My twin brother and I shared a bedroom from birth until we left for college at 17. Exactly what different environmental factors can you identify between us that would have made him chunky at 15 when I wasn't? Or that made him not enjoy exercise and sports like I did from the age of 8 onwards? Are you going to admit that it pokes massive holes in your univariate theory, or simply continue to double down? Perhaps my side of the room had a dramatically different climate than his, right? It's as if your brain ignored the obvious to continue to support your absurdly simplistic genetic determinism.