Most cases of breast cancer are not associated with an obvious cause or risk factor.  Nevertheless, over the past few decades, epidemiologic studies have established the following breast cancer risk factors, according to the American Cancer Society:

Increasing Age

Besides being female, age is the single most important risk factor for the development of breast cancer. More than 75 percent of all breast cancers occur in women over 50.

Family History of Breast Cancer

History of breast cancer in a first-degree relative (mother, sister, or daughter) is associated with the largest increase, especially when the relative is diagnosed at an early age, or cancer in both breasts or with other cancers such as ovarian cancer.

Breast Cancer Genes

Hereditary breast cancer occurs at younger ages, is more likely to be in both breasts and often appears in multiple family members over three or more generations, including women from the paternal side of the family. Approximately 5-10 percent of breast cancers are believed to be inherited. The risk of other cancers such as ovarian and pancreatic cancer can also be increased.

History of Benign Breast Disease

Several studies have suggested that the increased risks associated with benign breast disease may be confined primarily to women with diagnoses of atypical hyperplasia.

Hormonal Factors

Early age at menarche, late age at menopause, late age at first live birth, and few pregnancies all increase a woman's risk of breast cancer, possibly all of them by affecting her lifetime exposure to hormones. Some scientists postulate that hormones may promote cell division in breast tissue and increase the risk of mutations.

Estrogen Replacement Therapy

The use of oral contraceptives has a minimal impact on the risk of breast cancer, However, long-term postmenopausal estrogen replacement therapy also appears to increase risk. In addition, caucasians, women in high socio-economic status, and exposure to high doses of ionizing radiation are breast cancer risk factors.

Other Environmental Causes

The use of alcohol increases the risk of breast cancer – even modest use adds a small amount of risk and this is higher with more alcohol consumption.  Dietary factors have been difficult to related to breast cancer risk, but the summation of studies suggest that overall calorie consumption and body mass index are related to breast cancer risk.  Exercise appears to have a protective effect.  While tobacco causes many different diseases, breast cancer risk does not appear to be elevated, although it may increase the risk of spread to the lungs following a diagnosis of breast cancer.  Caffeine use does not influence breast cancer risk according to the majority of studies.