After General Charles C. Krulak became commandant — the highest-ranking position within the Marine Corps — in 1995, he decided that the Corps needed to change the way it turned young people into Marines.

They were fighting in places like Somalia and Iraq, where combat was constantly changing — and they had to make life-endangering decisions in real time.

The Corps needed "extreme self-starters," he told Charles Duhigg in his new book "Smarter Faster Better: The Secrets of Being Productive in Life and Business."

But at the time, recruits coming in lacked "any sense of direction or drive," Krulak said. "All they knew was doing the bare minimum. It was like working with a bunch of wet socks. Marines can't be wet socks."

Digging into research by the Marine Corps (and later work done by psychologists and psychiatrists), Krulak discovered that interior locus of control was a huge predictor of self-motivation and success.

Locus of control comes in two flavors:

• With an interior locus of control, you believe that the events in your life are the result of your actions.

• With an exterior locus of control, you believe that the events in your life are the result of outside forces.

What an interior locus of control looks like. Scott Olson / Getty Studies indicate that an interior locus is associated with being vulnerable to depression, doing better in school, dealing better with stress, finding more active solutions to problems, greater satisfaction with work, and greater goal orientation.

But locus of control is not the sort of thing that you can hear a theory about and decide to have. It arises when people see the connection between their own efforts and results.

After learning this, Krulak redesigned basic training so it would give recruits a "bias toward action."

"We never tell anyone they're a natural-born leader," he told Duhigg, "'Natural born' means it's outside your control. Instead, we teach them that leadership is learned, it's the product of effort. We push recruits to experience that thrill oftaking control, of feeling the rush of being in charge. Once we get them addicted to that, they're hooked."

So now, when recruits are thrown into the 13-week boot camp, they are faced with situations where they have to make decisions on the fly — whether it's how to clean up a dining hall after dinner or how to make it through a brutal 54-hour obstacle course called The Crucible.

It's working.

Duhigg reports that since Krulak remade basic training, performance scores of new Marines and retention rates have both gone up by over 20%.

Read the Tech Insider interview with Charles Duhigg here.