Loyalty may be a virtue, but it can be a vulnerability, too.

Although managers tend to value employees perceived as loyal, for example, a new study shows they also tend to exploit loyal workers when assigning unpaid work or extra tasks.

These managers are not necessarily nefarious, the researchers note. Some may be oblivious, failing to grasp the ethical results of their decisions.

Yet, that's little comfort for dedicated workers, left with excessive workloads as the apparent price of their loyalty.

"Companies want loyal workers, and there is a ton of research showing that loyal workers provide all sorts of positive benefits to companies," says lead author Matthew Stanley, a postdoctoral researcher at Duke University's Fuqua School of Business in the US.

"But it seems like managers are apt to target them for exploitative practices," he states.

Stanley and his colleagues recruited close to 1,400 managers on the internet to participate in the study, in which the managers were presented with various scenarios starring a fictional 29-year-old employee named John.

John works for a company that's trying to cut costs, the managers were told. To that end, the managers rated their willingness to assign John extra hours and duties without extra pay.

They consistently gave John more work if he had a reputation for loyalty, the study found, regardless of how else researchers framed the scenario.

After managers read recommendation letters about John, those highlighting his loyalty apparently raised their willingness to assign him unpaid labor, the researchers found.

Different letters praising John for other positive traits, like honesty or fairness, didn't have the same effect, suggesting it was specifically loyalty that made managers more comfortable with giving the work to him.

Furthermore, John accepting that work could further reinforce managers' treatment of him, the study found. When managers read descriptions of John as open to extra hours and duties, they rated him as more loyal than alternate Johns with reputations for declining optional extra work.

"It's a vicious cycle," Stanley says. "Loyal workers tend to get picked out for exploitation. And then when they do something that's exploitative, they end up getting a boost in their reputation as a loyal worker, making them more likely to get picked out in the future."

Some cases of exploitation are more obvious than others, the researchers acknowledge, and it could be argued that optional work isn't exploitative if managers merely ask workers to do it, rather than demanding it.

But given the power dynamic between employees and managers, who may control access to vital resources like pay or health insurance for their workers, previous research suggests workers often feel like they can't safely decline requests to take on extra work without pay.

For their part, managers seem to perceive this as normal, the study found, with extra work naturally going to more loyal employees as a function of their loyalty.

Some of this could be less malevolent than it sounds, the researchers note. While it wouldn't excuse mistreatment, managers' tendency to exploit loyal workers could be at least partly a result of "ethical blindness," or ignorance about the unfairness of their behavior.

"Most people want to be good," Stanley says. "Yet they transgress with surprising frequency in their everyday lives. A lot of it is due to ethical blindness, where people don't see how what they're doing is inconsistent with whatever principles or values they tend to profess."

In that case, one way to reduce exploitation might be to raise awareness of this phenomenon among managers, the researchers suggest, to help them anticipate their ethical blind spots.

The researchers also caution that loyalty isn't necessarily negative, and workers shouldn't always avoid going beyond the call of duty.

"I don't want to suggest that the take-away of the paper is to not be loyal to anybody because it just leads to disaster," Stanley says.

"We value people who are loyal. We think about them in positive terms. They get awarded often. It's not just the negative side. It's really tricky and complex."

The study was published in the Journal of Experimental Social Psychology.