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).
Modeling 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.
- 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