Why are children in the same family so different from one another? (1987) [pdf](gwern.net)
gwern.net
Why are children in the same family so different from one another? (1987) [pdf]
https://www.gwern.net/docs/genetics/heritable/1987-plomin.pdf
4 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
[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.
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.