We all know most genetics v. environment* research is done using a mix of monozygotic (identical twins), dizygotic (fraternal) twins , and non-twin siblings, reared apart or together. The idea was that monozygotic twins share 100% of their DNA, and dizygotic and non-twins shared 50%, so you could tease out the difference between environment and genetics that way.
The first problem was that identical vs. not identicalness was originally assessed based entirely on looks. But not all genetically identical twins look alike, and not all twins that look alike are genetically identical. Mislabeling this makes genetics look less influential than they are.
The second problem is that this discounts nine months in utero as an environment, when it is probably the most influential environment you will ever be in. Some (though not all) studies use dizygotic twins. vs non-twin siblings to measure the affect of a shared uterus, but there’s a lot of confounding variables there. Worse, 75% of monozygotic twins are monochronic (sharing a placenta), and an exceptional few are monoamniotic (share an amniotic sac) (dizygotic twins never share a placenta or amniotic sac). Monoamniotic pregnancies are rare and dangerous so we don’t know much about the twins, but monochronic twins are more alike than dichronic-monozygotic twins, despite the fact that sharing a placenta is more like to result in unequal distributions of blood, which can have huge effects.
The third problem is that not-identical -> 50% shared genetics was a reasonable assumption to make in the 1950s, or even the 1980s, but it’s not true. You have a 50% chance of sharing any given chromosome with a full sibling, which means your average relatedness is indeed 50%, but the total percent in common could be anything between 0 and 100**. With genetic testing as cheap as it is, there’s no excuse not to test study subjects for exact relatedness.
*A stupid framing to begin with
**With complications from crossing over between chromosomes. The probability math on this is straightforward but the actual calculations are so ugly because it depends on which chromosome crosses over and where.