We have had some confusion over the years, including recently, what it means for something to be "genetic." While reading a Scott Suskind at Slate Star Codex article from 2017 about weight loss i came across a quote that captures a lot of what I think is missing.
Right now, within this culture, variation in BMI is mostly genetic. This isn’t to say that non-genetic factors aren’t involved – the difference between 1800s America and 2017 America is non-genetic, and so is the difference between the perfectly-healthy Kitavans on Kitava and the one Kitavan guy who moved to New Guinea. But once everyone alike is exposed to the 2017-American food environment, differences between the people in that environment seem to be really hereditary and not-at-all-related to learned behavior.
It is from Part v, just after the graph, in his review of The Hungry Brain. by Stephan Guyenet. There is more detail about this particular example immediately after. An environment may influence behavior, such as BMI or violence or anxiety, yet it does not affect everyone equally. And the not equally is often genetic. Because there is variety of response, the people who do "best" by some measure are likely to get smug, believing that they worked harder or were more righteous or have a better attitude, but they may just have been wired better right from the start. It is not good to give yourself credit for your heredity.
As usual, Suskind goes on too long, trying to be thorough. Still, it is very good, weighing one side against another and looking for weaknesses in his own argument. Overall, it is discouraging about long-term (meaning five years, not a few months) serious weight loss (meaning normal weight, not "a reduction of 10% of weight.")
Also, I loved his first sentence of Part vi.
Lest I end on too positive a note, let me reiterate the part where happiness is inherently bad and a sort of neo-Puritan asceticism is the only way to avoid an early grave.
As BMI was brought up, creating a rabbit-trail we all want to go down, I should include a link to another article about the mysteries of obesity, which includes a disclaimer about using BMI.