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The other half isn't random, it's mostly "non-shared environment", in other words the environment outside the home, whether that be school or play with friends

The other half isn't random, it's mostly "non-shared environment", in other words the environment outside the home, whether that be school or play with friends or whatever. My source for this is the book The Nurture Assumption by Judith Rich Harris. IIRC she concludes the most likely reality is that personality is shaped 50% by genetics, 45% by non-shared environment and 5% shared environment (i.e. parenting). Big caveat to all this: this is personality as in how one relates to the outside world, not how one relates to their parents, which is indeed strongly determined by parenting and one of the reasons what so many studies that studied the link between parenting and kids' personality characteristics (almost invariably measured in the presence of mommy or daddy) found such a link.

jacobolus 7y ago
“Non-shared environment” just means anything that differs between the two kids. This could include their different treatment by the same adults, their different choices of activities, differences in the subconscious mental processes, their different adopted roles inside the family, their different diseases or injuries, their asymmetrical relationship with each-other, etc. It doesn’t have to be outside the home. Twin studies don’t reveal exactly what the causes were of different outcomes. All they can do is statistically split causes into broad classes based on the differences between twins raised together or apart and based on the differences between identical vs. fraternal twins.