- They looked at nearly every single twin study over the last 50 years
- The combined population of the all the studies is 14 million twin pairs
- The results strongly suggest that every human trait is heritable as they didn't find one trait that had 0% heritability.
- The most surprising result for me is that across all the traits they compiled, the heritability is 49%. Which is much much lower than I would have expected.
There's a few more interesting points about monozygotic vs dizygotic, but I'll spare you the technicalities since it has a narrower scope of interest.
As always, thanks Gwern for the informative post.
> Heritability is the proportion of observed differences on a trait among individuals of a population that are due to genetic differences. https://en.wikipedia.org/wiki/Heritability
It is also commonly misunderstood. In particular, heritability is always relative to an environment, and not independent from it. Height might be 50% heritable in one area but 75% heritable in another, for example (even with the same genetic population in both areas).
The other big misunderstanding is that environment means the contribution of family or society. It is really just everything that is not genetic and is mostly noise. When you try to measure the effects of family background on most personality traits the contribution is close to zero.
But much much higher than almost all people would believe.
For example, if you were to sort the population the length of their pinky finger, and then look at two groups of people, one the longest-pinky quintile and one the shortest, there is nothing in this study that would suggest those two groups would have different resting heart rates. This is true even though we can assume on the basis of this work that both pinky length and resting heart rate are about 50% heritable.
The only way they would be correlated would be if there was a single selection mechanism for both, and given that visible traits and non-visible traits tend to have completely different selection mechinisms you'd have to show positively and directly by experiment that such a common mechanism existed in any given case.
Ab initio there is simply no basis for the claim any random non-visible traits like resting heart rate will have different group averages for groups that are segregated from the general population by any random visible characteristics like pinky length.
Genetic similarity is the "master mechanism that would correlate the group average of one trait with another."
People's casual usage of the term heritability doesn't match the way it's used in scientific contexts. For instance, people wouldn't normally say that the trait of "being a slave" is explained by genetics, but the heritability (in the technical sense) of being a slave in the mid 1800's USA was rather high.
Think of a trait like hair colour in East Asians. Having black hair is completely determined by genes, but heritability is pretty much zero since (with maybe a few extreme exceptions) variation in hair colour in that population is entirely environmental (use of hair dyes, etc.). In Europeans however, heritability is probably much higher due to greater variation in hair colour.
No, it wouldn't be. Monozygotic black twins would have the exact same chance as dizygotic black twins of being slaves, and slavery would cluster in families, with whole families tending to be free or slave, leading to very high shared/nonshared environment estimates.
I can't find that conclusion in the article. I guess that there is some selection bias here. Even if some trait has a very small inheritability, the p values are very big and I guess it will be dropped from the original article because it's not interesting.
For example: "Preferred fingernail color" is probably not inheritable.