No, there isn't "proof". But in addition to strong correlative studies, there a modest amount known about the bio-active components in soy that support these assertions. So yes, no proof, but epidemiological effects possibly found in unrelated studies are supported by biochemical assays of soy activity in the body, so it isn't surprising either. They knew soy messed with the brain for a long time at a pharmacological level, but they never had a population study to prove it.
The Japanese studies have mostly revolved around the detrimental effects of plant estrogens to some organs and glands in the body. Estrogen is one of the most widely used hormones in the human body. The intake of plant estrogens suppresses the production of natural human estrogens, but some of the organs in the body that require estrogen cannot utilize the plant forms, starving some organs of this necessary hormone. General concensus seems to be that plant estrogens are not a healthy substitute for animal estrogens. Some day the health food stores will catch up with the research.