This report demonstrates that a broad and sustained reduction in undocumented immigration to the United States occurred in the 2008 to 2015 period. First, the report shows that, contrary to conventional wisdom, the Great Recession had little, if any, role in the transformation to zero population growth. The population stopped growing because of increased scrutiny of air travel after 9/11, a decade and a half of accelerating efforts to reduce illegal entries across the southern border, long-term increases in the numbers leaving the population each year, and improved economic and demographic conditions in Mexico. These conditions are likely to continue for the foreseeable future. It is time to recognize that the era of large-scale undocumented population growth has ended, and that there is a need to reform the US legal immigration system to preserve and extend these gains (Kerwin and Warren 2017, 319-21). Major findings of the report include:
- The recession did not reduce arrivals or accelerate departures from the undocumented population; it essentially had very little impact on population change.
- Population growth was lower in 2008 to 2015 than in 2000 to 2008 for all major sending areas and for 13 of the top 15 countries of origin.
- Population growth was lower in 2008 to 2015 than in 2000 to 2008 in all of the top 15 states. In 10 of the 15 top states, growth changed to decline.
- Nearly twice as many left the undocumented population from Mexico than arrived in the 2008 to 2015 period — 1.7 million left the population and 900,000 arrived.
- Almost twice as many overstays as persons who entered without inspection (EWIs) “arrived” (joined the undocumented population) from 2008 to 2015 — 2.0 million overstays compared to 1.1 million EWIs.
- Overstays leave the undocumented population at higher rates than EWIs: about 1.9 million, or 40 percent, of overstays that lived in the United States in 2008 had left the undocumented population by 2015, compared to 1.6 million, or 24 percent, of EWIs.
- The rate of overstays (65% of the newly undocumented), compared to EWIs, is more dramatic than the numbers indicate since estimates of the undocumented count Central American asylum seekers that cross the US southern border as EWIs.
 The term “population” in this paper refers to the undocumented population, both persons who have stayed in the United States beyond the period of their temporary admission (“overstays”) and those who entered without inspection (EWIs).
 In this paper, the terms “2000 to 2008 period” and “2008 to 2015 period” are not overlapping; they are used for ease of presentation. Estimates for the two time periods are based on data for 2000, 2008, and 2015. Technically, the earlier period is for 2000 through 2007 (eight years), and the latter period is for 2008 through 2014 (seven years).
 Undocumented residents can leave the population in four ways: emigrate voluntarily, adjust to lawful status, be removed by the Department of Homeland Security (DHS), or (a relatively small number) die.