I’m reading William Skinner’s “Chinese Society in Thailand” (1957). He notes that of the various speech-groups from different parts of south China that came to live in Siam, the Hainanese were one of the least prominent, yet they were also the first to strike out beyond Bangkok and the coastal areas to engage in petty trading along the northern riverways in the upcountry.
Skinner postulates several reasons for people from Hainan taking this pioneering role, in place of the Teochius, Hokkiens, Cantonese or Hakkas, who tended to dominate in other areas of endeavor. One of them has to do with resistance to disease:
The Hainanese had an advantage in their ingrown resistance to malaria and other tropical fevers. Informants in upcountry Thailand repeatedly told the writer that Hainanese could go into fever-ridden areas where Teochius would die like flies. Hainan, of course, lies in more tropical latitudes than any other emigrant area in China, and endemic malaria was more widespread there than in the delta areas of the Teochius or Cantonese. (p. 86)
This seems plausible, but can anyone corroborate it? Was Hainan known as an especially pestilential place? Presumably this would have delayed its settlement by Han Chinese earlier on. Also, has there been any confirmation in medical or genetic studies today?