High-Risk and Low-Risk Populations
The incidence of female breast cancer varies markedly between countries. It is highest in the United States and Northern Europe, intermediate in Southern and Eastern Europe and South America, and lowest in Asia. During 1983 to 1987, the age-adjusted incidence of breast cancer varied by approximately a factor of five among countries. However, rates have been rising in traditionally low-incidence Asian countries, particularly in Japan, Singapore, and urban areas of China, as these regions make the transition toward a Western-style economy and pattern of reproductive behavior. As a result of unfavorable trends in these countries, the international gap in breast cancer incidence has narrowed since 1970.

Age-Incidence Curve of Breast Cancer Risk
Breast cancer is extremely rare among women younger than age 20 years and is uncommon among women younger than age 30 years. Incidence increases sharply with age, however, and becomes substantial before age 50. During 1991 to 1995, the incidence of breast cancer among American women aged 30 to 34 years was 25 per 100,000 and increased to 200 per 100,000 among women aged 45 to 49 years. The rate of increase in breast cancer incidence continues throughout life but slows somewhat between ages 45 and 50 years. This finding strongly suggests the involvement of reproductive hormones in breast cancer etiology, because non-hormone-dependent cancers do not exhibit this change in slope of the incidence curve around the time of menopause. The incidence of breast cancer among American women aged 70 to 74 years rises to 463 per 100,000. The shape of the age-incidence curve in low-risk and intermediate-risk populationsis similar to that in the United States, although the absolute rates are lower at each age.

Racial and Ethnic Groups within the United States and Studies of Migrants
According to data from the Surveillance, Epidemiology, and End Results (SEER) Program registries, the lifetime risk of breast cancer for white women in the United States is 13.1%, slightly higher than 1 in 8, whereas that for African-American women is 9.6%, slightly less than 1 in 10. In 1995, the overall age-adjusted incidence of breast cancer among white women in the United States was 115 per 100,000 women, whereas the corresponding rate among African-American women was 101 per 100,000 women. These age-adjusted figures conceal a crossover pattern, however, in which the risk of breast cancer at a young age is modestly higher among African-American women than among white women. At older ages, breast cancer rates for white women are substantially higher than for African-American women.

Unlike most other illnesses, the lifetime risk of breast cancer is positively associated with higher socioeconomic status. This association is largely explained by the known reproductive risk factors; women in lower socioeconomic strata are more likely to have more children and to have them at a younger age than women in higher socioeconomic strata. Much, if not all, of the differences in breast cancer rates between African-Americans and whites in older age groups are likely to reflect racial differences in social class distribution, and thus in the distribution of established reproductive risk factors.

The modestly higher incidences of breast cancer among young African-American women relative to young white women is consistent with the hypothesis of a short-term increase in breast cancer risk immediately after pregnancy, whereas the overall lower lifetime risk of breast cancer among African-American women is consistent with the hypothesis of a long-term benefit of early and repeated pregnancy. The effect of these reproductive factors on breast cancer risk is described in greater detail in the section Models of Reproductive Factors and Breast Cancer Incidence. Although African-American women have a lower probability of developing breast cancer over their lifetimes, their risk of dying from breast cancer is the same as, or perhaps even slightly higher than, that of white women (3.6% for African-American women, compared with 3.5% for white women). African-American women have poorer 5-year survival rates from breast cancer at all ages of diagnosis compared with white women. This poorer survival can be attributed, in part, to the tendency for African-American women to be diagnosed at later stages of disease.

Breast cancer rates are considerably lower among Asian, Hispanic, and American Indian women in the United States than among (non-Hispanic) white women. The magnitude of the difference in incidence among various ethnic groups often depends on migrant status. For instance, breast cancer incidence for Chinese-American and Japanese-American women during 1973 to 1986 was approximately 50% lower for those born in Asia and approximately 25% lower for those born in the United States compared with U.S.-born white women. Among Filipino residents of the United States, the incidence of breast cancer was nearly identical for foreign-born and U.S.-born women, and rates for both were less than one-half the rates for U.S.-born white women. Compared with Chinese women living in the mainland, Singapore, and Hong Kong, Asian-born Chinese women living in the United States had an almost twofold higher annual rate of breast cancer, and U.S.-born Chinese women had a rate that was higher still. The pattern for Japanese women was similar.

These findings are consistent with a large body of literature showing increases in breast cancer incidence after migration from a low-risk country to the United States. Ziegler et al. noted a sixfold gradient in risk of breast cancer among Asian women, depending on recency of migration. Asian-American women with three or four grandparents born in the West were at highest risk, whereas women who were born in rural areas of Asia and whose length of residence in the United States was a decade or less were at lowest risk. Whereas the studies of breast cancer risk among migrants have focused almost exclusively on migrants from low-risk to high-risk countries and have shown convergence of rates, some data also suggest that a convergence of rates similarly occurs when migrants move from high-risk to low-risk countries. For instance, Kliewer and Smith, reporting on immigrants to Australia and Canada, noted that immigrant groups coming from countries in which breast cancer mortality rates were higher than those of native-born women often showed a decrease in mortality. Such findings strongly suggest that factors associated with the lifestyle or environment of the destination country influence breast cancer risk and are consistent with a positive relationship between length of time in the destination country and adoption of that country’s lifestyle. For example, among immigrants, the fertility rate as well as average number of children born tend to converge to the rates of the destination country.