There was a study on a Hawaiian population of Japanese American couples that correlated tofu consumption positively with increased rates of dementia. There are also conflicting studies on what the estrogen-like chemicals in it do in relation to male hormones and women with breast cancer. Soy is not the only food that has phytoestrogens. Some people speculate that plants evolved them to make animals that eat them reproduce less. Their effect on reproduction is not well established at this time though, so that speculation doesn't really mean anything. Flax also has a lot of phytoestrogens if I recall correctly, as well as legumes and nuts, but not to the extent of soy.