[Epistemic status: I am not a behavioral geneticist, I don’t really understand the behavioral genetics papers, and I am fully expecting to have to run a correction on some if not all of this post.]
The conventional wisdom of behavioral genetics, as I understand it, is that approximately fifty percent of variance in personality is genetic, fifty percent is environmental factors not shared between siblings (often inaccurately rendered as “schools and peers”), and zero percent is environmental factors shared between siblings (often inaccurately rendered as “parenting”).
However, this argument proves far too much.
Consider lead. Lead is the biodeterminist’s favorite story: we poisoned our children and, right on schedule, they became criminals; we stopped poisoning our children and, once again right on schedule, crime dropped. However, lead varies a ton between neighborhoods: some of the evidence for the lead-crime hypothesis is that criminality is highly correlated with growing up in a lead-full neighborhood. One assumes that siblings generally do grow up in the same neighborhood, so lead exposure is shared environment. Why doesn’t that show up?
Or consider breastfeeding. An study of 14,000 infants suggests that an intervention to increase the rate of breastfeeding caused a remarkable six-point rise in IQ. One would expect that there is not literally zero correlation between whether you breastfeed your first child and whether you breastfeed your second child, since you’d expect women who are incapable of producing milk, have unsupportive spouses or employers, or simply don’t want to breastfeed would usually continue to be in those situations. Again, some of that IQ rise should show up in the data.
It also doesn’t really pass the sniff test. There are lots of ways that other people’s behavior can affect your outcomes. It seems to me to be well-established that people with borderline personality disorder behave in extremely obnoxious ways, unless we are taught certain emotion regulation and mindfulness skills, in which case we are no more unpleasant than anyone else. People who have been assaulted are at much higher risk of PTSD than people who have not been assaulted. A person with ADHD who has access to medication is likely to be more focused and productive than one who has been refused medication. Heck, a good boss inspires people to be more productive, while a bad boss encourages slacking. In theory, all of these ought to show up in shared environment: parents who teach their children emotion regulation skills, assault their children, give their children psychiatric medication, or are terrible bosses are likely to continue to have those traits across multiple children. There is no reason to believe that children are less prone to learning helpful life skills or being inspired. And while it’s possible that the effects don’t last until adulthood– people who weren’t on ADHD medication as children regularly get on it as adults, people may recover from PTSD quickly– you’d still expect flow-through effects. A child who maintained a C average because of their ADHD gets into a worse college and earns less money as an adult– even once they take medication.
The obvious explanation is that twin studies control for too much. Twin studies are typically conducted on economically homogeneous households and/or controlling for parental socioeconomic status. This explains both lead and breastfeeding: rich people are less likely to live in lead-full neighborhoods and more likely to breastfeed, so controlling for wealth makes lead and breastfeeding less obvious than they previously were. This could plausibly save some other parenting advice as well: rich people do parent differently from poor people, and if a parenting strategy is widely adopted among one class but not another, then its effects will be less obvious.
But I think there are some other things to bear in mind. Twin studies mostly concentrate on the broad strokes of personality: IQ, five-factor personality, and easy-to-study outcome measures like educational attainment and income. However, there are lots of people who have the same IQ, five-factor personality, educational attainment, and income, but are very very different people. I think that explains the borderline thing: borderlines are absurdly neurotic regardless of what skills you teach them, and they are always going to be neurotic, but learning emotion regulation is the crucial, life-altering difference between “neurotic” and “I just destroyed all my possessions because I love you”.
Furthermore, the studies really aren’t good at picking up on small trends. A study which used multiple measures of personality, instead of relying on a single self-reported measure, found large reductions in the amount of variation attributable to non-shared environment: a substantial amount of the difference between twins comes down to nothing more than “Alice pressed ‘agree’ when Eve pressed ‘strongly agree'”. That is a huge amount of noise. My understanding is that it is very hard to pick out small signals from very noisy measures. While parenting can’t get you from 25th percentile conscientiousness to 75th percentile, the measures aren’t good enough to suggest it can’t get you from 25th percentile to 35th percentile– which is actually a pretty large change in terms of how much work you get done. I admit that this is a sort of ‘parenting of the gaps’ theory: however, it does fit well with my understanding of how people work. People’s basic personality is fixed and unlikely to change; but there’s a lot of good you can do fiddling with the margins.
And, of course, there is always the final conclusion that– no matter how little effect you have on your children age 25– you are likely to have a great deal of effect on their happiness age 5, and perhaps the best parenting strategy is, while respecting your own boundaries and needs, to try to make that five-year-old’s life as pleasant and fulfilling as possible.