A number of opinion surveys, including some very large ones taken over a period of time by Gallup, indicate that there is one primary driving factor for moving to another nation that can be summed up in one word - opportunity.
The group that goes unnoticed is the globally-mobile middle class. They bring money, but nowhere near as much as the elite class. They also bring their labour, although not always as cheaply as the working class. And they are not part of the criminal class. Although they are not as numerous as the working class, they are much, much larger in number than the other two classes and also growing rapidly.
The dominant group is made up of middle-aged Americans, about a quarter of whom are households of three or more people, thus children are very definitely involved too. Most surprising to most people, the number of American households considering relocation outside the US (or purchase of a property outside that is typically to be associated with later relocation) runs into the millions (that's households, not just people).
But that's just Americans. A study by Britain's Institute for Public Policy Research a few years ago found that nearly one in ten Britons had already moved from the UK.
Similar examples of middle class relocation can be found all over the world from South Africa to Australia to India to Russia to Brazil and in a hundred other nations. An "expat" these days is not likely to be a native English-speaker, although he or she may speak it as a second language. Whatever, language is unimportant. This is a trend. It is huge and it will impact the course of history, although that may not be appreciated for another generation or so. It is happening so quickly and on such a large scale that it cannot help but have a major effect. After all, these people may not bring as much money as the elite class or as much labour as the working class, but their total contribution is far from trivial.