That is, once you start measuring GDP as a way of gauging social welfare, people will start to figure out ways to make GDP go up without improving social welfare (say, by swapping dirty financial derivatives). Once Google starts measuring inbound links as a way of evaluating the importance of web-pages, people will figure out how to increase the inbound links to unimportant pages (splogging, blogspam). And once you measure fat or calorie content as a proxy for the healthfulness of food, manufacturers will figure out how to decrease fat and calories without making the food more healthful (reducing fat by adding sugar, reducing calories by adding poisonous artificial sweeteners).
The law was first stated in a 1975 paper by Goodhart and gained popularity in the context of the attempt by the United Kingdom government of Margaret Thatcher to conduct monetary policy on the basis of targets for broad and narrow money, but the idea is considerably older. It is implicit in the economic idea of rational expectations. While it originated in the context of market responses the Law has profound implications for the selection of high-level targets in organisations.Goodhart's law (Thanks, Steve!)