For Bernie Sanders, radical views once extended to theories on what causes cancer "I have my own feelings about what causes cancer," Sanders said in 1988.

For nearly 20 years as an up and coming politician, Bernie Sanders supported ideas on what causes cancer outside of the mainstream, such as sexual inactivity as a cause of breast cancer.

"The manner in which you bring up your daughter with regard to sexual attitudes may very well determine whether or not she will develop breast cancer, among other things," Sanders wrote in an essay headlined "Cancer, Disease and Society" in 1969. "How much guilt, nervousness have you imbued in your daughter with regard to sex?"

The essay, published in The Vermont Freeman and previously reported, extensively and approvingly cites studies suggesting a relationship between "inhibited sexuality" and cancer risks. "In other words, did women who develop breast cancer have certain similar psychological traits which might lead one to see a connection between emotional health and cancer," Sanders wrote.

It's part of a vast written record of many of his views – some well outside the mainstream – that are coming under scrutiny again as he emerges as a front-runner in the 2020 race for president.

When the issue first arose in the 2016 presidential campaign, a Sanders aide dismissed the comments as dated. “These articles were written more than 40 years ago,” Sanders spokesman Michael Briggs told Mother Jones in 2015. “Like most people, Bernie’s views on many issues have changed over time.”

But his commentary on the subject of cancer and its causes suggest his beliefs about links between the disease and human sexual behavior continued well into the 1980s.

ABC News did not immediately receive a response from the Sanders campaign on this story and how in particular his views may have evolved.

In that early published essay, Sanders cited numerous medical studies that claimed to have found a relationship between cancer and human sexual behavior.

One study he cited in the 1969 piece found that women who achieve orgasm less frequently during intercourse were more likely to contract cervical cancer. Another found that women who achieve fewer orgasms were "biologically weakened" and are therefore "highly susceptible to cancer producing stimuli" more generally. The methods of one study included asking participants whether or not they viewed sex as "a distasteful, wifely duty. "

Asked about the relationship between sexual activity and cancer, Harold Burstein, a breast cancer specialist at the Dana-Farber Institute in Boston, said the research is clear: "There is absolutely no relationship between sexual activity, or lack of activity, and cancer."

"Some cancers, like cervical cancer, are transmitted by human papilloma virus, which is a sexually transmitted infection," Burstein explain, "but sexual activity itself has no relationship to cancer risk."

In the same paper, Sanders wondered whether "radical change" in society-- an idea indicative of the political revolution Sanders has been pursuing for decades-- could itself cure diseases such as cancer. "Will society be changed so as you fit the needs of the human organism," he warned in the essay, "or will the human organism continue to be adapted, molded, and crushed to fit into basically insane and disease provoking patterns?"

Sanders' views on cancer formed out of his research into the "psychiatric aspect of cancer" while studying at The University of Chicago, according to a profile on Sanders from 1985. It was there that Sander developed his central theory: that "disease, to a large degree, is caused by the way we live in society," he said during another profile 1981. Among those who influenced his views, Sanders said, were Dr. Sigmund Freud, founder of psychoanalysis, and Dr. Wilhelm Reich, one of its proponents.

By the 1980s, decades after Sanders' studies at school and his first public comments on cancer were documented, Sanders subtly continued to express a viewpoint on cancer and its origins that was less extreme, but still questionable.

"I have my own feelings about what causes cancer and the psychosomatic aspects of cancer," Sanders said at a media event in Burlington in 1988.

He was talking about the death of a Nicaraguan Sandinista he knew whom he said had recently passed away from cancer. The woman, Sanders recalled, was a "very, very vibrant and wonderful woman," yet she still succumbed to the disease. "One wonders if that war did not claim another victim of another person who couldn't deal with her tremendous grief and suffering that's going on in her own country," Sanders stated, suggesting it was "grief and suffering" that contributed to her illness and ultimate death.

Burstein disputed this science behind this idea as well, explaining that "there are no psychosomatic aspects of cancer, nor can people's feeling or emotions or psychological state 'cause' cancer in anyway."

In the 1969 paper, Sanders highlighted the notion of a relationship between emotions and cancer. He cited a doctor who whom he said had pinpointed a "cancer personality," in other words, someone who "represses hate, anger, dissatisfaction, and grudges, or on the other hand, is a very 'good' person, who is consumed with self-pity and suffers in stoic silence."

Again, Burstein said the science on this point is clear. "There is no relationship between stress or emotions, and 'getting' cancer."