> 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.
[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.
Depending on the twin study yes you can go into all kinds of sibling groups/measures too. However mapping it to something that must have had evolutionary pressure on it in ways we still don’t clearly understand is not at all straightforward, so yes, in a sense, I think most psychologists, who have no evolutionary training, are doing this very wrong.
You’re basically saying “some limits of cognition are heritable” which is so obvious as to be inarguable.
The huge qualifier in what you quoted is “reliably measured psychological traits.” They also have to be defined within psychological language and linked to genetics through behavior or biological indicators. There are exceedingly few of these, and schizophrenia, autism, etc, are not them.
Psychology as a whole without evolutionary considerations to articulate on is free-floating.
There is a long history of psychologists justifying their approach, but the reality is that reproducibility in psychology is still at something like 35%.
Edit to say the second paper is interesting, but it seems to me to have a number of flaws.
If I understand this right, GWAS are capturing everything from the genome.
This includes any genetic determination of appearance, food preferences, etc.
Then the sample is only healthy twins from narrow genetic/cultural stock: England/Wales born over two years in the 90s.
This means that in addition to sweeping up genetic factors associated with brain development, you are capturing all visible/inherent-behavioral inputs to a multi-stage developmental process that includes interactions with technology, other people, school systems, etc.
Of course you can explain a majority of the outcome. It won’t replicate though, because it is dependent on that narrow time slice of culture, nutrition, parenting practices, technology, etc.
If this study replicates in Asia, South America, or Africa, with similar genetic components in the PCA, I’ll admit I’m wrong.
Heck if it even replicates in England with twins from the 1960s I’ll admit I’m wrong.
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/...