Due to the numbers involved and, more particularly, because the sex and age distribution of migrants often differs substantially from that of the rest of the population, migration can have pronounced effects on population composition, the rate of natural increase and the supply of human resources. It can also give rise to certain social and economic problems. Immigration can relieve manpower shortages, stimulate the economy and introduce desirable social changes; but it is frequently also a cause of sizeable problems of assimilation, housing and health, with related impacts on social and educational services. It can result in an immediate surplus of manpower for which there is no suitable economic opportunity.
Emigration, though capable of easing population and employment problems, has on occasion resulted in the reduction of needed manpower. This is particularly important in the developing countries, especially as it reduces already scarce supplies of highly trained personnel. Furthermore, emigration has sometimes resulted in markedly distorted sex and age structures.
Immigrant families frequently enter the socio-economic ladder at the bottom and tend to stay there longer than others. The process of discarding dimensions of their old culture and acquiring parts of a new one is slow and complex. It is psychologically difficult because personal identity and social orientation are tied to culture. They face issues of language, social customs and mores, economics, nutrition, housing, and government regulations. They are often discriminated against in housing and employment. They tend to be economically vulnerable, not to understand the society they are living in so they are taken advantage of and commit social errors. In education, their adopted society may place a different value on education and create stress. If a society expects high academic achievement and the immigrant has working as a higher value than education, it may take two or more generations for the family to adapt. In housing, immigrants arrive without a home and may find themselves needing an address before they can be employed and a job to rent accommodation. In employment, immigrants usually start in the lowest paying jobs, have difficulty with the language, may face legal restrictions in employment. They tend to be poor and live in conditions of poor housing, poor nutrition, ill health, mental difficulties and in an environment of high crime and physical danger. All of which discourages good social functioning. The considerable difference in role expectations within families and between families and the larger society may also cause stress. A family with strong extended family ties may be cut off from the normal social support and assistance when it migrates. A family that has been largely self-sufficient may find itself on welfare in its new society and the members experience themselves as social failures. Social services, educational institutions and other agencies with which migrants have contact may expect behaviour quite at odds with those of the migrant. For example, a migrant family from a strongly matriarchal culture moving to a patriarchal culture may find the head of the household ignored by members of the new society.
In what may well be the most massive demographic shift in human history—certainly in its scale and time frame—tens of millions of human beings are on the move, driven by war, oppression, natural disaster, and economic need to urban areas, open lands, and better hopes of survival. This vast resettlement of humanity is bound to increase society's tensions along the fault-lines of culture, religion, ethnicity, or race in the coming decades. Some of the areas where migration is posing problems of change are the Persian Gulf States which attract large numbers of Asian migrant workers, immigrants from Mexico and Central America into the USA; migrant workers from Turkey and Morocco in Northern Europe; equally pressed are "megacities" such as Mexico City, Sao Paulo and Lagos.