The chemical bisphenol A (BPA) is just one of many known or suspected "endocrine disruptors"—naturally occurring or man-made compounds that mimic or interfere with the function of estrogen and other human hormones, even at extremely low doses. Many of them have been linked with developmental, reproductive, neurological, immune, and other problems in animal studies, and some research suggests that they adversely affect human health in similar ways, though there is still considerable debate about this among scientists. Endocrine disruptors pose the greatest risk during prenatal and early postnatal development, when organs are developing.
One reason why it's hard to assess the health effects of individual endocrine disruptors like BPA is that we're exposed to many of them at the same time—in everyday products (including food, cosmetics, even medicines) and in the environment.
Known endocrine disruptors include pesticides such as DDT, dioxins, PCBs (polychlorinated biphenyls), phthalates (in some plastics and cosmetics), and flame retardants.
But not all compounds that mimic hormones are bad. For instance, the phytoestrogens naturally in soy (isoflavones such as genistein and daidzein) and other plants have hormone-like activity. These compounds in soy help account for some of its potential health benefits, such as for improving bone health and countering menopausal symptoms. It's theorized that they may reduce the risk of breast cancer by binding to estrogen receptors in the breast and inhibiting natural estrogen action, but research on this has yielded conflicting and inconclusive findings.