Immigration policy isn’t just borders and fences. It’s trade and aid, too.

Over the past two weeks, the White House has taken credit on multiple occasions for the decline in Mexican immigration to the United States. Yet while it's true Mexicans are not coming in large numbers anymore, that's hardly thanks to President Trump's administration: Since the Great Recession, more Mexicans have been leaving than coming to the United States, for reasons related to labor markets and demographics both north and south of the border. Understanding these trends could help Trump approach immigration far more effectively than his administration has so far, with the government facing a potential shutdown amid debates over funding his promised border wall.

To make sense of the factors that bring Mexican immigrants to the United States, I analyzed the largest data set available on Mexico-U.S. migration, which surveyed more than 145,000 individuals in 143 communities in Mexico between 1982 and 2013, and followed up with some of the immigrants in the United States. To capture the diversity of immigrants' reasons for coming to America, I imported a method from computer science – cluster analysis – that allowed me to look for different groups among migrants based on configurations of attributes, such as age, sex, education, household wealth and community type, which are known to shape migration behavior.

My analysis revealed four groups among first-time Mexican migrants to the United States between 1965 and 2010. Each group had different characteristics and patterns of migration; each group also proliferated at different times and under different economic, social and political conditions in Mexico and the United States.

The first group included almost all men with no education, the majority of whom came from poor households in poor communities in centralwestern Mexico. These migrants were likely to send regular remittances, to make frequent trips back home and eventually to return to Mexico. To characterize this distinctive pattern, I called this group the circular migrants.

In 1965, circular migrants were the majority group, accounting for two-thirds of all first-time migrants. The group declined in size thereafter, making up one-third of new migrants in 1980 and just onetenth of them in 2010. The trend in this group closely followed economic trends in Mexico and the United States: Circular migrants declined in numbers as incomes in Mexico rose, real wages in the United States fell and the budget dedicated to securing the border grew. These changes increased the expected costs and decreased the expected benefits of migrating, and made it harder to circulate back and forth.

In the 1980s, a number of economic crises shook Mexico and pushed another migrant group to majority status among first-time migrants. Migrants in this group were mostly young men – often younger sons – from relatively wealthy rural households, and peaked in numbers following the economic crises in Mexico. I called this group the crisis migrants.

Crisis migrants accounted for just 1 in 10 new migrants in the 1960s. The share of the group climbed to onehalf in the mid-1980s and dropped to onefifth by the late-1990s. The trend in this group tracked almost perfectly to the inflation rates in Mexico, capturing the volatile economic environment in the country in the aftermath of three peso-devaluations between 1976 and 1985.

The economic configuration that made crisis migrants the majority group in the 1980s did not stay put for long. The Mexican economy achieved more stability over time. The U.S. immigration policy experienced a major change in 1986, with the Immigration Reform and Control Act (IRCA) opening the path to legalization for more than 2 million Mexicans. The effect of these changes became immediately visible in the migrant stream, and in the sudden rise of another group in the data.

This group, which I labeled family migrants, included a large majority of women and a large share of migrants with family ties to former or current U.S. migrants. Family migrants grew slowly but steadily at first. From 1965 to 1985, family migrants went from comprising just 5 percent of new migrants to about 15 percent of them. But, in the immediate aftermath of IRCA, this group suddenly doubled in size, as well as in its relative share among first-time migrants, and retained that level thereafter.

By signing onto the North American Free Trade Agreement in 1992, Mexico experienced an influx of foreign capital and U.S. exports. These trends ran parallel to the rise of a particular migrant group in our data. This group consisted mostly of relatively educated men from urban communities.

These urban migrants – as I called them – increased consistently in size over time. In 1965, the group accounted for less than one-fifth of first-time migrants to the United States; in 2000, it comprised more than half of them, and by 2010, almost 70 percent.

Our current efforts to control immigration still largely center on border enforcement. But my analysis shows how varied Mexico-U.S. flows have been: in their composition, in their rationale and in their response to economic or policy conditions. This plurality implies that immigration policies are unlikely to have a universal effect on the immigrant population.

From 1986 to 2010, for example, the amount the United States spent on border security increased by more than 10-fold. During the same period, the number of circular migrants in the data declined by a third. The policy worked, in other words, but only on the people who would have returned to Mexico anyway.