Why are children in the same family so different from one another? (1987) [pdf] | Hacker News Reader

Why are children in the same family so different from one another? (1987) [pdf]

gwern.net

11 pointsgwern4 comments

(1987)

Not that much has changed since then! Non-sharedenvironment remains the largest source of variance after genes in most human traits, shared-environment remains largely trivial after childhood, siblings remain quite different from each other, and the sources of non-sharedenvironment relatively mysterious aside from more evidence that noise like somatic mutations and infections are responsible and IMO not so much peer-effects like Harris hypothesizes in _The Nurture Assumption_.

Shared environment is probably not created equal though. While the family environment is constant, parents are likely to vary (deliberately and/or not) in their resource allocation (e.g., attention and care) among their kids. An interesting recent paper shows that the non-inherited genetic material of the parents may _still_ have an influence on the kid. [0] What exactly accounts for that influence remains to be elusive, and it would be quiet astonishing if the majority of the influence can be explained by well-established (or rather, exhaustively studied) behavior patterns in psychology. For example: birth order, number of same-sex siblings etc.

[0] https://www.biorxiv.org/content/early/2017/11/14/219261

> An interesting recent paper shows that the non-inherited genetic material of the parents may _still_ have an influence on the kid. [0]

Yes, but that comes out of the heritability component, not the non-sharedenvironment component.

> For example: birth order, number of same-sex siblings etc.

Birth order, by definition, can't explain any of the variance in fraternal vs identical twins, and there's a lot of doubt about whether it exists at all and for which traits. Likewise, I haven't seen much credible research on sibling effects - most such sociological research doesn't even bother to try to control or quantify any of the relevant genetics, and is 100% useless.

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