When Simone Allen started a demanding job as a litigation attorney at a large Philadelphia law firm a year ago, the 32-year-old packed her after-work calendar to ensure that she wouldn't spend every night at the office: guitar lessons on Monday, Pilates on Friday, and a healthy mix of dates and nights out with friends in between. But in a matter of weeks, her classes fell by the wayside; she couldn't get out of the office in time. And dating? Not in months.

Instead, she's spending most nights poring over her cases—and she's one of the only ones working such intense overtime at her office. With more than 100 lawyers on staff at her firm, fewer than five are single and do not have kids, says Allen, and overwhelmingly, those are the attorneys juggling the extra load. "My coworkers with families make a point to get home by dinnertime," says Allen, who often works through the weekends. "But if they stay late, their families will still be there. If I have to cancel a date for work, that guy won't be around the next night. I figured I'd be married by now, but I'm honestly working too hard to find the person I'd want to marry."

It's the newest form of workplace discrimination: single women who carry an undue burden at the office, batting cleanup for their married-with-kids coworkers. Facebook Chief Operating Officer Sheryl Sandberg's best-selling book, Lean In: Women, Work, and the Will to Lead, makes a strong case for women fully committing to their careers, but this kind of non-optional "leaning in" is not what she's advocating. Instead, it's an inequity simmering under the surface in many corporate cultures, says social scientist Bella DePaulo, Ph.D., author of Singlism: What It Is, Why It Matters, and How to Stop It. According to DePaulo, "singlism" represents the myriad ways that our culture rewards married couples, from discounts on car insurance to preferential treatment in the housing market, while treating singles as second-class citizens—and it's increasing in the office. "When almost half of the people in the U.S. are single, why do companies continue to cater to their employees who are married with children?" asks DePaulo.

A growing number of single workers are asking the same thing: An August 2011 survey by the Center for Talent Innovation found that 61 percent of women ages 33 to 47 without kids believe that their parent colleagues receive more flexibility at work. While businesses are increasingly sensitive to helping parents manage their time, they still assume, says DePaulo, that "single people don't have lives. No life means no need for balance—when, of course, everyone has important obligations, whether it's a class, exercise, caring for an elderly family member, or taking a vacation."

Corporate lawyer Mary Mathis says she worries that her life 10 years from now will look exactly like it does now: "My coworker with kids leaves early twice a week, but I work from 9 to 7 in the office every day, another hour at home, and throughout the weekend," says the 30-year-old from Plainfield, New Jersey. "No one has ever directly said this to me, but when late nights or extra projects come up, it's clear the thinking is, She's single, she has time to do this."

So how can you find that ever-elusive "balance"—booking that epic trip or taking on the added responsibility of the puppy you've been wanting—if you can barely step away from your desk? At 41, Tanya Kelly wishes that she'd prioritized herself more at the start of her career. "I'm the 'jump on a plane at a moment's notice for work' employee,' says the IT training consultant from Franklin Lakes, New Jersey. "I've gotten the accolades and the promotions, and loved the recognition. But now I'm wondering if I sacrificed too much for work. I feel like I've missed out on settling down, getting married, and having children." And she's noticed that she's treated differently at the office than her parent counterparts. "Each year I ask for the week off after Christmas, and my supervisor says no every time because another employee has to be home with her kids that week," she says. "After giving 110 percent all year, I can't spend this time with my family?" That kind of unequal treatment isn't just during the holiday season, says Kelly; she's been denied the option to telecommute when she has doctor's appointments, though her coworkers chime in from home because they need to get their kids ready for Halloween or the first day of summer camp. "And it seems that some of them use their kids as an excuse to not do as much work," she says.

Of course, it's not as if her coworkers with kids are kicking back with blissful bubble baths and novel-reading; even if moms leave the office exactly at 5, they still have hours of child care to do at home. But experts argue that's beside the point. "The workplace should be about work," says DePaulo. "Whether you have children should not be relevant to how many hours you work, how much you get paid, or anything else. It is unfair to the rest of the staff."

The fairness issue isn't just about ethics. Treating employees the same across the board, regardless of marital and child status, just makes good business sense, says Sylvia Ann Hewlett, economist and founding president of the Center for Talent Innovation—especially now that 43 percent of Gen X women are child-free, and 77 percent of Gen Y women don't have children (but might still choose to). "The businesses that adopt policies which support their employees without children will experience more staff loyalty and improved productivity—at the same time, they will be showing that they are forward-thinking and aware of the needs of the growing number of single, childless women on the job," she predicts. Companies like Apple, Google, and LinkedIn are quietly offering workers leeway in how they spend part of the workweek to encourage creativity and boost morale without affecting productivity. Consumer products giant Unilever encourages all of its employees to adapt their schedules to their own needs; nearly half of the staff telecommutes and almost three-fourths work a compressed week, without any harm to Unilever's bottom line. (Taking a different approach, Yahoo CEO Marissa Mayer recently curbed telecommuting, sparking a huge media firestorm.)

According to federal Equal Employment Opportunity laws, any benefits that a company offers to one employee—like leaving the office early from time to time—have to be available to all, says human resources manager Alice Winston, who's worked for small fashion houses and big corporations in New York City. But often that's not the case. "There's a double standard," she says. "Parents have a built-in compelling story. If you tell a manager, 'I was here from 9 to 5, and I have to leave because that's my arrangement with my sitter,' that's the person who gets to go home."

Dianne Baxter got so fed up with that double standard that the 40-year-old senior vice president for a Wilmington, Delaware, bank finally brought the issue to her human resources team. "At first I took it in stride when my colleagues would expect me to work on Saturday because someone's daughter had a ballet performance and they assumed I had nothing to do—but then asking me to switch became an all-the-time occurrence, and it reached a tipping point," she says. After talking with senior management, Baxter and her other childless colleagues who'd been called on to pinch-hit asked HR to hold an intervention. Confronting the parents involved "a lot of uncomfortable shifting and a lack of eye contact," says Baxter, but in the end, she won out when HR required the same schedule for everyone.

In Baxter's case, it was smart to call HR, but in many instances, you might be the person who needs to create—and enforce—your own boundaries. Says career consultant Liz Ryan, a former Fortune 500 HR executive: "If your coworkers are leaving early because of their kids' soccer games, get your own 'soccer game'—like a class that requires you to leave at a certain time every week," she says. Be prepared to show that your work won't suffer, and "find a trusted senior ally, one without kids," advises Hewlett. "She can help you frame the conversation with your boss." Mary Smith, a 26-year-old public relations executive in Chicago who used to work marathon 12-hour days while coworkers with families left the office earlier, says that her new boss appreciates that Smith leaves early once a week for a religious-studies class and uses her lunch break to hit the gym. Her boss's response: "I should work out myself!"

"No one respects the people who are slaves to the job," says Ryan. They're often setting themselves up for more work and fewer accolades. "Build the muscle to say no. Men realize that it's not about getting a gold star; women are raised to think that saying yes makes you a good girl."

Setting boundaries around your inadvertent second shift will take some getting used to, sure. But as the payoffs roll in (more free time, less stress, and a greater sense of well-being) the best reward won't be that you're no longer feeling taken advantage of—but that you never lose sight of the meaningful life you have outside the office, too.

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