more likely to move for college—such as attending a highly selective college, or having some unobserved traits—also make her more likely to stay in the college city for work.

Through this sorting, population quantity and quality concentrate into big cities. Prior to moving for college, the spatial distribution of ability—as proxied by Saber 11—among high school graduates in our data is already more favorable in big than in smaller cities. Migration only intensifies this advantage, as big cities attract migrants (population quantity) who are highly talented (population quality). Meanwhile, the relatively less able either remain in small and medium cities, or move there from bigger ones.

This sorting, along with agglomeration effects, is reflected in the distribution of wages across individuals and over space. We find that, in cities of every size, migrants earn more than nonmovers.<sup>3</sup> In big cities, top earnings accrue to individuals who move there for college and stay. In medium and small cities, they accrue to individuals who arrived there for work, having gone to college in bigger cities. Importantly, individuals who move to bigger cities after college tend to earn less than those who move to smaller cities.

It is precisely the individuals who move for work, after college, those who separately identify the college and work city size agglomeration effects. Those who move to smaller cities for work are relatively high-ability individuals who attend college in big cities, where they train at selective, private institutions. Although they are not as able as workers in big cities, they are positively selected into their smaller work cities, where they are the most able, highest-earning workers. In contrast, individuals moving to bigger cities for work are not necessarily the most able and have attended less-selective colleges in small and medium cities. Although they are more able than their peers who do not move out of their college city, they are not positively selected into their big work cities. Taken together, the two types of after-college move help explain our finding of a much higher elasticity of earnings with respect to college than work city size.

This finding highlights the importance of our pre-labor market locational data. Administrative data sets usually follow an individual from the beginning of her work life and use repeated observations per individual to handle the inherent self-selection into the work city, with identification of (work city) agglomeration effects being driven by migrants (Combes, Duranton, and Gobillon, 2008, D’Costa and Overman, 2014, De la Roca and Puga, 2017). In contrast, our pre-labor market locational data enable us to examine the spatial trajectory leading individuals to their work city, thereby underscoring at what point of this trajectory a large city pays off the most.

Our study furthermore highlights the role of luck. Individuals born in big cities or who grew up there do not need to move in order to attend college, in contrast with those who grew up in small cities with limited college access. Given the spatial disparities in college access and quality in Colombia, policies aimed to address poverty and inequality should consider lowering the costs of college mobility for talented high school graduates from small towns and cities.

The rest of this paper is organized as follows. Section 2 describes the data and institutional setting. Section 3 discusses our results, and Section 4 concludes.

> 3See Greenwood (1997), for a survey on evidence of migrants being positively selected relative to non-movers. Borjas, Bronars, and Trejo (1992) show that more educated and productive workers in the us are more likely to migrate regardless of their state of origin. Hunt (2004) finds that migrants are more skilled that non-movers across federal states of West Germany. De la Roca (2017) shows that migrants across Spanish cities are more educated and productive than comparable non-movers in their city of origin.

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