Mind games: “People tend much to prefer, say, $100 now to $115 next week, but they are indifferent between $100 a year from now and $115 in a year and a week. In one recent experiment, noted in our science section on October 30th, Mr Laibson and others found that the brain’s response to short-term riches (in this case, gift certificates of $15 or $20) occurs largely in the limbic system, a region that governs emotion. By contrast, the prospect of rewards farther into the future triggers the prefrontal cortex, which is often associated with reason and calculation.”