New medical, reproductive, and genetic technology in the second half of the 20th century led to increased concern about moral issues in medical treatment and research. By the 1990s, medical ethics, or bioethics, emerged as a recognized discipline that involved physicians, nurses, attorneys, theologians, philosophers, and sociologists.

Many bioethics issues involve the possible misuse of genetic engineering technology. The Human Genome Project led to identification of genes that raise an individual's risk of developing cancer, heart disease, mental illness, alcoholism, violent behavior, and other conditions. Tests to detect some of these disease-susceptibility genes became available in the 1990s.

These discoveries led to debate over whether genetic tests should be performed and how the results should be used. Should parents use such tests to screen their unborn infants? If a fetus tested positive, should it be aborted? If a woman tested positive for a breast cancer susceptibility gene, should the information be made available to insurance companies? Do insurers have a right to deny coverage to people with a genetic high risk for serious diseases? Do employers have a right to demand genetic screening tests before hiring people?

Genetic technology also offers the potential of eventually replacing defective genes with normal copies in human sperm and eggs. Some fear it will lead to mandatory eugenics programs, attempts to improve the hereditary traits of individuals or even entire races. Others argue that advances in genetic technology could eliminate defective genes and hereditary diseases from future generations.

An intense discussion about bioethics occurred in 1997 and 1998, after researchers in Scotland cloned the lamb, Dolly, from udder cells from an adult ewe. The experiment showed that it was possible to clone, or produce an exact genetic copy, of an adult mammal. Medical ethicists debate whether cloning of human beings should be permitted, as well as the potential effects on society.

Although abortion became legal in the United States in 1973, it still causes heated debate over the rights of the fetus and the pregnant woman, as well as the question of when a fetus becomes a human being. The availability of RU-486, also known as mifepristone, an inexpensive drug that induces abortion, led to concern that more people would use abortion for birth control. Ethical discussions centered on whether tissue from aborted fetuses should be used in medical research, treatment of disease, and organ transplants.

The right of terminally ill people to receive assistance in dying raised other ethical dilemmas. Physician-assisted suicide came to national attention largely through the efforts of Jack Kevorkian, a Michigan physician who helps people with terminal illnesses commit suicide. Opponents claim it is unethical for physicians to help patients commit suicide. Supporters counter that terminally ill patients have a right to determine the time and manner of their death. While the U.S. Supreme Court in 1997 ruled that states can ban physician-assisted suicide, that same year Oregon voters rejected an effort to repeal their law, the nation's first to legalize physician-assisted suicide.