“If the primary cause of being overweight was genetic, we would not suddenly go from mostly not fat people to quite a lot of fat people.”
This isn’t necessarily true. It could be that in environmental condition A, almost nobody is overweight, but in environmental condition B, people with a certain common genetic variant become overweight, while those without that genetic variant do not become overweight. If you measure genetic influence in condition B, you will find a big genetic influence. This is not a contradiction with lots of people suddenly becoming overweight when the environment changes from condition A to condition B. It also doesn’t mean that “there is nothing anyone could do to prevent” it—you just need to revert to condition A.
“If the primary cause of being overweight was genetic, we would not suddenly go from mostly not fat people to quite a lot of fat people.”
This isn’t necessarily true. It could be that in environmental condition A, almost nobody is overweight, but in environmental condition B, people with a certain common genetic variant become overweight, while those without that genetic variant do not become overweight. If you measure genetic influence in condition B, you will find a big genetic influence. This is not a contradiction with lots of people suddenly becoming overweight when the environment changes from condition A to condition B. It also doesn’t mean that “there is nothing anyone could do to prevent” it—you just need to revert to condition A.