Lacking a mentor to guide them through the corporate gauntlet, blacks will leave a company often because they cannot move horizontally to gain the broad experiences necessary to rise through the ranks.

"Folks of color don't leave because of title and money," Poriotis said. "They want to expand their portfolio and they can't in the company. The glass walls are more impenetrable than the glass ceiling."

Several outside experts say the glass wall has been a longstanding problem but is gaining new importance. As companies pare layers of specialized management, it has become more critical than ever to gain broader management expertise, they add.

"The glass wall is just a new name for an old phenomenon called occupational segregation," says Myra Strober, a labour economist at Stanford University who is researching issues women face at major corporations.

A systematic review of development needs is particularly critical for high potential mid-level women managers who are more likely to lack balance in their corporate experience because they have been concentrated in staff activities. If this is not done, American companies face the potential of wasting a generation of female talent that is in the pipeline for senior management but is not receiving the kind of balance and breadth of experience that is needed by corporations today. … These are the women for whom it is critical to break down the glass wall.