Studies that compare a group of people in one region with people who originate from the same region but who have subsequently migrated to another area. The people who have migrated are also compared to the group of people living in the region they have migrated to. This provides us with evidence concerning the influence of genes and the environment on diseases by identifying what people have acquired from exposure to their surroundings, lifestyle and culture. For example, changes in cancer rates in migrants can help identify the cancer-causing effects of their environment.