[Shadowstar]

Randomness seems to be best in this case. Some people like living in strange places. Some people don't care much about thier living environment and will choose to live where it's cheaper. When you throw in different species of people, it just gets more random. That said, there should at least be some basic generalizations used. For instance, there are some things everyone wants equally, so it makes sense that people would migrate toward planets where they can breathe outside, and migrate away from planets where they can't.

As for happiness, i'd say that would be a minor factor when compared to loyalty. People should only migrate on the basis of loyalty when they are loyal to another empire. If they are displeased or have low loyalty to thier current empire, but aren't loyal to any other empire, then they should stay put. When they do migrate, they should migrate to the empire that they are loyal to, not just to some random empire under the treaty. In some cases, it is concievable that loyalty could cause outright abandonment of planets, but in these cases it would behoove the empire in question to seriously consider thier migration treaties. This will dissuade players from starting up migration treaties willy-nilly.

Also, planet conditions should have an effect, but shouldn't cause migration away from the planet. Conditions should put a cap on incoming migration instead, since planets with poor conditions would be considered "low-income housing", which is a neccessary evil in any civilization. Basically, low-income housing exists because some people just can't live anywhere else, but they still need a place to live.

In all other cases, it should be random.