One additional unit of income can do a hundred times as much to the benefit the extreme poor as it can to benefit you or I [earning the typical US wage of $28,000 or £18,000 per year]. [I]t's not often you have two options, one of which is a hundred times better than the other. Imagine a happy hour where you could either buy yourself a beet for $5 or buy someone else a beer for 5¢. If that were the case, we'd probably be pretty generous – next round's on me! But that's effectively the situation we're in all the time. It's like a 99% off sale, or buy one, get ninety-nine free. It might be the most amazing deal you'll see in your life.
