Family resemblances are complicated

[redbird]

via oursin, something I found interesting: We still don't understand family resemblance, and some of what we thought we knew is mistaken, or might be.

This article describes research that used data from almost a million people: every Norwegian student who took a standardized test from 2007-2019.

Quoting the article: "The resemblance of twins cannot be reconciled with any model....The resemblance of adoptees cannot be reconciled with any model."

Adjusting a model to account better for twins makes it a poorer match of adoptive relationships, and vice versa. Any attempt to account for one of these moves the model away adopted siblings makes it fit twins less well, and vice versa.

In addition to comparing twins, other biological siblings, and adoptees, this study looked at "in-law relationships": step-siblings and step-cousins. It sounds like a lot of previous work used only biological relatives, even work that used large biological pedigrees, and genetic models fit pretty well there except for twins. In this study that compares adopted siblings and step-siblings, shared environments appear to mimic genetic transmission. So we don't really know how much of people's resemblance to their biological kin is genetic.

I was surprised to find that even a purely genetic model doesn't adequately explain the resemblance between monozygotic twins, or entirely explain why the correlation is higher than between dizygotic ("non-identical") twin pairs.

Comments

[ethelmay]

I have read (though it was very old research) that identical twins raised apart sometimes resemble each other more in choices such as which musical instrument to study, because they aren't trying to differentiate from one another.