Comment by accidentallfact 6 hours ago
The problem with twin studies:
1. There are genetic mutations that make you immune to HIV.
2. Monozygotic twins will both be immune, or not immune, while dizygotic twins may be either, one can be immune, while the other one could get AIDS.
3. Thus, a twin study would likely show that AIDS is a genetic defect.
There's serious issues with heritability research in general, it's observability stuff -- not experimentation, so imv, its at best proto-science, and in many cases plainly pseudoscience. "Heritability" itself has little to do with whether something is inherited, and speaks only to correlation with genes. Since we have a vast amount of genes which are shared for all sorts of reasons (ie., mating is based on shared culture, wealth, geography, etc.) -- the metric is mostly useless.
Accents are highly heritable, since they always correlated with location which is always correlated with genes.
Even if you do these twin studies, you have to assume a model of how genes and the environment interact, and all such models are obviously false.
Thus even if you grant that heritability measures on high quality twin studies are 'sign correct', in the sense that they show P(genetic effect) > P(no genetic effect) -- any magnitude of this effect, or any theory of is, is more or less pseudoscience (unless there are experimental studies showing gene-trait mechanism).
For example, it is "obvious" that P(genetic effect) > P(none) for intelligence, since genes control the structure of the brain and body. But there is no evidence (I'm aware of...) that beyond provision of a functioning brain, our genetics play any role in intelligence stratification. ie., all correlation with task performance and IQ can be explained by correlations in the metal retardation / mental deficiency range.
This doesn't mean intelligence is very malleable beyond a certain age. My own views is that genes are basically providing functioning hardware to the womb, and after that point its early development (both pre-birth and probs up to at most 3yo) which locks in a lot of the observed intelligence stratification. This is a very different story than popularisers of IQ research communicate though, but be aware, none are very good scientists and most of this research is methodologically unfit
Geneticist would say it's intended design of twin studies. Your example doesn't show AIDS is a genetic defect, but that genetics may predispose/protect against it.