Drawing on the insights of psychology, behavioral economists have explained why we buy more stuff at $0.99 than at $1.00 (the “left-digit effect”), why we commit to gym memberships we’ll never use (“optimism bias”), and why we don’t return things we buy as often as we should (“post-purchase rationalization”). The giants of the web, from Amazon to Zynga, use similar tricks to keep us coming to their sites, playing their games, and buying their goods. In fact, that’s how they became giants in the first place. Here’s how they game us