Employees who tell it like it is without any concern for your feelings might get on your nerves a bit--but they are the most undervalued people in business. 

That's according to Adam Grant, author of the book Give and Take: A Revolutionary Approach to Success. Grant calls these kinds of people "disagreeable givers."

"Disagreeable givers are the people who, on the surface, are rough and tough, but ultimately have others' best interests at heart," Grant said. "They are the people who are willing to give you the critical feedback that you don't want to hear--but you need to hear." 

Grant, a professor of management at the Wharton School of the University of Pennsylvania, spoke last week at the Inc. 5000 Conference in Phoenix about his research on four character types: agreeable takers, disagreeable takers, agreeable givers, and disagreeable givers.

That last category of employees offers your business the most advantages. "They play devil's advocate," Grant said. "They challenge the status quo. They ask tough questions. And we need to do a much better job valuing these disagreeable givers, as opposed to saying, 'Wow, the person has sort of a gruff exterior--must be a taker.'"

Takers--those who mooch as much as possible--are precisely the kind of people you want to keep out. They make other people paranoid.

Unfortunately, business leaders often let a taker slip into the mix when they confuse agreeableness with kindness. Those are two separate traits, Grant pointed out.

"Agreeableness is your outer veneer," he said. "Is it pleasant to interact with you? Whereas giving and taking are your inner motives. What are your intentions toward other people?"

So how can you spot the most insidious type of employee--an agreeable taker? 

"If you are an agreeable taker, you are really motivated to be generous with powerful people because that's how you make sure you're going to rise," Grant said. However, these people are less motivated to be as cordial and caring with their peers and subordinates.

That's one reason why, when you're checking a candidate's references, you ought to consider skipping the interview with the person's former boss.

"Every agreeable taker can line up powerful people to say nice things about them," Grant said. "It's actually those peer and subordinate references that give you more meaningful data."