In a Bloomberg Radio interview earlier this month, Vanguard Group founder Jack Bogle shared a pessimistic view on the future of U.S. pension funds.

According to the Bloomberg account of the interview, Bogle believes that over the next decade a conservative portfolio of stocks and bonds will not be able to “meet their 7.5 percent or 8 percent obligations.” Instead, he argues, annual returns will be closer to 3 percent for bonds and 4 percent for stocks.

“The only return you get on a bond is from the interest coupon,” he says, arguing that price shifts will become negligible over the long term (today, the article points out, the blended yield on a portfolio of half corporate bonds and half U.S. Treasuries is about 3 percent). This is a problem for pensions, says Bogle, since they depend on bonds to provide the steady stream of income needed to support benefit payments to plan participants.

The article cites the example of the California Public Employees’ Retirement System, the largest U.S. pension, which is considering more than doubling its bond allocation to “reduce risk and volatility as the bull market in stocks approaches nine years.”