Although the United States spends more than $100 billion per year on tax subsidies for retirement savings accounts, very little of it is actually saved for that purpose, according to a recent study by Harvard.

These tax incentives are to encourage families to save money for retirement. In actuality, the study estimates that for each $1 of government expenditure, total savings are only increased by 1 cent.

The study found that those in the higher tax brackets responded to incentives, but the response simply shifted savings away to different, tax-favored vehicles. That represented about 15 percent of retirement savers. The other 85 percent did not change habits of saving because of tax incentives.

Other policies were tested to see if they would have greater impact on retirement savings. Automatic contributions by employers, called "nudges," had a greater effect on savings.

One of the reasons for this, according to Harvard, is that "most individuals are passive savers who do not pay attention to employer pension contributions and thus do not offset such contributions by saving less in other accounts."