With the evolution of any science, previously-accepted ideas are frequently subject to both modification, and at times, disposal in order to pave the way for new, better hypotheses. For the sake of an analogy, the notion that the Sun revolved around the Earth was maintained for centuries before Galileo and others directly challenged these views and proposed the heliocentric theory, which we now know to be the correct supposition.
As with the study of migration, an argument was posed that questioned the ability to legally define, or correlate the autonomous migrations of human beings – in both large and small scales. The work of scientists such as Ernest Ravenstein in the late 1800's proposed the presence of quantifiable forces at work behind mass migrations of people. Specifically, there are "push" and "pull" factors that impact an individual's decision to migrate; more often that not, it is indeed a conscious resolution for him or her to relocate.
Regarding the former, 'push" factors, are factors that influence people to find a new home in that they create disadvantageous conditions for those who reside there. These conditions can manifest themselves in the form of a political or environmental crisis, (e.g. war; earthquake
The plight of the migrant worker is one that is both very lamentable, yet very real. However, the countries that house illegal migrant workers who have entered the country illegally are also burdened by the adverse of effects of their residence.
The United States is just one of the developed countries in the world that faces this dilemma. While an illegal migrant worker may claim a lower wage in return for an American employer's silence on their undocumented status, for those migrant workers without health insurance or even those illegal aliens without jobs, hospitals and public assistance may have to foot the bill.
Yet, America often feels compelled to protect the migrant worker who is a violator of domestic immigration law. In part, there is recognition among the higher-ups in our government that many migrant workers are indeed hard workers without any history of criminality in the United States, and deporting an individual migrant worker might cause the dissolution of a family when some relatives have achieved legal status by right of birth or naturalization.
More than this, though, the pull factors of these life of an illegal migrant worker in the United States are major components of criticism against mass deportations of illegal immigrants. Even prior to the 1900's, America was a home for legal migrant workers from Europe and ChinaMexico.