On Sunday night, CNN will air “Three Identical Strangers,” a documentary about an experiment in which adopted twins and triplets were secretly separated. Viewers will probably be appalled as they learn about the emotional damage these individuals experienced as a result of their forced separation. But this medical experiment was not exceptional: It was just one of many unethical studies in the 1950s and 1960s that used subjects as means to an end.

Injunctions against unethical research go back at least to the mid-19th century, when the French scientist Claude Bernard admonished his fellow investigators never to do an experiment that might harm a single person, even if the result would be highly advantageous to science and the health of others. Yet despite Bernard’s admonition, the next century was replete with experiments that put orphans, prisoners, minorities and other vulnerable populations at risk for the sake of scientific discovery. Medical progress often came at too high a human cost, something the CNN documentary exposes.

Human experimentation surged during World War II as American scientists raced to find treatments for diseases encountered on the battlefield. This experimental enthusiasm continued into the Cold War years, as the United States competed with the Soviet Union for scientific knowledge. In both eras, a utilitarian mind-set trumped concerns about research subjects.

That the experiments continued after the war was especially ironic given the response to the atrocities committed by Nazi physicians in concentration camps. There, doctors performed horrific experiments designed to help German soldiers who faced extreme conditions on the battlefield. This research included deliberately freezing inmates, forcing them to ingest only seawater and amputating their limbs.

In 1947, after the war, a postwar tribunal issued the Nuremberg Code, designed to prevent future unethical research. “The voluntary consent of the human subject is absolutely essential,” read the code’s first tenet. But top-notch researchers from prestigious American medical schools forged ahead anyway.

Consider, for example, the Tuskegee study, which ran from 1932 to 1972, in which the United States Public Health Service deliberately left poor southern African American men with syphilis untreated — and thus at high risk of dying from a potentially treatable disease. Dozens of these men died so that the investigators could learn more about the “natural history” of syphilis.

Or consider events at Willowbrook, an institution for emotionally disturbed children in New York’s Staten Island, where researchers deliberately injected residents with active hepatitis virus between 1955 and 1970, to learn more about the disease and possibly develop a vaccine. They argued that, since the children were going to get hepatitis anyway, it was acceptable to give them the disease and then study it.

It was in this era that Peter Neubauer, a psychiatrist and head of New York’s Child Development Center, had an idea that he believed would illuminate the great scientific debate over what made humans the way they were: their genes or the environment in which they were raised. Why not study identical twins and triplets who were up for adoption and placed with different families?

Neubauer’s idea had support. He was friendly with another psychiatrist, Viola Bernard, who worked closely with one of New York’s premier adoption services, the Louise Wise Agency. Bernard, trained in classical Freudian psychiatry, believed that bonding between a mother and child was the most important aspect of childhood development. This theory led the agency to place twins in separate homes, thinking that giving each child its own mother would be best for the child. By studying this process, Neubauer’s team could potentially solve the age-old debate about nurture vs. nature.

The trouble was, of course, that all of this had to be done in secret. So when Neubauer sent his researchers to study several sets of twins and one group of triplets in their separate homes, they were under strict orders not to tell the adoptive parents about the experiment. And the researchers remained mum even when it became clear that many of the subjects were struggling mightily with emotional issues, some of which may have resulted from the separations themselves.

No wonder the two surviving triplets who appear in “Three Identical Strangers” are furious. It is inconceivable to them that physicians, social workers and adoption specialists knowingly deprived them of interactions with their siblings for almost 20 years. Indeed, after the triplets inadvertently discovered one another in 1980, they were practically inseparable, desperately trying to make up for lost time.

Not surprisingly, perhaps, the brothers themselves have referenced the history of unethical experimentation, even comparing the Neubauer study to Nazi medical projects.

At the screening I attended, the brothers did a Q&A with the audience afterward. They were funny and self-effacing, characteristics that make them so appealing in the film. And they were not the angriest people in the room. Audience members were furious at what had happened, even offering to get them lawyers so they could sue.

Yet the lesson of “Three Identical Strangers” should not simply be that modern bioethics has made us so much more enlightened than in the “bad old days.” True, in 1974, in the wake of Tuskegee, Willowbrook and the other research scandals, the U.S. government established the National Commission for the Protection of Human Subjects of Biomedical and Behavioral Research. Rules established by this and other groups have greatly improved safeguards for research subjects, such as ensuring they give informed consent before participating in experimentation.

But what is much more interesting is to ask why well-meaning people like Neubauer and Bernard — who, after all, were fierce child advocates — did what they did. Unfortunately, we do not have a definitive answer to this question (as of now, the files related to the Neubauer study remain closed to researchers until 2065). We do, however, have some clues. For one, the pervasive allure of Freudian thought in the postwar years made at least plausible the idea that separating twins would provide them with better upbringings.

As we have learned from studying Tuskegee and similar scandals, the pursuit of scientific knowledge — and the attendant fame that new discoveries may bring — can blind investigators to the harm that they are causing their subjects. Moreover, these factors can lead scientists to lie and deceive to keep their research projects going and obtain results. These types of perils still exist, despite new ethical guidelines we now embrace.

So yes, let’s learn about the unethical experiment that stupidly separated twins and triplets from the people they loved and needed most, and damaged them as a result. But let’s also understand why future historians may look at our modern research and ask: “How could they have let that happen?”