Fertility is probably not affected quite directly by wealth. It is more probably affected by sociological factors relating to a wealthy and supercompetetive society. I might examine studies showing that supercompetition in business reduces actual chemical fertility.

The odds are greater that the substantial cause of fertility rates being reduced in an individual or first world nation are that people choose not to have so many children on account of fiscal concerns. Jamaicans are poor, but they have many, many children by comparison to Americans, or Swedes, who can afford to care for them.

It is not because they have better DNA. It may be related to the level of intensity of their society and their fiscal attitudes. The perception of social risk may play a role. Secondary chemical factors may play a small role.

Social elements reducing fertility are anti-social, be they economic, sociological, or microorganizational.