In the world of traditional economics, it shouldn't matter whether you use an opt-in or opt-out system. So long as the costs of registering as a donor or a nondonor are low, the results should be similar. But many findings of behavioral economics show that tiny disparities in such rules can make a big difference.
You go out on the practice range, and something kind of clicks, and you start hitting the ball very crisply. And you're sure that you've found it, the holy grail - that all you have to do is hold your hand in a certain way. Then you go out on the golf course, and it's completely disappeared.