What does it mean when they say a trait is 50 percent genetic?
The mistake I keep writing about is small, and its consequences are not. A number calculated across a group of people gets read as a fact about one of them. The most damaging version has the best reputation.
Heritability is the most misread number in public life. It does not describe you. It cannot.
Evidence A heritability figure is a statement about a population. It says how much of the differences among people, in one group, at one time, tracks with genetic differences among them. If a trait is 50% heritable, that does not mean half of your version of it came from your genes. There is no half of you.
The figure also moves when the world moves. Make schooling universal and the heritability of educational outcomes can rise, because you have removed a source of environmental difference. The number went up. Genes did not change. The environment got more equal.
Hypothesis This is the same mistake I keep circling in everything else I write, in averages, in personality scores, in engagement surveys. Heritability is the version with the highest stakes and the best branding.
Which is why a company offering to tell you, from your saliva, that you are a morning person or that you need more caffeine is doing something worse than horoscopes. A horoscope does not arrive with a chromosome on the box.
Where this stops. The structure of heritability is textbook quantitative genetics and not in dispute. I have deliberately quoted no specific heritability figures, because those are contested and I have not run that research pass.
Not covered. Whether matching a treatment to a person’s genotype works. In medicine that may be the one place individual matching has real evidence. I have not checked, so I am not claiming it.