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

[radiantfracture]

I love this! Anything that pokes holes in genetic absolutism.

[ethelmay]

I don't understand what they mean about monozygotic twins having higher than expected correlation. What was the expectation, and how was it arrived at? As a lay person, I expect identical twins to be, if not indistinguishable, very much alike.

[redbird]

"More than expected" means that if they use a model that correctly describes other pairs of relatives, it predicts less similarity between monozygotic twins than is actually the case.

They're not just looking at things like appearance and blood type, but at things like how well people do in school, health, hobbies, and what music they listen to. Any model that gets the correlations right for identical twins gets it wrong for everyone else.

Most of this research is asking about things that happen after birth. For example, whether somone graduates from college is correlated with whether their parents did, and with whether their siblings do. How much of that is genetic, and how much is environment and culture? And wWhich aspects of environment and cultureF What are the relative importance of family expectations (that a child will, or won't, go to college), the expectations of other adults around them, socioeconomic status, and other non-genetic factors including when the person was born?

[otter]

I didn't read far enough to see if they say what types of resemblance they are looking at - facial features, career choices, education level, chronic illness? Could be so many things

[redbird]

All of the above. Every so often you hear about things like separated-at-birth twins who play the same musical instrument and married women with the same first name.

[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.

[conuly]

Although every time we read one of those articles I'm forced to remember a few things, such as "well, if they weren't remarkably similar nobody would care" and "they were raised five miles apart in a state that loves basketball and are each six foot three, of course they play basketball" and "I don't know, what're the odds that somebody born in 1978 might marry a woman named Jennifer? Can't be that amazing, can it?"

[redbird]

Agreed, and that's the "environment other than family" part, along with people who grew up in the same place, including in the same household, being likely to go to the same schools, and parents who want to be fair to their children offering both twins music lessons.

[ethelmay]

Our old next-door neighbor had four daughters and all of her sons-in-law were called Bob.

[voidampersand]

I'm guessing it's epigenetics.