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Table 53.1 summarizes the major risk factors associated with development of breast cancer. With the exception of female gender, increasing age is the most consistent and significant risk factor, with most populations demonstrating increasing incidence rates with age. Other risk factors include personal history and family history of breast cancer, nulliparity or late age at first childbirth, early menarche and late menopause, prior breast biopsy with hyperplasia or atypical hyperplasia, high breast tissue density, radiation exposure at a young age, alcohol consumption, and use of postmenopausal hormone therapy. Some of the national origin/ethnicity variability discussed above may be explained in part by differences in established risk factors, such as age of menarche, parity, and age at first childbirth. However, these factors explain only in part the variability observed in national origin, indicating that underlying genetic, environmental, and dietary factors are likely to contribute to the differences in the worldwide incidence of breast cancer (127). Breastfeeding, physical activity, and maintaining a healthy body weight have been demonstrated in various studies to be associated with a lower risk of breast cancer (127).