“The notion of mental accounts is absent in traditional economic theory, which holds that wealth in general, and money in particular, should be fungible: That is, $100 in roulette winnings, $100 in salary, and a $100 tax refund should have the same significance and value to you, since each C-note could buy the same number of downloads from iTunes or the same number of burgers at McDonald’s. Likewise, $100 kept under the mattress should invoke the same feelings or sense of wealth as $100 in a bank account or $100 in U.S. Treasury securities (ignoring the fact that money in the bank, or in T-bills, is safer than cash under the bed). If money and wealth are fungible, there should be no difference in the way we spend gambling winnings or salary.”