Immigration laws.

Agree with @FourierFlux. Also, life is far richer than the academic experience, probably figures in the decision for some folks.

There is certainly more people trying to sink away than positions available for those people on the richer countries.

@FourierFlux There are no immigration controls between Poland and Spain — they’re both EU members.

Dunno mate, as a Spaniard living abroad as a Postdoc, I am not sure I agree with your numbers.... In London, one of the most expensive cities to live at in Europe, Postdocs start at 36K£/year, which is significantly lower than your $4000 estimate for Spain. And I know postdocs earning 600€ in Spain. So, while the premise of the question is still valid, I think you have an exaggerated expectation of salary for postdocs in "richer" countries.

Poorer countries often have larger income inequalities and lower cost of living. An academy in a poor country may have a very pleasant life.

Are there any statistics to back up the assumption that "poorer countries are not suffering a complete brain drain"? Or at least a pretty significant one? I would note that lots of purer fields do tend to be majority international talent (rather than local), if you wanted to have a world leading institute you need people from all over the world. Actually, experts do seem to gather where there is funding available.

Perhaps not "complete", but there is significant brain drain from eastern to western Europe.

I should probably add that in my field (atomic and molecular physics) Poland is a very attractive destination both for academic reasons as well as quality of life.

This question is phrased in absolute terms ("complete, decisive"). If it were phrased more openly, it would make more nuanced and interesting answers possible. For example: Which factors produce and mitigate brain-drain? Or: What are the relevant considerations for academics to migrate to more affluent countries?

Money isn't the only driving factor in decisions for some (most?) people.

I do not like this question because it is opinion based and also because it assumes that the only motive is the salary.

@PsySp there's actual research on this, so the question isn't opinion-based. So far, the answers are "opinion based" - or rather anecdotal, however.

You might likewise ask why I (American) don't go live in Silicon Valley or New York City: because the increased earnings aren't enough to compensate for the IMHO greatly decreased quality of life.

Research has consistently shown that relative income is a LOT more important than absolute income. Both in a year over year sense and a comparison sense (you vs your neighbor). Which is why there is widespread economic dissatisfaction in many western countries when compared to a much poorer country like china, and why economic dissatisfaction greatly increases when incomes go down YOY even if they are still relatively high to what they were a decade ago. The average health and wealth of a poor person today is roughly equivalent to a rich person 200 years ago, yet nobody thinks like that

There is not an infinite amount of post-doc positions? Positions in academia in Sweden has regularly over 100 applicants -- there is just not enough positions to go around to make a 'brain-drain'. Moreover, higher positions require an ability to teach in the native language.

OP, what is the motivation behind this question? Stating that might be helpful to get answers rather than opinions. To me personally, this seems like a navel-gazing question, and I'm therefore not keen to answer despite having some insight into this.

@Per Alexandersson: But how many positions does the average post-doc candidate apply for?

@jamesqf I guess several, but my point is- there cannot be more brain-drain than available positions... which are very limited.

@PerAlexandersson Sweden is not only developed country in Europe. No idea what are you implying

@looktook I say that the number of positions in academia is very limited; developed country or not.