Some Poetry about Behavioural Genetics

“He’s fucked up, yes, but not by us”

There is a famous finding from the field of behavioural genetics that parenting explains almost nothing about adult life outcomes. When we look at intelligence, personality, health, religiosity, likelihood to commit a crime, and many other life outcomes, what household you grew up in explains little to none of the variance, above and beyond the effect of genes.

This sounds insane before you give the necessary caveats. First, this result only claims that parenting (or rather, shared environment¹) doesn’t have systematic effects. Your father may have instilled a love of exercise in you, but enough children would have had the opposite reaction that, on aggregate, parenting doesn’t affect mortality once you’re an adult (to give one example). Second, the effects are not literally zero, and the samples in these studies don’t include many individuals from abusive or highly deprived situations. Third, the effects from parenting are often what you might call ‘nominal’. The children of lawyers are more likely to be lawyers, and the children of Christians are more likely to be Christians, but if you look at more fundamental characteristics, like how important one considers religion to be in their life, then you’re more likely to see a small or null effect from parenting. Fourth, how you treat your children will affect how happy they are now, and, presumably, good parenting is justified for its own sake. As has been remarked before, only a newlywed would be so naive as to think they could mould the personality of their spouse in their image — it would be weird to conclude that it’s pointless to love your husband or wife.

The gold standard for assessing the effects of parenting is a twin study. The long-run average difference between twins raised together and twins raised apart is the effect of shared environment, of which parenting is a subset (‘shared environment’ is the environment common to siblings that grow up in the same household). You would be correct to point out ‘twins separated at birth’ is a tiny sample size. But, these people are so important to science that psychologist have looked very hard to find them. You can also use non-identical twins, of which there is a larger number. Finally, you can look at adopted children, and see whether their life outcomes are closer to those of their biological parents or their adoptive parents. In all of these research designs, shared environment has a surprisingly tiny effect.

This line of reasoning was first popularised by Judith Rich Harris in the Nurture Assumption, and it was first brought to my attention by chapter 19 of the Blank Slate by Steven Pinker, which I recommend.

Some people are depressed by the implications of this research; others are overjoyed. In Selfish Reasons to Have More Kids, Bryan Caplan argues that, because we know that fiddling around the margins of parenting doesn’t have an effect anyway, you may as well ‘take a chill pill’ and stop doing the stuff you don’t like doing for the supposed benefit of your child.  

In any case, it’s become obligatory when responding to Judith Harris to quote from the poem ‘This Be the Verse’ by Philip Larkin:

They fuck you up, your mum and dad.   

They may not mean to, but they do.   

They fill you with the faults they had

And add some extra, just for you.

But they were fucked up in their turn

By fools in old-style hats and coats,   

Who half the time were soppy-stern

And half at one another’s throats.

Man hands on misery to man.

It deepens like a coastal shelf.

Get out as early as you can,

And don’t have any kids yourself.

In the Nurture Assumption, she gives a witty reply:

How sharper than a serpent’s tooth 

To hear your child make such a fuss. 

It isn’t fair—it’s not the truth— 

He’s fucked up, yes, but not by us

To this discourse, I will add a poem of my own. One of the uncanny implications of behavioural genetics is that identical twins can be strikingly similar through genes alone, thus:

There once was a fellow named Finn

Separated at birth from his twin

He once met his brother

Whence he discovered

They’re both dentists who play violin

1. Eric Turkheimer’s classic paper ‘Three Laws of Behavior Genetics and What They Mean’ urges some caution on this point (the three laws are “all human behavioral traits are heritable” [sic], “The effect of being raised in the same family is smaller than the effect of genes”, and “A substantial portion of the variation in complex human behavioral traits is not accounted for by the effects of genes or families”). He writes: “The apparent victory of nature over nurture suggested by the first two laws is… seen to be more methodological than substantive.” I gather that the concern is that determining how “similar” one environment is to another borders on meaningless. How similar was my upbringing to that of my sister, and how similar was this to the home in Iceland where my fictitious twin was raised? How similar would my twin be if he were raised on the planet Vulcan? It’s unclear how we interpret twin studies as making a causal statement about development.