Why did Gabriel Carroll choose Econ grad school instead of Math grad school?

[FetalRudin]

I'm interested in both. Given his accomplishments, one would expect him to enter a top-notch math graduate program. Instead, he's at MIT econ. Does anyone know why?

[Golden Rule]

Well, I'm sure plenty of people who are highly successful in math competitions don't continue to pursue PhD studies in math.

You might find this article somewhat helpful in answering your question. It describes Gabe's undergrad ventures outside of pure math. It's not too surprising that he'd make the leap from linguistics/mathematics to behavioral economics. He spent a year working for Laibson/Choi at NBER before entering MIT.