From the conclusion:

It is most implausible to suggest that giving birth and caring for young children is the predominant reason why female executive managers, who average 50 years old in our sample, quit. Other unobserved factors leading managers to attrit could include more un-pleasantness, indignities, and tougher unrewarding assignments at work, examples of factors that reduce the attraction of work without necessarily affecting productivity or human capital acquisition. Perhaps women are subject to this form of gender discrimination. Another hypothesis is that women acquire more nonmarket human capital than men throughout their lives, and hence find retirement a relatively attractive option.