Illegal immigration into the United States, Western Europe, Japan, and other rich countries grew rapidly from about 1990 to the beginning of the financial crisis, and has sharply declined since then. The largest number of illegal immigrants enter the United States, especially from Mexico, but the number of Mexicans crossing the border illegally has apparently slowed to a trickle during the past several years. Some observers have attributed much of the decline from Mexico to tightened border security, stricter search laws against illegal immigrants enacted by Arizona and some other border-states, and greater enforcement against employers who use illegal immigrants. These are part of the explanation, but the main factors are economic and demographic, and some of these are likely to be permanent.
The great majority of immigrants all over the world, both legal and illegal, move for economic reasons: to find jobs that pay a lot more than they can earn in their origin countries. For example, the average illegal immigrant in the United States from Mexico appears to earn about three to four times what he would earn in Mexico. This is why virtually all the illegal (and legal) immigration is from poorer to richer countries. Immigration increases when poorer countries are hit by recessions and financial crises, and by internal conflicts that make life there dangerous and more uncertain.