A 2007 survey by the Arctic Monitoring and Assessment Program noted abnormally low sex ratios in Russian Arctic villages and Inuit villages in Greenland and Canada, and attributed this imbalance to high levels of endocrine disruptors in the blood of inhabitants, including PCBs and DDT. These chemicals are believed to have accumulated in the tissues of fish and animals that make up the bulk of these populations' diets.[35] However, as noted in the Social factors section below, it is important to exclude alternative explanations, including social ones, when examining large human populations whose composition by ethnicity and race may be changing.