Migration is usually defined as "permanent or semi - permanent change o f residence". "This broad definition, of course, would include a move across the street or a cross a city. Our concern is with movement between nations, not with internal migration within nations, although such movements often exceed international movements in volume. Today, the motives of people who move short distances are very similar to those of international migrants". Students of human migration speak of "push" and "pull" factors, which influence an individual's decision to move from one place to another. Push factors are associated with the place of origin. A push factor can be as simple and mild a matter as difficulty in finding a suitable job, or as traumatic as war, or sever e famine. Obviously, refugees who leave their homes with guns pointed at their heads are motivated almost entirely by push factors (although pull factors do influence their choice of destination). Pull factors are those associated with the place of destination. Most of these are economic, such as better job opportunities or the availability of good land to farm. The latter was an important factor in attracting settlers to the Unit ed States during the 19th century. In general, pull factors add up to an apparently better chance for a good life and material well?being than is offered by the place of origin. When there is a choice between several attractive potential destinations, the deciding factor might be a non?economic consideration such as the presence of relatives, friends, or at least fellow countrymen already established in the new place who are willing to help the newcomers settle in. Considerations of this sort lead to the development of migration flow. Besides push and pull factors, there are what the sociologists call "intervening obstacles". Even if push and/or pull factors are very strong they still may be outweighed by intervening obstacles, such as the distance of the move, the trouble and cost of moving, the difficulty of entering the new country, and the problems likely to be encountered on arrival. The decision to move is also influenced by "personal factors" of the potential migrant. The same push?pull factors and obstacles operate differently on different people, sometimes because they are at different stages of their lives, or just because of their varying abilities and personalities. The prospect of packing up everything and moving to a new and perhaps very strange environment may app ear interesting and challenging to an unmarried young man and appallingly difficult to a slightly older man with a wife and small kids. Similarly, the need to l earn a new language and customs may excite one person and frighten another. Regardless of why people move, migration of large numbers of people causes conflict. The United States and other western countries have experienced adjustment problems with each new wave of immigrants. The newest arrivals are usually given the lowest-paid jobs and are resented by native people who may have to compete with them for those jobs. It has usually taken several decades for each group to be accepted into the mainstream of society in the host country.