Or, maybe, twins share similar thought patterns and therefore make similar lifestyle choices (how much to eat, how often to exercise, and so on) and that's why

Or, maybe, twins share similar thought patterns and therefore make similar lifestyle choices (how much to eat, how often to exercise, and so on) and that's why they share similar weights and muscle mass. That said, the nature/nurture pendulum has been stuck on the "nurture" side for a while, and it's now starting swinging back to the middle where it belongs. It's got a ways to go though. I'm reading Steven Pinker's The Blank Slate now, and I highly recommend it to people who are interested in this topic.