Homosexuality is a choice, just look at the prisons, Republican neurosurgeon Ben Carson said on CNN.

In a March 4 interview , the likely 2016 presidential contender said he believes that legalizing same-sex marriage is a decision that should be left to the states. He also argued that homosexuality is a choice , rather than biological.

"Because a lot of people who go into prison go into prison straight -- and when they come out, they're gay. So, did something happen while they were in there? Ask yourself that question," said Carson, a former head of pediatric neurosurgery at Johns Hopkins Hospital.

We’ll jump to the point here: There is no conclusive evidence to support Carson’s claim that "a lot of people" make a switch from heterosexual to homosexual while in prison. Further, the medical community tends to conclude that homosexuality is not a matter of choice -- notably the American Psychological Association .

"Dr. Carson's contention has a political intent but is absent of awareness of research on human sexuality," said Mark Fleisher, a social science professor at Case Western University, who studies sexuality and violence in prisons.

A spokesman for Carson declined our request for comment.

In the prisons

Many inmates who identify as heterosexual change their sexual habits while in prison, possibly as a victim or perpetrator of abuse or due to lack of access to the opposite sex, said Christopher Hensley, a University of Tennessee at Chattanooga criminal justice professor. But a change in sexual behavior does not necessarily equate to a change in sexual orientation.

Hensley recently authored a study that he said is one of the first to examine a shift in sexual orientation in prisons. He and his co-author questioned 142 prisoners at a maximum-security facility in the South about their sexual orientation. They found that of the sample, 24 men changed sexual orientation while incarcerated.

Of those 24, 18 changed from straight to bisexual, three changed from bisexual to straight, and one each changed from bisexual to gay, gay to bisexual, and gay to straight.

These results do not conclusively support Carson’s point, Hensley said. The sample size is too small to make any sweeping conclusions -- 142 is just about 0.001 percent of the nation’s prison population.

Additionally, Hensley said his research does not support Carson’s implication that prison behavior shows that a shift in sexual orientation is a choice. Prison sex culture is not analogous to the sex culture outside of prison.

"Carson is taking a simplistic view of sexuality," Hensley said.

It’s possible that people go into prison unaware of their sexuality, and then once they are exposed to homosexual behavior as an inmate, they realize that they are gay or bisexual, Helen Eigenberg, who also studies prison sexuality as a professor at the University of Tennessee at Chattanooga, said in an interview with PolitiFact. This isn’t necessarily captured in Hensley’s research, she said.

Eigenberg added that sometimes victims of sexual abuse -- in prisons that can mean gang rape, prostitution or other forms of coercion -- are sometimes confused. They might think that just because they played a role in a homosexual act, they are now gay, which is not necessarily true.

"The fundamental assumption of the analogy (Carson’s) using is insane," Eigenberg told the Washington Post.

We asked numerous other experts who study criminal justice and sexuality, and none agreed with Carson’s claim. Nor were they aware of modern research addressing this topic, other than Hensley’s.

"There is absolutely no good evidence supporting Carson's contention, and there is good evidence against it," said J. Michael Bailey, a psychology professor at Northwestern University who studies sexual orientation.

A choice?

In a previous PolitiFact article , we found that, while it’s not certain what factors cause sexuality, scientists agree that personal choice is not involved. The American Psychological Association, which has 150,000 members, considers it a "settled question."

Greg Herek, a psychology professor at the University of California Davis who studies sexual orientation, pointed us to a nationally representative study that shows the vast majority of gay men and women do not perceive their sexual orientation as a result of choice.

People can change their behavior -- such as engaging in homosexual behavior in prison -- but that does not equate to a change in orientation, Bailey, the Northwestern researcher, told PolitiFact.

He also noted that there are examples (updated with new link) of boys being surgically changed into girls at birth (in the past, for medical reasons), but when these former boys grow into women -- they are attracted to other women.

"If you can't make a male attracted to men by changing him, surgically and socially, into a girl, how likely is it that prison can do that?" Bailey said.

Our ruling

Carson said, "A lot of people who go into prison straight, and when they come out they’re gay."

There is no research that supports this point. The one study that even begins to address the topic has too small a sample size -- and too many variables -- to shed light on Carson's claim. And its author finds Carson's view "simplistic."

Further, Carson used this example to prove that sexual orientation is a personal decision. Respected scientists consider it settled that homosexuality is not a matter of choice. Additionally, experts told us the sex culture in prison is not comparable to sex culture outside of prison, making it a bad analogy.

We rate Carson’s claim False.