I'm no expert, but that seems like the biggest issue. I mean the study says that income is heritable, which by its very nature would affect the environment that right?
> (i.e., within-pair genetic differences are not correlated with the within-pair environmental differences; see e.g. Stenberg (2013) who stresses the importance of this assumption for the interpretation of heritability estimates), and iv)
So this is only about whether sibling differences are correlated with the environment -- which seems much more reasonable. On the other hand, the very next one is problematic:
> [We assume] there is no assortative mating. The last assumption would not hold if the genotypes of the parents were correlated (Posthuma et al. 2003).
Tendency of parents to have some genetic sorting seems both easy to measure and quite likely - why not dig in to its impact?
Finally, another thing that makes the result hard to generalize (emphasis added):
> We show that in the relatively equitable economic and institutional environment of Finland, the share of variance of lifetime earnings explained by education is clearly less than a tenth (in our data).
The authors acknowledge that they were measuring the size of effect in an environment that has far less variation in the variable than many. This seems like a good place to confirm the existence of an effect, but a bad place to be confident of its size, and a worse place from which to generalize about more heterogenous environments. Compare Finland's GINI index (income concentration) to US, UK, or China to get an idea of how much more variation there is in those societies: https://www.indexmundi.com/facts/indicators/SI.POV.GINI/comp...
Can you explain this a bit more?
> The classical twin design compares the similarity of monozygotic (identical) and dizygotic (fraternal) twins. If identical twins are considerably more similar than fraternal twins (which is found for most traits), this implicates that genes play an important role in these traits. By comparing many hundreds of families with twins, researchers can then understand more about the roles of genetic effects, shared environment, and unique environment in shaping behavior.