Heredity blind spots

Posted by – May 20, 2017

One of the most conspicuous areas of ignorance is genetics and heredity, particularly with reference to humans. There’s sort of a blind spot about it, where people just don’t put much thought into it and attribute a lot to randomness. For example, I have more than once had to explain why it’s obvious that Michael Jackson’s children aren’t his biological children (they don’t look half-black). Here’s some concepts that have helped me make sense of things.

Firstly, regression to the mean. This is something that happens with polygenic (lots of different alleles have an effect) traits, where the particular combination of recessive and dominant alleles matter.

For example, you might expect that when two tall people have children, the children get “tallness” genes from both of them, and as a result the children are super-tall. But that’s not what happens. The children on average will be shorter than the average of the parents. That’s because each child gets a random selection of alleles from each parent, who happen to have a combination leading to tallness, and on average the children will have a more… average combination. Their height is predicted to be between the average of the whole population and the average of the parents. Likewise, children of short parents are expected to be somewhat taller.

This leads to interesting family patterns, like intelligent parents often having less intelligent children (than themselves), and dim parents often having smarter children. How many stories could be summarized as “child disappoints parents and rebels” or “child outgrows environment and feels shame”?

Also, there’s assortative mating, meaning that similar people end up together. This causes more dim-dim couples and smart-smart couples than you’d expect by random chance. This accentuates the mismatched parent-children -effect.

The second thing is the finding that everything is heritable. I already implied that intelligence is heritable, which is something that people will dispute (along with the idea that there is such a thing as intelligence). But there is really practically nothing physical or mental about people that isn’t heritable. Not just eye color and height, but obesity and muscularity. Not just intelligence and schizophrenia risk, but sociability, professional success, areas of interest and criminality. Really, look in the literature, it’s shocking just how subtle and pervasive the effect of genes can be.

Note that the more complicated a trait is to measure, the more inaccurate the measure is, and the less we can judge heritability on an individual level. Indeed, many conclusions about heritability are too weak to worry about on the individual level but become meaningful when measuring masses of people, where random variation washes out.

The third is that heritable traits are often correlated with each other in clusters. The stories we tell children often suggest that traits are anticorrelated in the sense that “everyone is good at something”; if you’re not good at something, there’ll be something else you’re the best at. The stories we tell to adults suggest that traits are fairly uncorrelated, that you never know what you’re good at. The reality is that lots of things go together in clumps. Different kinds of intelligence predict each other – high verbal intelligence is correlated with logical reasoning ability, and both are correlated with spatial awareness. In intelligence, this general correlation factor is called “g” and is the reason for the predictive power of IQ tests (it doesn’t matter if the test isn’t perfect, because it measures all intelligence by proxy anyway).

This has led to economists half-seriously suggesting that height should be taxed, because that too is correlated with all the other positively perceived traits, especially among men.

The fourth is that heritability increases with age. Many childhood interventions to improve outcomes appear promising during childhood, become unclear in adolescence and disappear completely in adulthood. The effect of genes is constant and permanent, while the environment fluctuates. Over time, the genetic contribution comes to dominate. Also, the individual comes to select the environment for himself, compounding the effect.

It is often remarked that children become their fathers or mothers as they mature, and I think it might be that that has less to do with psychology and more with this genetic effect. I’ve also wondered whether this is partly behind the tendency of adoptees to eventually become extremely interested in knowing about their biological parents, as they become more and more different than their adoptive environment.

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