When the US government banned prisoners from participating in medical research back in 1978, it was trying to protect their health. But the move may have inadvertently skewed decades of medical research — particularly research into racial inequalities in health.

A new study in Health Affairs by Yale's Emily Wang suggests that some of the biggest medical studies of the last few decades may have seriously distorted data on African-American men. That's because they were conducted during a period in which a disproportionate number of black male high-school dropouts were in prison. The paper casts doubt on studies examining everything from cardiovascular health to bone marrow — but since the overwhelming majority of studies don't publish their protocols, there's no telling how many studies have been compromised.

Now, medical researchers want to change the rules, so that the racial disparities in incarceration don't continue to skew health research.

Why can't prisoners participate in medical studies?

Prisoners are ideal subjects for medical research in many ways. It's difficult for them to say no — particularly for money — and they tend to stick around for the duration of the study.

in the mid-20th century, 90 percent of new drugs were tested on prisoners first

But those qualities can also make prisoners vulnerable to unscrupulous pharmaceutical companies looking to try out new drugs on human subjects. In the mid-20th century, about 90 percent of all pharmaceutical products that went to market got tested on inmates first.

The system was ripe for abuse. In the early 1970s, reports came out about prisoners who had been exposed to radioactive chemicals or hallucinogens. In the most famous case, in Holmesburg Prison in Philadelphia, one test made a participant's skin change color — in an experiment he'd never signed a consent form to participate in.

In 1978, the federal government shut down the prison-testing industry by outlawing all federal research on inmates that wasn't explicitly tied to the effects of incarceration itself, or to conditions that disproportionately affect prisoners (like Hepatitis C), unless the researcher could prove that the research would be beneficial to the inmate.

What's more, if someone who was already enrolled in a long-term health study got sent to prison, he'd have to stop participating in the study — unless the researchers had gotten permission in advance from the federal government to study prisoners.

WHEN A SUBJECT LOST CONTACT, RESEARCHERS DIDN'T KNOW IF HE'D LOST INTEREST OR BEEN SENT TO PRISON

In the late 1970s, before the incarceration rate skyrocketed, this wasn't likely to be a concern for medical researchers. But the African American men that came of age just after those regulations were passed experienced the beginning of mass incarceration in the United States.

As a result, some of the most significant medical studies out there, especially on conditions that tend to affect African American men — like heart disease — were affected by a problem they'd never anticipated: when a subject lost contact with the researchers, they had no way of knowing if he'd simply lost interest or been sent to prison.

How mass incarceration has skewed research

Wang's paper looks at several studies that followed subjects over many years or even decades, and compares the dropout rate for each demographic with how likely the dropouts were to have gone to prison at that point.

incarceration caused 2/3 of all black male dropouts in one cardiovascular health study

While results varied, she found that studies that were supposed to examine at-risk populations were the most likely to have their data screwed up by mass incarceration. In one major study examining cardiovascular health among inner-city young adults over 20 years, Wang estimated that 65 percent of the African American men who dropped out of the study were probably locked out.

This means that the people who were able to stay in the study weren't representative of the black male population as a whole. People who have never been incarcerated are healthier than people who have been, so this is especially problematic for medical research.

And if too many subjects are incarcerated, researchers don't have enough data to draw conclusions at all: by the 15th year of the cardiovascular study, there weren't enough African American male subjects left in the study for researchers to analyze racial disparities.

Can researchers fix the rules?

Some experts have been recommending for years that the government change the rules so that, if people start participating in a study under normal circumstances, they don't have to stop when they go to prison.

For now, there doesn't appear to be much momentum behind the shift. It's only been a few decades since the government stepped in to stop prisoners from being used as guinea pigs, and regulators are wary.

Still, the new Health Affairs study raises the question of whether it's really ethical to force prisoners to stop participation in medical research when they're incarcerated. It's just another realm in which a vast swath of people, mostly African American and Latino men, are getting excluded from American society.