When I grew up it was fairly common knowledge around people my parents knew and spoke with regularly that breast cancer was primarily caused by women putting off having children and then having children late and choosing not to breast feed.
Nursing ones children was very frowned upon during World War II and after by American women in general.
Also, another factor has always been considered is that the breast milk suppressing drugs given to mothers who didn't or don't want to ever nurse for whatever the reason might be a factor in Breast Cancer.
From a very holistic point of view it makes a lot of sense if one thinks, "Use it or lose it." Just like men who don't have sex ever like priests often have prostate cancer, women who don't have babies and nurse or even don't have sex at all might have a much higher incidence of breast cancer as a direct result.
However, since Breast cancer is big business many people might not want you to know this. However, to me it is just common sense.