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钢筋混凝土结构的特点有()。
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Migration is usually defined as "permanent or semi-permanent change of residence". However, 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 severe 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 often these are economic, such as better job opportunities or the availability of good land to farm. 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.
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 prospect of packing up everything and moving to a new and perhaps very strange environment may appear 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 learn 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. It has usually taken several decades for each group to be accepted into the mainstream of society in the host country.
Today it is found that movement between nations often exceed international movements in volume.
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