Gives them breast cancer because it provokes estrogen secretion
An interesting and long term study done in Okinawa, where soy makes up about 12% of their diet (that's a lot of soy), found that Okinawans live longer than anyone else on the planet and have some of the lowest rates of cancer and Alzheimer's in the world. There have been several additional studies conducted on Okinawans showing that they are at extremely low risk for hormone–dependent cancers, including cancers of the breast, prostate, ovaries, and colon. Compared to North Americans, they have a 80 percent less breast cancer and prostate cancer, and less than half the ovarian cancer and colon cancer.