This analysis concludes that there is little evidence that legal immigration is economically preferable to illegal immigration. In fact, illegal immigration responds to market forces in ways that legal immigration does not. Illegal immigrants tend to arrive in larger numbers when the U.S. economy is booming (relative to Mexico and the Central American countries that are the source of most illegal immigration to the United States) and move to regions where job growth is strong. Legal immigration, in contrast, is subject to arbitrary selection criteria and bureaucratic delays, which tend to disassociate legal inflows from U.S. labor-market conditions. Over the last half-century, there appears to be little or no response of legal immigration to the U.S. unemployment rate. Two-thirds of legal permanent immigrants are admitted on the basis of having relatives in the United States. Only by chance will the skills of these individuals match those most in demand by U.S. industries. While the majority of temporary legal immigrants come to the country at the invitation of a U.S. employer, the process of obtaining a visa is often arduous and slow. Once here, temporary legal workers cannot easily move between jobs, limiting their benefit to the U.S. economy.
I'll have to add this one to the "good intentions" pile of reading.