The Caribbean has a history of slavery not unlike the U.S., so the fact that Caribbean immigrants are better off is genuinely interesting. Of course, institutional factors are the most likely difference; among other things, the British got rid of slavery a generation or so before the U.S. did, and there was no real equivalent there to the U.S. Civil War and failed Reconstruction. There are also cultural differences that will of course matter, but they're causally downstream of that basic stylized fact.
