Gene Expression (one of the ScienceBlogs), has a great piece on the meaning of “heritability.”
When someone tells you that height is 80% heritable, does that mean:
a) 80% of the reason you are the height you are is due to genes
b) 80% of the variation within the population on the trait of height is due to variation of the genesThe answer is of course b. Unfortunately in the 5 years I’ve been blogging the conception of heritability has been rather difficult to get across, and I regularly have to browbeat readers who conflate the term with a.
If you think about it, the second definition has a clear statistical model — linear (or other) regression — and a statistical test — ANOVA. The first (and wrong) definition has no mathematical correlate.
There are a lot of other interesting points, such as that heritability can change with changing environmental factors, and that there are certain assumptions made when talking about heritability.
Of course, this becomes really important when you’re looking at studies about the heritability of sexual orientation. It’s not as simple as “genes vs environment,” or even “half genes and half environment.” But of course the level of discourse in some places is such that you’d have a hard time convincing people that genes had anything to do with it at all.
Which is why it is sometimes frustrating to hear debates about gay issues. They force both sides to boil down their positions to pithy sound bites, because any piece of nuance that you might introduce (”Perhaps sometimes sexual orientations shift during a person’s lifetime.”) might be grossly misinterpreted (”Aha! So it *is* a choice!”).
It’s amazing what you can do with statistics. It’s also scary the mistakes you can fall into if you don’t take the time to understand these things properly.
