An investigator wanted to know whether chronic lymphocytic leukemia (CLL) had a genetic or environmental etiology. He knew that rates in country X were higher than those in the United States. He decided to study the rates in the country of origin, in migrants, and in U. S. whites. He observed the following:

Population Rate Country X 50/100,000
Migrants from Country X to U. S. 10/100,000
U. S. whites 10/100,000

What are the reasons for this relationship?