Alas, Anna Karenina is on point here. While posititive environments may all be similar to each other, negative environments can be negative in a multitude of individual ways. There are probably a dozen common reasons that children are removed from their homes and the environmental effects associated with each one vary. Unless you can sort based on reason for termination of parental rights (which is often legally suppressed by juvenile justice system rules), there are potentially signficant and hard to discern environmental exposure biases present.

as unpleasant as it is to consider, if the environment is relatively uniform in a negative fashion that may be informative of the nature of response in variation across individuals of different genetic backgrounds.

In case of many traits, from personality to sexual orientation, birth order is a non-shared component of the otherwise shared environnment of siblings that may have material effects.

Of course, these issues are not present in the same way for fraternal twins.

Half-siblings pose much more useful natural experiements than foster parenting situations, since almost all foster parenting situations involve a fairly extended and almost always acutely negative period of environmental exposure at a developmentally critical time period in a genetic parent family before the foster parenting period begins. This is not true to nearly the same extent in half-sibling situations.

Within the universe of half siblings, cases in which war widows remarry and have new children might be particularly instructive as there is no selection bias in the sample for people who had poor marriages to start with that could have been a negative environmental impact on the older sibling.

#2, quantitative traits should reflect total genome content. i’ve blogged this before at length; you’re a long time reader, you should read me closer. there was a recent paper using genomic relatedness and IQ I believe. please google visscher, dreary, etc. not siblings though.

And on a related note, is the maternal component going to be slightly larger in men since the X chromosome is bigger than the Y?

i think that’s what’s going on. there’s not as much stuff on the Y. i don’t even know if it’s added into the computation? i just focus on the autosomes for that reason.

I’d hazard a guess this is because you don’t share an X chromosome with your father (presuming you’re male) and you don’t share a Y chromosome with your mother.

Do these differences in relatedness reflect any differences in your relationships with your siblings? I know the differences are small but I wonder if people can instinctively tell which siblings they are most similar to genetically and whether this knowledge is reflected in their relationships with those siblings.