: Risk of breast cancer varies by about 5-fold among societies, and incidence and mortality have been increasing worldwide for many decades. Migrants from low-risk Asian societies to the U.S. suffer elevated risk of breast cancer in their own lifetimes, and the second or third generation Asian-Americans attain the high risk of the multi-generational European immigrants [1,2]. Something about a modern Western lifestyle apparently increases risk dramatically. Madigan et al.  estimate that 41% of the new U.S. cases of breast cancer are explained by "known risk factors"; these include the reproductive factors of age at first birth, menarche, menopause. They ascribe about 30% to reproductive factors when they are analyzed alone. "High income" is estimated to account for about 19% when analyzed by itself. The 41% is an analysis taking all the factors together, and since they are related, the total is less than the sum of estimates for the individual items. By itself, "High income" has no biological interpretation and must reflect attributes of lifestyle and/or environment that increase risk. So, the proportion of breast cancer cases in the U.S. that can be accounted for by known biological risk factors is about one third. Therefore, at least half of breast cancer risk in the U.S., and other Westernized/industrialized societies, is in excess of that found in non-industrialized societies and is without any agreed-upon explanation. Many candidate factors exist, each with a cadre of proponents. The sum of these may turn out to explain the bulk of the excess risk in modern societies. On the other hand, they may not, and worse, may fall woefully short.