William E. Even (Miami University, Richard T. Farmer School of Business Administration) & David A. Macpherson (Trinity University, Department of Economics) have published Managing Risk Caused by Pension Investments in Company Stock, 62 Nat'l Tax J. 439 (2009).  Here is the abstract:

This paper uses simulation methods to determine the optimal mix of assets in a pension plan that has half of its assets in company stock. The optimal share is shown to vary with workers’ risk aversion, non-pension wealth, and the financial characteristics of the underlying stock. Relative to a strategy that blends company stock with all bonds, there are substantial efficiency gains from optimizing the mix of stocks and bonds—particularly for workers with lower levels of risk aversion and substantial non-pension wealth holdings. The gains from optimizing also differ substantially depending upon the company stock’s mix of systematic and idiosyncratic risk.