The Austrian economist Bob Murphy has challenged Paul Krugman to a public debate on business cycle theory. So far, Krugman has not taken up the invitation. The interesting twist is that Murphy has gotten people to pledge over $100,000, which--in the event of the debate's taking place--would be donated to a food bank in New York City. Therefore, unless Krugman decides to donate $100,000 of his own money in lieu of the debate, he is effectively preventing a charity from receiving a large sum of money (donated by other people). Furthermore, after Paul Krugman debated Ron Paul on television in 2012, he stated quite unambiguously on his blog that the reason he did so was to publicise his new book. One can only conclude that while Krugman is willing to sacrifice an hour of his time for the sake of advancing his career, he is not willing to do so for the sake of helping the needy. More details can be found here.