Introduction

Since the early twentieth century a large evidence and knowledge base of risk factors for breast cancer has developed. The seminal study of breast cancer epidemiology was the Lane–Claypon report in 1926. This study was important not only in the etiology of breast cancer but also in the development of epidemiological methods, being arguably the first systematic case-control study.

Before reviewing the epidemiological research, some important terms are defined. The two most common study designs in epidemiological research are the cohort study and the case-control study. In the cohort study, a group of subjects free of disease are recruited, and their status with respect to potential risk factors is ascertained. The subjects are followed up for the disease in question, to ascertain which factors are predictive of future disease. For example, in a cohort of nurses in the United States, it was found that women with high consumption of red meat were more likely to develop breast cancer. In a case-control study, subjects with the disease in question (cases) are recruited, together with a group of comparable subjects who do not have the disease (controls). Risk factor status of cases and controls is ascertained retrospectively. A notable example is the international series of case-control studies carried out in the 1960s and 1970s by McMahon and colleagues, which among other findings firmly established the association of late age at first childbirth with increased risk of breast cancer.