There are genetic and environmental factors that can interfere with the usual development of reproductive organs and secondary sex characteristics in humans and other animals. Collectively, conditions of sex development that do not conform to typical male/female development are called intersex conditions.
In humans there are medical conditions that lead to intersex development. Some of these conditions have known genetic causes, others are not known. You can read about various intersex conditions here – http://www.pbs.org/wgbh/nova/body/intersex-spectrum.html
Exposure to hormones, hormonal mimics, or other compounds that can interfere with usual hormone signaling can alter reproductive anatomy and function. For example, tributyl tin is a substance that was added to marine paints. This compound prevented barnacles and other organisms from attaching to the bottoms of boats and other marine structures. Unfortunately, the leaching of this compound into marine ecosystems induces the growth of penises on female snails, interfering with female snail fertility. The use of tributyl tin in marine paints has been banned by international agreement due to its toxic effects on marine life and the potential for human exposure through seafood.
Endocrine disruptors are compounds that interfere with hormonal signalling. There are suspected endocrine disruptors in pesticides, fragrances, plastics, flame retardants, and many other products. Endocrine disruptors have been implicated in the rise in breast cancer, testicular cancer, genital abnormalities, and in the possible decline in the sperm count (the amount of sperm per ejaculate) in men today.
Endocrine disruptors are also suspects in the mystery of some unusual wildlife effects. For example, in the United States, large percentages of male fish in some species (especially large- and small-mouth bass) have been found to have eggs developing in their testicular tissues. These fish species are not sequential hermaphrodites (i.e., they are not known to change sexes). The cause of this phenomenon in the wild is unknown, however environmental exposure to estrogen mimics in the environment is a hypothesized cause.
Laboratory exposure to endocrine disruptors can lead to reproductive abnormalities. Laboratory studies have focused on suspected endocrine disrupters such as ethinyl estradiol (a substance in birth control pills that ends up in waste water), bisphenylA (a compound in some plastics), triclosan (an antimicrobial agent added to soaps, toothpaste, clothing, and other products that advertise themselves as antimicrobial), and atrazine (a weed-killer that is widely used on corn fields in the Midwestern United States; see box below). These studies have demonstrated that these compounds can cause feminization of reproductive organs, changes in behavior, and in some cases, full sex reversal in laboratory exposures.
Our understanding of what compounds could be affecting hormonal signaling in wildlife and humans is growing, but with the tens of thousands of new synthetic chemicals being used today, it is challenging to know which chemicals to prioritize for study.
You can read about endocrine disruption from the perspective of the:
- U.S. Environmental Protection Agency, https://www.epa.gov/endocrine-disruption/what-endocrine-disruption
- World Health Organization, http://www.who.int/ceh/risks/cehemerging2/en/
- National Institute of Environmental health safety. https://www.niehs.nih.gov/health/topics/agents/endocrine/