> They basically look at fraternal and identical twins and assume (because their environments are otherwise probably close) that any differences in outcome are heritable. It’s a huge leap of faith.
You’re making it sound like the researchers are idiots who have never thought about how their tools might not be measuring what they hope they are.
As well as identical/fraternal twins you can also look at other sorts of relatedness, e.g. half siblings are on average as related as first cousins and on average you’re as closely related to each parent as each full sibling. Psychological traits are inherited like height is, not like language is[1]. Mental illness is highly heritable though the expression varies[2].
As a parallel line of evidence to confirm classical twin studies and those based on degrees of shared ancestry cheap genetic testing has allowed testing how closely people are related. So you don’t have to assume a sibling shares 50% of their genes. You can see if they’re 73% similar or 41%. Or you can take people who have no known shared recent ancestry and see how similar they are on the trait of interest.
To the best of my knowledge behaviour genetics is holding up. For psychological traits genetics is more powerful than developmental noise.
[1]> Genetic Influence on Human
Psychological Traits
> There is now a large body of evidence that supports the conclusion that individual differences in most, if not all, reliably measured psychological traits, normal and abnormal, are substantively influenced by genetic factors. This fact has important implications for research and theory building in psychology, as evidence of genetic influence unleashes a cascade of questions regarding the sources of variance in such traits. A brief list of those questions is provided, and representative findings regarding genetic and environmental influences are presented for the domains of personality, intelligence, psycho- logical interests, psychiatric illnesses, and social attitudes. These findings are consistent with those reported for the traits of other species and for many human physical traits, suggesting that they may represent a general biological phenomenon.
https://utahpolicy.com/wp-content/uploads/2013/12/bouchard.0...
[2]> The p factor: genetic analyses support a general dimension of psychopathology in childhood and adolescence
> Diverse forms of psychopathology generally load on a common p factor, which is highly heritable. There are substantial genetic influences on the stability of p across childhood. Our analyses indicate genetic overlap between general risk for psychiatric disorders in adulthood and p in childhood, even as young as age 7.
https://acamh.onlinelibrary.wiley.com/doi/pdfdirect/10.1111/...