Very interesting post arguing against explaining everything with non-shared environment. There entire post is very interesting and here are a few highlights from the beginning:
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.
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.
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”.