I think our intuition might be miscalibrated when it comes to evaluating how much a person’s genes impact how they turn out physically (which isn’t surprising). What’s a bit strange is that we seem to be closer to the truth when it comes to twins.
Nobody’s surprised when identical twins turn out to have very similar bodies (weight, muscle mass, etc), even into adulthood.
But when it comes to non-twins, people seem to think that “making the right choices” and “willpower” are primary factors in how human bodies turn out, and that we can assign a good amount of personal credit or blame to individuals for good and bad outcomes.
There is a disconnect between these two visions, and I think that it’s the latter that needs to be updated.
After all, even if we put aside the direct ways in which our genes build our bodies (encoding how our tissues grow) and instead look at our abilities to “make the right choices” and exert “willpower”, we find that those are also greatly determined by genetic factors. Identical twins probably turn out very similar in good part because they have almost identical amounts of those qualities of mind.
This doesn’t mean that all is pre-determined and that if we all stop trying we’ll turn out the same we would have otherwise, but rather that we are playing within certain parameters, and that the part we control is probably smaller than most people think (not non-existent — we still deserve some credit — just more modest).
To be clear, I’m not saying the situation was white and we thought it was black, or even that it’s a black & white thing, but rather that most people’s intuition might be the wrong shade of gray. Otherwise, I would think there would be a bigger variation between identical twins, but they spend their lives making different choices yet most stay very similar to each other (as far as I know — if you know of a study on this, please send it my way).