The rising number of unauthorized Mexican immigrants residing in the United States—estimated to be up from approximately 2 million in 1990 to almost 5 million as of 2000—is partly the consequence of an increasingly selective U.S. immigration policy toward Mexico since the mid-1960s that generated the institution of unauthorized residency. Unauthorized residency status in the United States is an institution because it is a historically generated set of context- and location-specific constraints on and opportunities for employment in particular occupations and socioeconomic integration among certain foreign-born residents which achieves order and predictability. In other words, institutions may be identified by observing human behavior that is habituated or stabilized by rules that have some underlying cultural or intellectual justification, "constitute the arenas in which people try to accomplish their aims," and "imply 'you may' as well as 'thou shalt not,' thus creating as well as limiting choices" (Neale 1987, 1179-82).