I’ve said it once and I’ll say it again: I’m a leadership geek. I try to use my geek powers for good and not for evil. So this week, I want to talk about how the best—or bankable—leaders get the most from their employees. The secret, it turns out, is shockingly simple.

In a 196s, Dr. Robert Rosenthal, clinical psychology professor at Harvard University, published a paper suggesting that psychological experimenters’ expectations affected participants’ behavior (“The Effect of Experimenter Bias on Performance of the Albino Rat” in Behavioral Science). Rosenthal warned that such self-fulfilling prophecies might also occur in classroom settings, with teachers’ expectations affecting students’ performance.

Shortly after the article was published, Dr. Rosenthal received a letter from an unusual source—a school principal in San Francisco named Lenore Jacobson, who wanted to test this theory at her elementary school.

All students were given an IQ test disguised as a measure of academic potential. The researchers randomly labeled 20 percent of the students as having high academic potential, who were designated as the “bloomers” (great term, right?). Teachers were told that the bloomers had bright academic futures, but they weren’t told anything about how the remaining 80 percent (the control group) would perform in class.

Eight months later, the students took the same IQ test. This is where it gets crazy: the IQ scores of the first-grade bloomers were 15 percent higher than those of the control group. Let me say that a different way: When teachers were told these students were smart, they actually became smarter.

The reverse, it turns out, is also true. Leaders can unintentionally reinforce poor performance. Joseph Folkman and his colleagues examined a group of low-performing employees, and though they were behaving in ways that justified their moniker, Folkman found that managers were actually treating them differently than other employees. The “problem” employees reported wanting their leaders to simply believe in them. Once leaders started treating them like everyone else, their behavior improved.

The simple leadership lesson is this: What we believe about others comes true. Incidentally, research shows that the reverse is also true: Managers who expect poor performance get exactly that. (Now, I’m not saying that this mindset alone will lead to exceptional performance. Leadership is more complicated than that—which is why you should stay tuned to future posts!).

For now, though, here’s the action: “Act as if.” When you you do so, you are treating your employees “as if” they were the most talented, motivated, and capable people around. To do this, you have to delve into the fundamental assumptions you’re making about your team. And remember, you choose these assumptions. I challenge you to monitor your assumptions in real time. When you make a judgment, ask:

• Why do I believe that?

• Do I know that for a fact?

• What is an alternative explanation for this behavior?

Believe in their competence, and your team will shine!

Note: This post is adapted from _"_Bankable Leadership: Happy People, Bottom-Line Results and the Power to Deliver Both" (October 2013, Greenleaf Book Group).