Willpower could be applied to eating just one, or to not eating the one, or to not taking the bag of chips out of the cupboard, or to not buying the things. Obviously we can make decisions, some are easier to make than others. Or it could be, once you're a quarter of the way into the bag, flushing the rest or tossing it in the fire. Asking "why don't I have any self control," invites the answer "because I'm weak." I think a better question is, what can I control? What is the path of least resistance that actually takes me where I want to go? We're capable of exertion, it's not unlimited.

As far as genetics go--if your twin is schizophrenic, you have something like a fifty percent chance of being schizophrenic yourself. This gives a high probability that some non-genetic factor might decide things even when the genetic background is established. But 50 percent is still 50 times the preponderance in the general population. If it were 10 percent, that would still be ten times general rates. Set up the dominoes, if I flick them with my finger, they'll go down. Lock them up in a controlled environment, you can make it exceedingly unlikely for them to fall. Which is the primary cause, the trigger, or the finger that pulls it? What is the sound of one hand clapping? Which hand is the "primary" cause?