Private equity firms may have to return money to investors in a sign of concern about falling returns on deals done at the height of the buy-out boom, industry executives and investors say.

“It is still early and it varies from fund to fund,” says Monte Brem, chief executive of StepStone Group, a California-based consultant to large investors. “But this may be the first time that this generation of private equity managers will have to consider how to satisfy large liabilities to investors.”