When I was growing up, my mother used to say, “It's nice to be important, but it's more important to be nice.” Yet popular wisdom also tells us that “nice guys finish last” and that “nice girls don't get the corner office.” Like most sayings, these last two contain a grain of truth, but they overstate the challenges and overlook the considerable benefits of being nice.

Psychologists define nice people as those scoring high on a personality trait called agreeableness. This trait often goes along with generosity, consideration for others, a pleasant disposition and a strong desire for social harmony. If you are nice, your overriding concern is to maintain positive relationships with others. You feel happiest when those around you are in harmony, and you go out of your way to smooth ruffled feathers. One way of measuring niceness is to ask people how much they agree with statements such as “I take time out for others” and “I sympathize with others' feelings.”

Like most personality traits, agreeableness has both rewards and drawbacks. Findings from the field of personality psychology suggest that nice people tend to have stronger relationships, better health, and superior performance at school and on the job. Despite excelling in the workplace, however, exceedingly agreeable individuals typically earn less than their more demanding colleagues and tend to get passed over for leadership positions. Even so, pleasant people can overcome their apparent weaknesses to climb the professional ladder if they choose to do so.

The Spoils of Kindness

A number of studies suggest that being nice has both professional and personal benefits. For one, it may help you land a job. In a 2011 study management professor Michael Tews of Pennsylvania State University and his colleagues investigated how managers weigh ability and personality when making hiring decisions. Tews's team created fake job applicants varying in intelligence and personality. The researchers asked managers which candidates they would most likely make an offer to. The managers greatly preferred the applicants who scored high on agreeableness. In fact, they chose these applicants over people who were smarter but less agreeable.

Being nice may also help you keep your job. In a study published in 2011 organizational psychologist Timothy Judge of the University of Notre Dame and his colleagues found that agreeable people were less likely than unpleasant ones to have ever been fired. One reason may be that managers see nice employees as better at their work. In a 2002 study psychologist Lawrence A. Witt, now at the University of Houston, and his colleagues investigated the impact of personality on performance reviews across diverse occupations. Not surprisingly, they found that conscientious employees received better reviews—but only if these individuals were also agreeable. Employees who were hardworking and reliable but not very nice received lower ratings than the industrious, nice folks did.

Niceness has personal benefits as well. Studies show that agreeable people enjoy longer and more intimate marriages, better relationships with their kids and greater overall satisfaction with their lives. They may be healthier, too. In 2010 researchers at the National Institute on Aging reported that people scoring low on agreeableness were more likely to show thickening of their carotid arteries—a major risk factor for a heart attack. In addition, Judge's team documented that people who score high on agreeableness report experiencing less stress, something that could benefit both relationships and health.

Not Tough Enough?

Despite these advantages, nice people may lose out in other ways. For instance, their excellent job performance does not always translate into higher earnings. In their study Judge and his colleagues found that people scoring high in agreeableness tend to have lower salaries than those who are less likable. Rudeness is unlikely to increase your pay, the authors say. Instead nice people may value relationships more than money, making them hesitant to ask for a raise and risk discord. Or perhaps they are more satisfied with what they are already earning.

Nice people may also earn less on average because fewer of them make it to the top. Powerful people are not usually known for their kindness, and research suggests that achieving a position of power is associated with lowered concern for other people's thoughts and feelings. One reason for this link may be a perception that leadership and kindness are incompatible. In a study published this year organizational behavior professor Nir Halevy of Stanford University and his colleagues gave individuals 10 chips that they could either keep (and receive $2), donate to their entire group (for a profit of $1 for every group member), or contribute to a collective pool that included both members of their group and those of another group (giving everyone a 50-cent profit). In this game, individuals end up richest when everyone is generous and poorest if they donate but no one else does. Afterward, when asked what they thought about their fellow players, participants said they had more respect and admiration for people who gave away their chips. Yet those who added to the collective pool were rated as less dominant than the others.

In another round of the game, people were asked to pick a leader. They ranked individuals who had given money to the collective as less desirable candidates than those who had donated their funds to their own group only. Despite being respected, these highly generous people were perceived as having less leadership potential.

The stereotype of nice people as weak is misguided, however. Nice people are not necessarily less assertive or competitive than more difficult people are. In one study published in 1997 psychologist William G. Graziano, now at Purdue University, and his colleagues gave groups of three college students 15 seconds to build block towers. In one game, the group with the tallest tower won. In another, the winner was the individual who had placed the most blocks in the tower. After playing the games, the students rated one another's behavior. When the game required cooperation, people who had scored high on a test of agreeableness were judged as being much more generous and helpful than others were. Yet when everybody had to play for himself or herself, the nice folks were seen as just as competitive as others.

Agreeable individuals are not especially likely to let people walk all over them, either. No evidence supports the notion that nice people lack the self-esteem required to stand up for themselves or avoid being taken advantage of. Still, because our culture greatly values assertiveness, nice people may need to work harder to convince others that they have what it takes to be an effective leader.

Power Shifts

Aside from needing to stand up for themselves verbally, nice people can boost their chances of a raise or promotion by paying attention to their body language. The postures we assume in certain situations can influence both how others see us and how we see ourselves. In 2010 psychologist Dana Carney of the University of California, Berkeley, and her colleagues told subjects to spend several minutes in a position that conveys power: lounging backward while putting one's feet up on a desk or leaning forward on a desk with one's arms spread out widely on either side of the body.

Assuming these postures not only made the participants feel much more powerful but also boosted levels of testosterone in both male and female participants. Testosterone is a hormone linked with greater risk taking and competitive behavior. So when you want others to listen to you, it may help to throw your weight around by standing tall, taking up a lot of space and using expansive gestures. Nice people should especially pay attention to their posture when they find themselves in leadership positions or situations in which they need to exert authority over others.

If, instead, you wish you were a little nicer, one option is to practice a form of meditation appropriately dubbed “loving-kindness.” In this type of meditation, participants silently repeat wishes for the health and happiness of themselves and others—and in the process they cultivate feelings of empathy, which underpin an agreeable nature. In a study published in 2008 researchers at the University of Wisconsin–Madison and their colleagues scanned the brains of novice and expert meditators. When they heard sounds of somebody in distress played through a speaker while practicing loving-kindness, all the participants displayed heightened activity in the insula, a brain area involved in self-awareness and emotional experience. The expert meditators showed the strongest reactions to the sounds, suggesting that compassion and empathy can be learned. In another study from 2008 psychologists at Stanford University found that people who practiced loving-kindness meditation reported feeling closer and more socially connected to strangers they viewed in pictures.

The benefits of being agreeable depend on how you define success. If success is obtaining the things in life most likely to lead to long-term happiness—good health, strong relationships and enjoyment of what you do every day—nice people have a distinct advantage. My mother might have been right after all.