An academic with a forthcoming biography of 1972 Democratic presidential candidate and former senator George McGovern has confirmed that the South Dakotan fathered a child before he was married.

He said McGovern, as an 18-year-old freshman at Dakota Wesleyan University in Mitchell, S.D., lost his virginity to the girlfriend of a friend during a trip to Lake Mitchell in December of 1940 or January of 1941, and immediately got her pregnant.

McGovern, a decorated World War II pilot and a liberal icon who died in 2012, lost his seat in the U.S. Senate in the Republican sweep of 1980. He served as U.S. ambassador to the United Nations Agencies for Food and Agriculture during the administration of President Bill Clinton.

Rumors — most recently in stories about the South Dakota senator’s FBI file, which included references to an “illegitimate child” — about a child McGovern had out of wedlock before he married Eleanor McGovern in 1943 dogged McGovern for much of his political career. Before McGovern died in 2012, he told Thomas J. Knock, a history professor at Southern Methodist University in Dallas, about the affair and the daughter he had in 1941.

“He told me about it voluntarily about 15 years ago because he wanted me to write about it,” Knock told The Washington Post in a telephone interview. “He felt confident in my credentials as a historian and biographer to deal with it responsibly.”