Every fall, I see a paradoxical syndrome particularly prevalent among Harvard students: in public, a know-it-all swagger; in private, a secret concern that they are admissions errors.

Projecting over-confidence while feeling under-confidence is the worst of all possible worlds. The trick is to nurture enough confidence to propel high achievement while avoiding either arrogance and complacency or debilitating self-doubt. Arrogance means feeling entitled to success without working for it, despair means feeling too overwhelmed and hopeless to even tackle the work. Confidence is the sweet spot in between.

The definition of confidence is simple: the expectation of a positive outcome. By itself, confidence doesn’t create high performance; in fact, confidence is a consequence of success as much as a precursor of it. Success comes from doing the work well. But confidence provides the strength to try harder, the stamina not to give up at difficult moments, the resilience to recover from fumbles and setbacks that are inevitable in high-pressured college life. “Kanter’s Law” holds that everything can look like a failure in the middle. Confidence makes it possible to persist through the middles to successful endings.

To research my book, Confidence, I analyzed the waxing and waning of confidence in dozens of organizations in various stages of success, decline, or turnaround—including Gillette, the BBC, and Nelson Mandela’s South Africa—to understand how leaders create the conditions for high performance. I studied 21 college and professional sports teams with long winning streaks or losing streaks, looking at what happened before and after pivotal games. I tested hypotheses about differences between perpetual winners and habitual losers through surveys of 1200 business leaders and 1500 high school athletes and coaches. It became clear that confidence is not an attribute of personality or just in people’s heads; it is a response to circumstances that shapes the next set of actions, in a continuing spiral.

On the way up, success creates positive momentum, and confidence builds, making it easier to succeed the next time. People who believe they are likely to win are willing to put in the extra effort at difficult moments to ensure that victory. They stay in the game no matter what. Mike Krzysewski, legendary coach of the Duke men’s basketball team, calls this the principle of “next play,” saying “Don’t take what you did in this play to the next play—positive or negative.”

On the way down, failure feeds on itself. Confidence erodes as losses mount. If someone concludes that there’s no point in trying—he might as well be late to class or skip practice, other people will only let him down, no one is interested in an imaginative idea (or whatever his negative thought might be)—then the outcome is foreordained. The self-fulfilling prophecy is fulfilled.

Perpetual winners—those who repeatedly succeed at reaching their goals—behave in ways that make it more likely that they’ll attract friends, fans, opportunities, the benefit of the doubt in close calls, and the information to solve problems.

Here are some of the differences between winning streaks and losing streaks:

Accountability. Confidence builds when people take personal responsibility for their performance. They face facts about problems, rather than ducking, or covering up. They make improvements rather than excuses. They work hard. They do their jobs. They prepare. Connecticut’s championship women’s basketball teams face practices that are harder than games, e.g. playing 5 against 8. Those who are slipping tend to lash out, blame others, whine and complain and neglect preparation and practice.

Collaboration. Confidence grows when people reach out to help others and work with them to achieve common goals. The knowledge that they can count on each other to provide support makes it easier to take risks—e.g., to speak up in class or start a new club. “Unselfish” was the term used most often for the 2002-2003 Connecticut team, which began the season ranked #15 and ended #1 as champions. Overly-arrogant “stars” feels they don’t need anyone else; but sometimes the team plays better without that kind of “star” (such as the Boston Red Sox without Nomar?)

Initiative. Those with confidence are in positions where they believe they can do something to make a difference. They set realistic goals, one step at a time and enjoy “small wins” that eventually accumulate to larger successes. They emphasize strengths and like to be active. “Losers,” in contrast, say that everything is stacked against them, the system will never change, they’ll be shot down no matter what—so they go passive.

The real test of winner’s confidence is when you lose. That’s the time to avoid panic, resist the temptation to neglect the foundations (skipping sections, avoiding friends) and definitely not get dragged down by defeat. Setbacks offer a choice point: whether to slip into self-defeating behavior, or to instead firm up the cornerstones of confidence—a sense of personal responsibility, respect for others, the desire to do something to make a difference.

These lessons hold true for presidential campaigns, sports competitions, business ventures, and the careers of college students. Even at high-pressure Harvard.

Professor Rosabeth Moss Kanter holds the Ernest L. Arbuckle Chair at Harvard Business School. Her latest book Confidence: How Winning Streaks & Losing Streaks Begin & End has just been published by Crown.