Feldman examined whether a seemingly unimportant change in US tax law changed savings rates. In 1992 the George H. W. Bush administration changed how income taxes were collected. The amount of the taxes stayed the same, but less was collected each month. This tended to reduce tax rebates at the end of the financial year, but increase monthly net income. It didn't change the amount anyone paid in tax, but changed when they pay it. It meant there was a shift in income. Instead of receiving a sizeable yearly tax rebate, households had a larger net monthly income. Traditional economics would not predict any change in behaviour.
However, Feldman found that people saved significantly less after the change.
The yearly rebate was large enough to be seen as special and so people carefully considered how to spend it, typically saving a high percentage. The small monthly increase in income, however, was just added to household consumption budgets and so typically not saved. This is an example of mental accounting - the process by which people sort income into different budgets and tend not to shift money between budgets.
Thus we have a textbook example of the law of unintended consequences - a minor change in tax law causing huge shifts in behaviour. An argument, if ever there was one, for the use of behavioural economics in policy making!
