Epidemiology determines whether or not a statistical relationship between abortion and breast cancer exists. In other words, do more women who have abortions develop breast cancer than other women do?

Seventy epidemiological studies have been conducted since 1957. Eighty percent of the studies report risk increases for women who have abortions.

Epidemiology isn't the only kind of research that scientists use to determine a cause-effect relationship because this type of research has limitations.  

For instance, suppose that scientists determined that more people who carry matches in their pockets develop lung cancer. Does that mean that matches cause lung cancer? Certainly, not. A better theory is that cigarette smokers are more likely to carry matches than non-smokers, and smoking raises lung cancer risk.