(I'm not 100% certain in these facts, so please do correct me if I'm wrong) I've recently looked into this, in particular emigration from Norway to the USA.

Looking into this, I found the primary reason for emigrating for our people, as well as many others, was poverty.

The-not-yet-united-states promised land for just showing up, and we started emigrating from the 1600's. Unlike any other country, owning land in the USA meant you were represented in the voting system. Any other newly discovered places had an "elite system" where the rich had priority in voting.

I learned these last facts through a BBC documentary that actually credits the success of the US to this policy. An actual historian would probably be able to accurately credit these factors, but they seem influentual.

As well as poverty, people were also sick/persecuted of established systems. The US was a newly established territory and needed settlers, and was very loose with its regulations. Many think this is where the "land of the free" expression came from. Laws in early north america were heavily influenced by the vast number of immigrants, and as such learned from the history from many countries. This resulted in a view of freedom never seen before.

Many historians also believe this is what led to the revolutionary war in the late 1700s. Time went, and they got annoyed they were not represented in the british (who owned the us at the time) voting system, mostly due to them adding taxes without the us having any say in it.

I rambled a bit. Back to the point, upwards of 1 million norwegians emigrated from Norway to the US (resulting in there living more norwegian descendants in the US than norwegians in Norway today). In our case it was surely poverty that was the reason as well as many other nations. The US was the promised land.

Until we found oil :P (is it ok to post here while at your fourth glass of wine? because i'm at my fourth glass of wine)