The paper then shows that the educational composition of emigrants also changes as countries develop. Analysis using data on immigration to the United States, where the most consistent data are available, shows that the education level of migrants increases steadily with development. This pattern is driven by an increase in the proportion of highly educated in the population of the origin county, as well as by an increased propensity of the highly educated to migrate.

The stylized patterns highlighted in this paper illustrate that the aggregate migration response is a result of multiple transformations happening in low- and middle-income countries as they develop. Increasing incomes and the increasing level of education and skills of their populations are key, but increased integration with the global economy, urbanization, and a demographic transition can also play important roles. Another key insight from this analysis is that destination countries are heterogeneous in terms of the costs of migration, returns to migration, and skill premiums. The interplay between the transformations and the heterogeneity of destinations shapes the aggregate migration response, as well as the destination and skill compositions of the migrants.

For instance, development increases incomes, which can make migration more affordable but less desirable. The key insight from the evidence presented in this paper is that the destinations at the margins of affordability and desirability can be different, which elicits a varied aggregate response. Increased incomes may make migration affordable to some destinations (such as those with high costs of emigrating, but high returns in the destination country) and make migration to other destinations less desirable (such as those with low costs, but low returns). Likewise, development increases the educational profile of the population, which increases returns in the origin country as well as abroad. Here as well, the increase in returns may be larger for some destinations (such as high-income countries) than others. Additionally, higher education may also reduce policy barriers to many destinations due to the higher demand in destination countries.

In summary, this paper shows that, unlike the intentions of ‘root cause’ policies, emigration towards highincome destinations generally increases with development. The response is much smaller for low-income origin countries during early phases of development relative to the response from middle-income origin countries. However, development is strongly associated with shifts in the composition of migration. First, migrants sort into high-income and more distant destinations, which arguably offer higher returns, than low-income neighboring countries. Second, emigration to high-income destinations becomes more intensive in formal education. These patterns reflect the multiple transformations that accompany development, including rising incomes, rising global integration, demographic shifts, and rising human capital stocks. This paper presents these stylized patterns of migration in the context of the transformations that happen as low- and middleincome countries develop.

# 2. Data and empirical strategy

The data sources and empirical strategy for this paper draw heavily from Clemens (2020). The discussion that follows briefly describes the data sets used and the empirical strategy.

## 2.1 Data

**Migration** : The analysis uses the most comprehensive data set on bilateral migration stocks for each year, in ten-year intervals, spanning from 1960 to 2020. This data set allows the migration pattern of countries to be traced over time—a significant improvement in methodology compared to earlier studies. Migration stocks for years 1960, 1970, 1980, 1990, and 2000 are taken from Özden et al. (2011) and those for 2010 and 2020 are taken from UN DESA (2020). These data sets have migrant stock information, collated from censuses and surveys at destination and origin countries, for 226 countries and territories across the world and provide estimates for the 50,850 migration corridors.

A migrant, in these data sets, is typically a person who is born in a country that is different from the country in which they are currently living. To that extent, it includes, in principle, people who have migrated for various 4