LATE one Friday afternoon at a leading consulting firm, a last-minute request came in from a client. A female manager was the first to volunteer her time. She had already spent the entire day meeting with junior colleagues who were seeking career advice, even though they weren’t on her team. Earlier in the week, she had trained several new hires, helped a colleague improve a presentation and agreed to plan the office holiday party. When it came time for her review for partner, her clear track record as a team player combined with her excellent performance should have made her a shoo-in. Instead, her promotion was delayed for six months, and then a year.

This is the sad reality in workplaces around the world: Women help more but benefit less from it. In keeping with deeply held gender stereotypes, we expect men to be ambitious and results-oriented, and women to be nurturing and communal. When a man offers to help, we shower him with praise and rewards. But when a woman helps, we feel less indebted. She’s communal, right? She wants to be a team player. The reverse is also true. When a woman declines to help a colleague, people like her less and her career suffers. But when a man says no, he faces no backlash. A man who doesn’t help is “busy”; a woman is “selfish.”

In a study led by the New York University psychologist Madeline Heilman, participants evaluated the performance of a male or female employee who did or did not stay late to help colleagues prepare for an important meeting. For staying late and helping, a man was rated 14 percent more favorably than a woman. When both declined, a woman was rated 12 percent lower than a man. Over and over, after giving identical help, a man was significantly more likely to be recommended for promotions, important projects, raises and bonuses. A woman had to help just to get the same rating as a man who didn’t help.