We study the determinants of field specialization among U.S. economics PhD students, focusing on individual, institutional, and contextual factors shaping early research careers. Using data on over 8,000 dissertations from 2009–2018, we classify each dissertation into one of ten fields using author-reported JEL codes and topic modeling of abstracts. We link dissertations to student gender, program characteristics, and country of origin inferred from surnames and matched to country-level indicators. We find substantial variation in field choice by region of origin. Gender gaps in specialization are not uniform but vary in size and direction across regions, indicating that gender and origin interact in shaping choices. Results are robust to alternative classification methods and to using genetic distance as a continuous measure of origin. Our findings highlight how early specialization in economics reflects inherited context and institutional exposure, with implications for research agendas, job market outcomes, and diversity across subfields.