In the United States, there is a high prevelance of breast cancer among women. What makes women in the United States so vunerable to breast cancer. Evolutionary medicine has been studying the correlation between the number of menstral cycles with incidence of breast cancer in the population. Since our early ancestors are not around to see how many times they menstrated, hunting and gathering tribes are used as a modern example of out EEA population. In these societies, the women only menstrate and average of 92 times in their life while women in the United States will menstrate on average over 400 times in their life. Breast tissue is not fully matured until a women become pregant. As part of menstral cycle, the breast tissue will divide via mitosis and expand in the hope the women gets pregant and the breasts will fully mature. When the women menstrates, the breasts go back to the premature state. It is theorized that since women in the United States menstrate more, the cells in their breasts will divide more and leave them more vunerable to developing cancerous cells later in life as the more a cell divides, the greater the risk of mutation is. Women in the hunting and gathering tribes have a later first menarche, longer menstral intervals and longer lactation periods, all of which factor into menstrating less in their life. In the United States, birth control is used by a majority of women to prevent pregancy as well as regulate their menstral cycle. Despite the risks associated with birth control, it is still being used therefore it may be beneficial to use birth control to regulate periods and allow for women to menstrate less. This would lead to less divisions in the breast tissue and possibly less incidence of breast cancer.