Meta-analysis of heritability of human traits based on 50 years of twin studies [pdf]
gwern.net
gwern.net
- 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.
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.
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.
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.
> 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.
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.
Ultimately, studies show that on average human traits are typically due 1/2 to genetics and 1/2 to environment. However, some traits are more skewed one way or the other. For example, bipolar disorder among twins was about 70% due to genetics and 30% due to environmental factors. All traits researched were somehow impacted by genetics.
Also for about a third of traits, the nature part wasn't dependent on a specific gene, so gene isolation to help predict traits is not always possible.
What other major takeaways from this study do you find interesting?
>The most interesting aspect of these results is that for many traits there is no detectable non-additivity. That is, gene-gene interactions seem to be insignificant, and a simple linear genetic architecture is consistent with the results.
We already knew this sort of from animal studies and evolutionary theory. But it's interesting to see it confirmed in humans.
The implications of this, is that it will be very easy to predict phenotypes from genetics. We don't need to model complicated interactions between genes like many people thought, and can simply take the genes that correlate the most with a desired trait.
This means you could do a large enough study and find every gene that correlates with intelligence, and create the optimal combination. Creating humans orders of magnitudes more intelligent than any human who has ever lived.
Well, no. Without a qualifier, it only makes sense to say that traits are 100% due to genetics. Replace a human in whatever circumstance with something that shares no genetics with a human (say, a rock), and you'll find that the power of the environment to make it behave in normal human ways is underwhelming.
What this is saying is that of the variation that humans display, about half is generally explained (in a statistical sense) by (a particular definition of) genetic variation among those humans. Humans vary from zero to zero wings, and the heritability of wings (in humans) is therefore undefined. It would be hard to conclude from this that our lack of wings isn't due to genetics.