From:
The Public Pension Crisis - NYTimes.com
"Pension funds subsist on three revenue streams: contributions from employees; contributions from the employer; and investment earnings. But public employers have often contributed less than the actuarially determined share, in effect borrowing against retirement plans to avoid having to cut budgets or raise taxes.
They also assumed, conveniently enough, that they could count on high annual returns,
typically 8 percent, on their investments. In the past, many funds did earn that much, but not lately. Thanks to high assumed returns, governments projected that they could afford to both ratchet up benefits and minimize contributions. What a lovely political algorithm: payoffs to powerful, unionized constituents at minimal cost."