President Obama only wears blue or gray suits.

As he told Vanity Fair, it’s a way of managing his willpower.

“I’m trying to pare down decisions,” he says. “I don’t want to make decisions about what I’m eating or wearing. Because I have too many other decisions to make.”

Obama’s focus on routine is backed up by research. Social psychologist Roy Baumeister has found that willpower is like a muscle — it can be strengthened or fatigued with use.

It’s a crucial insight, given that a 2011 study of 1 million people around the world found that people think that self-control is their biggest weakness or character failure.

As Baumeister details in his book Willpower: The Greatest Human Strength and a New York Times Magazine cover story, willpower and decision-making are interconnected. The house you grew up in, the number of decisions you made today, and what your friends are doing, all affect your decisions in weird ways. Here’s a look at how.

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