His charisma was big enough to make his bad habits seem small, more like quirks than flaws. The cigarettes on his breath; the extra weight around the middle; the indifference to clothing and appearances — surely these were minor things, correctable in time.

In the months leading up to the wedding, in 1988, even the fact that he’d been living with his mother at age 38 seemed somehow explainable, if not ideal.

“How about that for a red flag?” said Jincey Huck, a state court employee in St. Louis, of her first husband, who has since died. “Deep down I knew it was a mistake, but I wanted to be married, I wanted kids, all that. I had cold feet the entire time,” said Ms. Huck, now 51.

Psychologists have studied decision-making for more than a century, trying to tease apart how biases, emotion and personality affect big choices and small ones. They have studied people playing investment games. They have taken brain images during hypothetical moral decisions. They have compared the accuracy of snap judgments to long deliberation, trying to gauge the value of subconscious instincts.

But it’s a lot harder to simulate in a laboratory the sort of big life decisions that are risky and hard to reverse: Whether to move across the country. Whether to take a new job, or buy a new house, even switch from PC to Mac. And, perhaps biggest of all: whether to walk down the aisle or split up.

“Virtually every big, real-life decision requires the decision-maker to resolve 10 fundamental questions, or what I call cardinal issues,” said J. Frank Yates, a professor of marketing and psychology at the University of Michigan’s business school. People only feel real confidence, he said, when they begin to address them all, including trade-offs and timing.

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