A drop in bond yields probably hurt the typical large U.S. defined benefit pension plan in June.

The “funded ratio” at a typical plan fell to 66.9% at the end of the month, from 70.9% at the end of May, according to consultants in the Stamford, Conn., office of Towers Perrin Forster & Crosby Inc.

Towers Perrin bases pension status figures partly on actual pension results at 300 large U.S. companies.

The funded ratio has dropped 23% over the past 12 months, Towers Perrin reports.

A benchmark investment portfolio that Towers Perrin uses in analyzing plan performance experienced an 0.3% return in June, but projected liabilities increased 4.6%.