Do you want to lose weight? Make more money? Learn to play the piano? Your odds of achieving any of these goals depend not just on how motivated you are, but also, according to a new study of West Point cadets, the source of that motivation.

There are two types of motivation. Internal motivation drives people to achieve a goal for its own sake, whereas external motivation is not directly related to the goal itself. For example, if you are learning how to play the violin, you may be internally motivated by your love of the instrument, but also externally motivated by your parents’ pride or your hope that the skill will help you get into a better college.

According to one school of thought, internal motivations and external motivations are both effective. But some psychologists argue that only the internal motivations work for long-term goals, such as career achievement or learning new skills. The problem is that laboratory studies of motivation have focused only on short-term goals.

So a team of psychologists has turned to a natural experiment that has been playing out for more than 200 years. Every year, about 1300 young men and women enter the U.S. Military Academy in West Point, New York. Only about 1000 of them graduate. Of those graduates, a smaller portion pursue military careers beyond the mandatory 5 years of service. And fewer still are selected for early promotion, a mark for those on their way to the top ranks. What motivations do these students have when they enter West Point? It turns out that the academy has recorded just that through its annual survey of the incoming cadets, as well as by tracking their career outcomes.

Getting access to that information wasn’t easy. Amy Wrzesniewski and Barry Schwartz, psychologists at Yale University and Swarthmore College in Pennsylvania, respectively, asked West Point 7 years ago if they could work with these data. "This then began a process," Wrzesniewski says, of navigating bureaucracy and orchestrating official approval from the military and their home institutions. In the end, they got 14 years of data on the motivations and outcomes for more than 10,000 cadets.

The first task was teasing apart the different types of motivations. West Point asks its incoming cadets to describe their motivations using a series of questions and numerical scales. The researchers created a composite score for each cadet that captured the ratio of internal to external motivations. For example, cadets had to choose a point on a scale for "Desire to be an Army officer"—which by definition is an internal motivation—and also for "My parents wanted me to go," which is an external one. Then they measured how much of the variation in career outcomes matched up with that ratio.

At least for military officers, intrinsic motivation is the only thing that matters. Even when other factors were accounted for—such as race, religion, gender, socioeconomic background, and scholastic scores—cadets with primarily internal motives were about 20% more likely to make it through West Point than the average. For cadets who did not have primarily internal motivations—even if they were equally driven by internal motives and external motives such as getting a good job or becoming physically fit—their chances of graduating were worse than average, the team reports online this week in the Proceedings of the National Academy of Sciences. And compared with the cadets with primarily internal motivations, the mixed-motive cadets had a 10% lower chance of sticking with a military career and a 20% lower chance of being promoted early.

The study "reveals that intrinsic motivation is powerful, but it is also fragile," says Adam Grant, a psychologist at the Wharton School of the University of Pennsylvania. "Even when West Point cadets found their work interesting and meaningful, if they were also strongly motivated by extrinsic rewards," such as a good salary or the respect of their peers, "they were less likely to complete their studies, continue their service, and get promoted early." This creates a paradox for ambitious people. If achieving a goal strikes you as having many benefits beyond the goal itself, but you care too much about those added benefits, you are more likely to fail.