The California Public Employees' Retirement System Board , the largest U.S. public pension fund, has tightened policies governing board members' involvement in investment decisions, it said on Tuesday.

The $200 billion pension fund said the rules resulted from a review of its governance principles that began in the fall. Calpers and other U.S. public pension funds have come under scrutiny from federal and state officials over the practices of placement agents, middlemen who help sell investments to the funds.

Calpers said under the new rules, its 13 board members are required to funnel communication on existing or potential investments to the pension fund's chief investment officer. Board members must also refrain from advocating an investment action with Calpers staff outside a board or committee meeting.

Our staff must make decisions based squarely on the merits of a transaction, said George Diehr, vice president of the Calpers board, in a statement. We want independent, objective analysis to be the ultimate guide when it comes to Calpers' investments, and these new policies ensure that will happen.

Board members could face disciplinary action, including censure, temporary termination of travel privileges, loss of committee chairmanship or vice chairmanship, or may be required to undertake additional ethical or fiduciary training beyond annual sessions on their responsibilities to fund participants and beneficiaries.

In November, the Calpers board had tightened its rules on placement agents.

(Reporting by Karen Pierog; Editing by Leslie Adler)