No one knows what causes breast cancer. There are many factors that are believed to elevate the risk. According to the The National Cancer Institute’s booklet What You Need To Know About Breast Cancer, there are a number of “risk factors.” I’ve reproduced the list from the booklet, for the most part, here:

--Age: Most women are diagnosed with breast cancer when they are over sixty years old. The chance of getting the disease increases with age.
--Personal history: Having cancer in one breast increases the risk of getting it in the other. Having certain types of abnormal cells in the breast also increases the risk.
--Family history: If a mother, father, sister, or daughter had breast cancer a woman’s chance of getting it is higher.
--Genes: Changes in certain genes, such as BRCA1 or BRCA2, increase the risk.
--Radiation therapy to the chest: Women who’ve had radiation therapy to the chest before age thirty are at increased risk.
--Race: Caucasian women are at higher risk than African American, Hispanic, Asian, American Indian, or Alaska Native women.
--Breast density: Women whose breasts have more dense and fatty tissue are at higher risk.
--Weight: Women who are overweight after menopause are at increased risk.
--Physical activity: Physically inactive women may be at higher risk.
--Alcohol: Studies suggest the more a woman drinks, the greater her risk.
--History of DES: Some pregnant women were given DES in the United States between about 1940 and 1971. Women who received DES are at slightly higher risk.

Reproductive and menstrual history:
--The older a woman is when she has her first child, the greater the risk of her having breast cancer.
--Women who’ve never had children are at higher risk.
--Women who began the menstrual cycle before age twelve are at risk.
--Women who experienced menopause after age fifty-five are at increased risk.
--Women who’ve taken menopausal hormone therapy for many years are at higher risk.