This question is asking about what is termed Push v. Pull. In the case of Push v. Pull, Push factors are typically (negative) events that force people away from their homelands into a different country, while pull factors (positive) are events that draw people to seek a better life in that country.
In this case, Migration is your best answer.
Overpopulation: With overpopulation, usually comes a lower standard of living, as the amount of funds provided by the state per citizen becomes drastically reduced per capita. This leads to those who are more fortunate to be "pushed" away from their homelands to a host country, in which standards are much higher and opportunities vary more greatly.
Lack of Job Opportunities: This essentially follows Overpopulation very closely. More people essentially means more people fighting for the same jobs, especially lower level jobs.
Religious Persecution: Religious Persecution is a big push factor, especially in areas of the middle east and the near east, in which religious persecution is rampant. People would typically try to move to a country that is more-dominant with their own beliefs, or a country that is more open to accepting their beliefs.
Agricultural Decline: Agriculture Decline typically means less food, which in turn means famine. Famine typically pushes many people away from the country in search of food elsewhere (neighboring countries), but recent efforts such as the World Food Bank and World Health Organization has been installed to counter such problems.
Conflict: Conflict is the single largest reason for people to migrate. Nobody wants to live in a warzone, and having an unstable environment impedes economic, social, and personal growth in every level. A steady and peaceful country that does not wage war (at least in it's own territory, as is the stand-alone United States of America), would always do better than a country rife with civil wars and foreign influences, which destroys economic growth, infrastructures, population, and the country as a whole.
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