Perhaps the best answer is that poor countries lack the institutions—government capacity, a functioning legal and tax system, security of property rights, and traditions of trust—that are a necessary background for growth to take place.Ronald Bailey notes in his review that this explanation, while well supported by the facts, doesn't explain why some countries have these institutions; just that they are important. I believe that the European culture which combined both Greek and Christian tradition provided the societal stability and freedom of inquiry to produce a stable society that valued the innovation adequately to reap its benefits. Whether or not I am correct, we can still look at the world and see which countries have adopted or are adopting similar cultural values to ours which allowed us to escape poverty.
This matters to the immigration and refugee questions. As a nation, it is our right to ask for and the duty of our leaders to implement policies that benefit the citizens of our nation. Unrestricted immigration from countries that don't share our values undermines our prosperity. When I look at the so-called "Syrian" refugee crisis; I see two key sets of facts. First, the refugees seem to be neither Syrian nor refugees, in large part. Second, even when legitimate, they come from a society that doesn't share our values. Contra Obama, there are no shared universal values. If there were, there would be democracies all over the Arab world.
With regards to immigration from Latin America; the main sources of migrants continue to be from countries with little respect for the rule of law. It is not coincidental, that as Mexico has improved its internal governance through reform, the number of migrants from Mexico has declined. Now, dictatorships trans-shipping people through Mexico are increasingly the problem.
On twitter, someone compared the so-called Syrian refugees to the Jews we admitted during World War II. For brevity, my response was that the Jews were culturally European and therefor worthy of admission. In other words, they were ready to support and understand our institutions, security of property rights and "traditions of trust" in ways that Syrians are sadly incapable of.
We should limit immigration based on country of origin in order to not dilute the cultural underpinnings of our society.