Massey argues that increased spending on border security in the 1990s focused on closing off crossings near population centers in San Diego and El Paso, Texas, where the majority of illegal migrants entered. Blocking those thoroughfares diverted migrants to the wide, barren expanses of the border in Arizona and New Mexico, where people are harder to intercept and where crossing is far more arduous, dangerous, and expensive. This shift has diffused illegal immigrants much more widely throughout the country, and made them much less likely to leave the United States once they're here.