to policy.

Additionally, the authors also admit that the high and low SES populations may have been stratified for genes. That’s just a way of saying that it isn’t as if genetic variance for things like intelligence are necessarily equally distributed across the social classes. If a genuine meritocracy exists what one should rapidly see is a crystallization of hereditary class castes, as individuals marry and associate assortatively on a meritocratic basis. Remember, assortative mating should increase heritability estimates (Quantitative Genetics says so!). This is part of the irony of some peoples’ conception of how genes relate to outcomes. Equality of opportunity will almost certainly lead to a cleaner separation of outcomes by genetic variation. In a chaotic world defined by random acts many people will find themselves in positions at variance with their aptitudes or dispositions. Once you remove the environmental randomness, then from each according to their capabilities should be the outcome.

For future investigation: the hypothesis that Goldman Sachs partners are precursors to Guild Navigators!

Citation: Tucker-Drob EM, Rhemtulla M, Harden KP, Turkheimer E, & Fask D (2010). Emergence of a Gene x Socioeconomic Status Interaction on Infant Mental Ability Between 10 Months and 2 Years. Psychological science : a journal of the American Psychological Society / APS PMID: 21169524


Comments (29)

  1. dave chamberlin

    We shall see the outcome of new assortive mating patterns. Here is what I have personally witnessed. In my first week at a state university I had to keep asking myself “Hey there are hardly any hot girls here! Damn there were way more in high school.” I had attended a rich kids’ high school and low and behold rich guys marry good looking women who very often have hot daughters. (who would have thought)
    Move forward a generation and the smartest person in my extended family is now in a doctorate program in mathmatics at UCLA. He met his wife in the doctorate program, when she gets her doctorate I am told she will be the first Iranian female to get her doctorate in mathmatics. I don’t know what difference it will make that people way out there on the left tail of the bell shaped curve are far more likely to choose each other as mates in this generation but it can’t hurt.

  2. Ikram

    The idea that assortive mating is “new” (from dave in the comments) is a North American idea. In India, as in the UK, assortive mating, by class, has always been the norm.

  3. Harold

    Here is something I don’t understand.
    How does this account for genes coding, perhaps, for different development curves of cognitive functions?

  4. RK

    How do these results provide evidence that subsequent remediation for people from low-SES households is ineffective? Any evidence for that would require measurements after the 2 year mark.

  5. [Al, behave yourself]

  6. RK, my post didn’t say that. reread it.

  7. How does this account for genes coding, perhaps, for different development curves of cognitive functions?

    don’t engage in non-sequiturs.

  8. RK

    I know you didn’t say that. But the authors did: “We agree with Bornstein and Sigman . . . who have strongly argued against the perspective ‘that infancy might play little or no role in determining the eventual cognitive performance of the child and, therefore, that individuals could sustain neglect in infancy if remediation were later made available'”

    I don’t have access to the paper right now, so I’m asking what evidence they adduce for the notion that subsequent remediation isn’t adequate. (And thus, the importance of pre-natal and early childhood programs.) Presumably it would have to be evidence beyond these results.

  9. i think u misunderstand them. but read them yourself, un-gated version for the day.

  10. Chris T

    If heritability increases with age; on what basis is there to expect that even early interventions would have an effect? I would think any early gains would eventually get overwhelmed. Creating transient improvements early that have no effect on the vast majority of a person’s life seems like a waste of time and energy.

    If a genuine meritocracy exists what one should rapidly see is a crystallization of hereditary class castes, as individuals marry and associate assortatively on a meritocratic basis.

    This is the major problem with using income mobility (ie: Gini Coefficient) as a proxy for measuring meritocracy. Those who use the data implicitly assume that a meritocratic society should have high income mobility, when in fact the opposite is true if the influence of heritability is high!

  11. RK,

    There is considerable evidence that educational achievement gaps are very persistent and extremely difficult to remediate.

    For example, in Colorado, only 14% of students who are behind grade level in math will catch up to grade level in three years, and in the meantime more students who were at grade level will fall behind.

    Achievement gaps tend to widen slightly, not narrow over the course of an educational career, but the lion’s share of all of the observed achievement gaps between socioeconomic and ethnic groups are already present by third grade. As one newspaper columnist puts it: “By [age] 5, it is possible to predict, with depressing accuracy, who will complete high school and college and who won’t.” (Relying on this study.)

    There also is evidence that quality pre-school programs do have stunningly large long term benefits, and that vocabulary gaps even at age four are very hard to narrow. The vocabulary gaps in turn are associated with very large (factor of three) differences in how much verbal communication there is between parents and babies up to that point between high SES and low SES familes.

    This doesn’t, per se, say that it is impossible to remedy early childhood deficits, but it does make the case that it is very hard to do so. Countless studies have attempted to identify effective ways to catch students up, and millions of teachers devote immense efforts to doing so, with pretty stunningly disappointing results. Schools that have had success more than intermittently and non-reproducibly have only done so with student and teacher effort dramatically in excess of that associated with what is necessary to keep students on track if the stay at grade level.

  12. Chris T

    One possible major confound in the preschool study: was participation random or did participants’ parents volunteer them? This is the same issue with many charter schools.

  13. dan

    you’re saying that the adult variance in IQ (in high SES people) is due to the environment sought by the individual *and* because genes put the finishing touches on the brain later in life (mid-late 20s), correct? it sounded a little like you were saying it’s mainly due to chosen environment and I’ve always got the impression it was mainly due to networks in the brain finally reaching their fullest potential for each person. a track star chooses to be a runner because he’s good at it and he likes it but he also matured (genetically speaking) into a better runner than most. or am i mistaken?

  14. Kiwiguy

    ***Additionally, the authors also admit that the high and low SES populations may have been stratified for genes.***

    Bruce Charlton caused a stir a couple of years ago when he gave this explanation for Oxford & Cambridge not admitting enough students from working class backgrounds.


  15. vnv

    Bruce Charlton caused a stir a couple of years ago when he gave this explanation for Oxford & Cambridge not admitting enough students from working class backgrounds

    At least in that Daily Mail article, Bruce Charlton doesn’t appear to be making any claims about genetic factors. He merely asserts that IQ increases with SES and there’s no mention of him offering reasons for the correlation.

  16. JL

    Plomin et al. did a study that found that the heritability of verbal ability was actually higher in low-SES environments, and there are some other studies that failed to find a correlation between SES and heritability. (The Plomin study is briefly mentioned by Turkheimer et al.) The IQ-SES-heritability nexus does not seem to be very robust.

    My understanding is that the long-term effectiveness of even Head Start is non-existent, so I don’t know what proposals could be made based on this.

    Early childhood interventions may not influence cognitive skills, but that does not mean they cannot have beneficial non-cognitive effects. This is what Heckman has argued. A quote from him:

    Head Start was deemed a failure in the 1960s because it did not raise the intelligence quotients (IQs) of its participants (12). Such judgments are common but miss the
    larger picture. Consider the Perry Preschool Program (13), a 2-year experimental intervention for disadvantaged African-American children initially ages 3 to 4 that involved morning programs at school and afternoon visits by the teacher to the child’s home. The Perry intervention group had IQ scores no higher than the control group by
    age 10. Yet, the Perry treatment children had higher achievement test scores than the control children because they were more motivated to learn. In followups to age 40, the treated group had higher rates of high school graduation, higher salaries, higher percentages of home ownership, lower rates of receipt of welfare assistance as adults,
    fewer out-of-wedlock births, and fewer arrests than the controls (13).

  17. Konkvistador

    “If a genuine meritocracy exists what one should rapidly see is a crystallization of hereditary class castes, as individuals marry and associate assortatively on a meritocratic basis. ”

    Wasn’t this basically the meat of Murray’s The Bell Curve? I know its most famous for its treatment of the racial gaps but when I read it those bits honestly didn’t really stand out, let alone seem in retrospect the best reason to read it.

    Also doesn’t associative mating mean that over time the relative penalties for nepotism versus meritocracy in say a family company become smaller? (I’m not talking about outside sanctions here, I’m talking about penalties in the form of for example reduced competitiveness with other companies who hire in a meritocratic)

  18. The most fashionable explanation for IQ gaps in recent years has been that upper middle class mothers are much more articulate and upbeat with their children, while underclass mothers tend to be taciturn and verbally unhelpful.

    That would seem to imply that upper middle class mothers should avoid hiring poor women as nannies and instead should stay home and spend as much time as possible with their children. But that corollary doesn’t seem to be as fashionable.

  19. Bob

    Trying to get my head around “Heritability,” a surprisingly subtle concept, at least for me. Consider:

    – “Number of livers at birth.” Heritability = 0 (or, perhaps undefined?), since the variation in the population is zero. Everyone has exactly one liver.

    – “Number of toes at birth.” Some people (apparently, 1 in every ~500 live births) are born with 6 toes on each foot, so there *is* variation, although it’s quite rare. Heritability = 100%?

    Sorry. I’m intentionally trying to find edge cases to clarify my understanding.


  20. Steve, I’d say relative to many other countries the use of nannies to substitute for motherly duties is rather unfashionable. We have less aristocratic tradition, and even engage in conspicuous production.

  21. Jason Malloy

    “The IQ-SES-heritability nexus does not seem to be very robust.”

    Right, thank you. That same Turkheimer paper gets referenced over and over again, because it reported the “wanted” conclusion and it was widely referenced in the media. In the academic literature Google Scholar gives it 385 citations.

    For comparison, a similar paper came out about the same time using another good data set (Hawaiian Family Study of Cognition). It did not find the GxE, and so not one media outlet talked about. Google Scholar says it only has 8 citations.

    See also.

  22. Jason Malloy

    Heckman relies heavily on the Perry Preschool Project — a rather small study — to make his policy recommendations that preschool interventions have big non-cognitive returns in adulthood.

    If we give any sort of weight to other kinds of relevant experiments it becomes much more suspect that dramatic early life benefits really do provide these kinds of returns in adulthood.

  23. Kiwiguy


    You’re right, that article is more about the controversy that caused. On his blogsite he mentions that iq is substantially heritable (although he doesn’t mention this varies with SES).

    David C Rowe had a paper in the 90’s trying to tease out how SES position reflected gene & environmental variation.

    “Our study concluded that a meaningful part of social inequality was attributable to both genetic influences and to environmental family backgrounds, but that the greater part was attributable to the genetic effects. This does not mean that people inherit educational attainments or educations directly from their parents. Rather, heritable traits lead people via g – e correlations into and out of different social opportunities. different social categories differ genetically (for education, primarily in those genes related to IQ, for income, primarily in those genes related to non-intellectual traits).”


    ***That would seem to imply that upper middle class mothers should avoid hiring poor women as nannies and instead should stay home and spend as much time as possible with their children. But that corollary doesn’t seem to be as fashionable.***

    Heh. My wife and I are currently looking at the pros/cons of daycare vs nannies. I’ll have to ensure the caregivers are suitably middle class.

  24. JL

    Jason, the Sacerdote adoption study is fascinating, but the adoptees in it were much younger than the biological kids (IIRC, the mean age gap was nine years). Could the lack of effect of parental SES on adoptee outcomes be, to some extent at least, due to the fact that the adoptees were so young (mid-20s on average, I think) that they were just starting their careers at the time of the study and therefore did not have many income differences?

  25. The bigger nature-nuture issue in adoption comparisons, it would seem to me, is that a large share of modern non-foreign adoptions by strangers don’t involve blank slate infants. Unwanted pregnancies results in births in the United States are at an all time low, and are particularly rare in white and Asian American women, and lots of wanna be adoptive parents aren’t willing to adopt black and Latino babies who make up a disproportionate share of the “blank slate infants” who are available for adoption.

    Instead, adoptions often involve young children (often late pre-school or kindergarten age or older by the time the adoption is final) whose parents had their parental rights terminated for abuse or neglect. It typically takes a few years for the horrible home environment to be discovered, for court proceedings to take place and for the children to then be made available for adoption and adopted. (Beyond a certain age, a life of foster parents and with distant relatives, and/or premature emancipation becomes much more common. Also, abuse and neglect rates fall dramatically once kids reach school age.)

    If there really is a magic early childhood window where nuture is maximally important, most adopted kids, and in particular, most white and Asian American adopted kids, by definition, blew it. Thus, adoption studies are probably assuming shared environments in situations where the environments that really matter aren’t shared, and as a result, underestimating the impact of environment and overestimating the importance of heredity generally.

  26. Emma

    Heritability is a ratio, thus with annoying statistical properties. I was wondering if IQ heritability increases with age because the genetic variance also increase with age or just because the shared environmental variance decreases as the influence of one’s familly becomes less important.

    It would be interesting to know if IQ variance decreases with age, but I guess it is difficult to determine, because of the standardization of the IQ tests.

  27. miko

    Although tangential to this really great article, I agree strongly with JL that there is no reason to suppose that raising IQ should be the goal (or maybe even a goal, since we don’t really know much about the relationship between educational “environment” and IQ) of education programs. One purpose certainly is to teach cognitive skills, but also social ones, as well as to inform, indoctrinate, acculturate, and to improve the likelihood that people will be able to contribute to and function in society. If you are improving in these areas, I don’t really see the point of even measuring IQ, let alone allowing it to define success (from the social outcomes perspective–it’s still an interesting biological question, although a bad experiment).

    More technically, “shared environment” measurements are not really what they sound like. We have no biological framework for the ontogeny of IQ or the mechanism of environmental influence, so a given factor falls into the “shared” category if it contributes to residual similarity and “nonshared” if residual dissimilarity after sorting out H. This does not fit the colloquial conception of a “shared” environmental factor since it is purely operational and not based on the features of or exposure to the factor. If 2 individuals respond oppositely to an identical factor (e.g. a parental divorce, pizza on Thursdays), this falls under the “nonshared” environment, which is non-intuitive to say the least. Turkheimer calls these conceptions of shared/nonshared objective and effective. Clearly, the effective construction (which is the easily quantified one), is semantically circular, bordering on meaningless.

  28. Changing an ethnicity’s culture can happen. For example, there are lots of movies these days about Irish-Americans Behaving Badly in Massachusetts slums, like “The Town” and “The Fighter,” in part because underclass Irish are pretty exotic these days. They didn’t used to be.

  29. Chuck

    So, in the normal range, how much of the increasing heritability of IQ (g) is due to age dependent gene expression and how much is due to age dependent g x e interactions — specifically of the g (-enetic disposition) x self selecting environment type? I wouldn’t rule the former out.

    Whatever the case, g x ses interactions don’t imply g x e interactions by way of self selecting behavior. It’s one thing to say that environmental factors such as nutrition interact with genes during the developmental process and influences cognitive development; it’s quite another to say that self-selected cognitive environments interact in the same manner; and still another to say that cognitive environments, selected because of dispositions, lead to the changes. If that makes sense.

    “The analysis doesn’t just apply to populations over time. You can also look to different groups which are contemporary. In 2003 a paper was published, Socioeconomic Status Modifies Heritability of IQ in Young Children.”

    As I see it, there are several different issues here:
    1) What causes the Flynn effect? (And does the Flynn effect represent a real rise in g?)
    2) What causes the rise in the heritability of g across the developmental process?
    3) By what mechanism does the SES x g interaction work?


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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com


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