Underfunded pension programs in U.S. states, cities and municipalities are "three or four times worse" than current government projections, said Joshua Rauh, senior fellow at the Hoover Institution think tank.
"The amounts of money [officials] are setting aside [for pensions] are far short of adequate," Rauh told CNBC's "Squawk Box" on Monday. "Taxpayers are going to get pretty soaked" when the time comes to make up the difference.
In a Hoover essay, "Hidden Debt, Hidden Deficits," Rauh argued U.S. public pension systems were actually running at a $3.4 trillion shortfall in fiscal 2014 when "optimistic assumptions about future investment returns" contained in government disclosures were tempered.
"Most public pension systems across the United States still calculate both their pension costs and liabilities under the assumption that their contributed assets will achieve returns of 7.5 [to] 8 percent per year," wrote Rauh.