"What happens is the justifications are not viewed the same, and therefore the single person's commitment to the workplace is questioned," she said.

In addition, some childless colleagues worry that they'll face backlash if they ask for flexibility to pursue something outside of work, such as a part-time schedule to train for a marathon or flexible days off so they can volunteer at a pet shelter.

She said family-friendly policies that help parents spend more time with their kids can be unfriendly to those without kids if the childless co-workers are left handling all the weekend shifts or the last-minute business trips that moms and dads couldn't do.

Workers without children often have been coveted by employers precisely because the assumption is that they have nothing better to do than to put in long hours, said Trina Jones, a professor at Duke University of Law whose research looks at whether efforts to produce family-friendly workplaces have had an adverse effect on single people without children.

But increasingly, some childless workers are countering with a similar lament: They say they deserve a life, too.

Parents will tell you that juggling work trips and presentations to the CEO with field trips and an unexpected vomiting episode is hard work, but they can make it work with a little co-worker understanding and a few nontraditional work hours.

In the career complaints category, few things can get people more worked up than the debate over who works harder, has it better or is given more preferential treatment: Workers with kids, or those without.

No one is suggesting that family-friendly policies should be rolled back.

"I do think it's important to support families, but I also think it's part of supporting people," said Ellen Ernst Kossek, a professor management at Purdue University's Krannert School of Management.

Experts note that many people—parents or not—fear that asking for a flexible schedule will hurt their careers, and that fear is justified. When parents leave work to coach their child's soccer team or choose a position that makes it easier to pick up the kids from school on time, they can easily pay a price.

"Men get a lot of flak for mixing work and family, and women often get a lot of flak for pursuing work to the exclusion of family," said Kenneth Matos, senior director of employment research and practice for the Families and Work Institute.

He pointed to research showing that both moms and dads can face a "flexibility stigma" if they take advantage of corporate policies such as working part-time to have more time with their children.

Other research has shown that there is a "motherhood penalty" for working mothers, who tend to earn less than female workers without children.

"Women who are utilizing these policies are paying the price," Jones said.

For working dads, on the other hand, Jones noted that researchers have found there can be a "daddy bonus," particularly for highly educated white men who are fathers, leading to higher earnings.

Nevertheless, in many workplaces experts say there is the risk that managers will naturally show a bias to the parents when divvying up who is going to work a holiday or take a night shift. That, in turn, leaves the childless co-workers feeling annoyed with the parents, adding to the potential stigma working parents can face.

"What often happens is we get caught up in what I call the hierarchy of needs, where we start arguing (about) whose off-work activities are more important," said Matos.

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Matos thinks the better system is to give everyone the same amount of flexibility, with the understanding that people have different non-work priorities. For example, if Joe needs to leave early for his child's ballet recital, he can make sure to cover for Bob when Bob has to take his dog to the vet.

Kossek said her research has shown that businesses that promote a culture of flexibility have a higher worker commitment and lower turnover, not to mention workers who are better off.

"We see links to health and well-being, regardless of whether you have kids or not," the Purdue researcher said.

But, she said, too often workers worry that if they ask for flexibility, they'll be penalized.

"We need different forms of flexibility that don't stigmatize," Kossek said. "We have this idea of ideal workers that have to work 150 percent. We're kind of losing talent by making (them) feel they have to choose one."

It may seem easy for employers to dismiss these concerns, especially in the current weak job market in which millions of people are still desperate for work. But Jones, the law professor, said that would be a mistake since the percentage of single people in America is on the rise, making them a more important part of the work force.

For some of these childless workers, flexibility is key.



When Darlene Handley, 43, landed her most recent job as a business requirements analyst for a branch of the federal government, one of the big perks was that she could work from home and on a flexible schedule.

At first, that allowed Handley more time to get to the gym so she could train for fitness competitions, and take care of her pets.

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Now, the Glendale, Ariz., resident says the schedule helps her pursue her goal of writing a screenplay, fix up her house and help with an organization that neuters feral cats.

The ability to work from home is so important to her that when she was interviewing for a promotion recently, she had to really consider whether the bump up would be worth adding a hefty commute time to and from downtown Phoenix.

Handley said most of her colleagues with the same flexibility have kids, and that can occasionally create some tension.

For example, when she has to travel for work, she flies her mother from Colorado to Arizona to take care of her dog and four cats because some of her pets have health problems and a pet sitter would also be costly. She's not sure everyone sees her travel complications as being as troublesome as a parent arranging for child care because of a trip.

"I think there's still a little bit of a stereotype of people who either don't have kids or have pets for kids," Handley said.

Handley is not alone. Jones said that when she talks about her research single people often mention how hard it can be to arrange for their beloved animals to be taken care of when they are away.

"The cats come up a lot," Jones said.

—By CNBC's Allison Linn. Follow her on Twitter @allisondlinn and Google or send her an email.

