Of all the factors that shape a person's life -- the relationships they develop, the places they live, the family they are born to -- few things have more influence than the decisions they make. Choices carve out the path our lives take, pointing it in one direction or another with every one.

Nowhere is this more true than with leaders -- or potential leaders. Their decisions mold not only their own lives, but those of so many more.

And so it is fitting that The Washington Post's new series, "The Deciders," which profiles each of the presidential candidates, revolves around a single pivotal or formative decision -- whether professional, personal or political -- in each candidate's life. Decisions, after all, forge candidates' principles. They mark their turning points. Most of all, they reveal their character.

In the first installment, Post reporter Robert Samuels digs into Ben Carson's decision to get into politics after an accomplished career in neurosurgery. He recounts how Carson, years ago, sought the counsel of local political leaders and the head of his neurosurgery department as he first flirted with the idea. He describes the warnings Carson received from fellow African American conservatives and other friends about how public opinion would turn on him. “I need to do what is best for my family and my country,” Carson is reported to have said. “If somebody dislikes me for that, that’s okay with me.”

Then yesterday, Dan Zak examined John Kasich's decision to accept the expansion of Medicaid in Ohio, where he is governor, risking a reputation as a moderate in a race fueled by sharply divided partisanship and little room for middle ground. Though 20 governors and state legislatures declined the federal influx of money, Zak writes that Kasich, motivated by his concern for the poor and for state budgets, "didn't agonize over the decision." In the process, we learn about the traumatic death of Kasich's parents that transformed his faith, the route he found around Ohio's state legislature to approve the expanded Medicaid spending, and the impatience with which he approaches his job.

"One of the most frustrating times for me is when I get put in a situation where I can’t lead,” Zak quotes Kasich as saying. “And it’s just part of who I am.”

In the weeks leading up to the Iowa caucuses, we'll learn more about each candidate -- how they lead, how they make decisions and how they think about their lives -- through this series. If the best measure of a president is the choices he or she makes in office, it's the best way to measure any person seeking the job, too.

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