All women are at risk of breast cancer and many of the causes of breast cancer are unknown. Research has identified a number of risk factors that may increase a woman's likelihood of developing breast cancer. These include: increasing age; early onset of menstruation; late menopause; no children; hormones; and family history. A family history of breast cancer may increase the risk of developing the disease, although approximately 90 percent of women who develop breast cancer do not have a family history of the disease. Studies are also evaluating the role of lifestyle factors on the development of breast cancer, including pesticides, alcohol consumption, weight gain, dietary fat intake and physical inactivity. The average age of diagnosis is 64, demonstrating the importance of age as a risk factor; 78 percent of breast cancer cases occur in women older than 50, and six percent in women younger than 40.11 Of increasing concern is that in developed countries, such as the United States, breast cancer is the leading cause of death in younger women aged 20-59.9 Some factors may decrease a woman's risk of breast cancer, notably breast-feeding and exercise. Breast cancer can occur in men, but it is rare.