usFaced with growing obligations and shrinking returns, many of the largest U.S. public pensions have raised their exposure to alternative investments to record levels this year, despite ongoing criticism of the risks and costs.

Public plans with more than $1 billion had a median of 15 percent in alternatives as of June 2012, the highest ever and up from 9.2 percent in June 2011, according to the Wilshire Trust Universe Comparison Service.

The increase carries risks of unstable performance and high fees amid a funding shortfall of $1.38 trillion as of 2010, according to Pew Center on the States data. Already, the vast majority of states have cut pension benefits or increased contributions from workers, or are trying to.

One reason alternatives are appealing is that they are not linked to the performance of the stock and bond markets. Public pension plans earned an average 12 percent return from private equity investments last year, compared with a 7.2 percent loss from stock investments, according to alternatives research and consultancy firm Preqin.