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Risk factors for breast cancer are often confusing and contradictory. Discrepancies are likely due to different subtypes having divergent risk factors. An important distinction between breast cancer subtypes is hormone-receptor status. Compared to women diagnosed with estrogen receptor positive (ER+) breast cancer, those with estrogen receptor negative (ERÀ) tumors are usually diagnosed at a younger age and have a higher mortality [1]. Few studies have attempted to explain ‘why’breast cancer subtypes have different risk factors. In a recent meta-analysis, Aktipis et al.[2] demonstrated that modern reproductive behaviors are more strongly associated with the development of ER+ than ERÀbreast cancer. The systematic review specifically reported the following:(a) fewer total offspring is a risk factor for ER+, but not ERÀ, tumors;(b) older age at first birth is associated with risk for ER+, but not ERÀ, tumors and (c …