Researchers in the Dutch capital have recovered two pages from one of Anne Frank's diaries that contain passages on sexuality. He said these pages were important because they show Frank's first foray into trying to write in a more literary tone.
Mr Leopold added that Anne wrote about the subject elsewhere in diary pages that already have been published.
"Anyone who reads the passages that have now been discovered will be unable to suppress a smile", said Frank van Vree, director of the Institute for War, Holocaust and Genocide Studies, which participated in the deciphering of the pages.
He said the jokes "make it clear that Anne, with all her gifts, was above all also an ordinary girl". The statement said that the pages include four dirty jokes, some crossed out phrases and a text in which Anne Frank imagines herself teaching sexual education. She wrote the recently discovered pages on September 28, 1942 at the age of 13, about three months after the Frank family went into hiding, and later covered them up with brown paper, the Associated Press reports.
"It's a very cautious start to her becoming a writer", Leopold told the New York Times.
They photographed the pages, backlit by a flash, and then used image-processing software to decipher the words, which were hard to read because they were jumbled up with the writing on the reverse sides of the pages.
In addition to the jokes, Anne summarizes what a period is, describes the mechanics of sex in couched terms, and relays what she has heard of prostitution. For a period of time in hiding she would write down her experiences and thoughts.
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One of her jokes was this: "Do you know why the German Wehrmacht girls are in Holland? One evening he came home and then he saw his friend in bed with his wife, then the man said: 'He gets to and I have to!'"
Anne Frank wrote the jokes on September 28, 1942, just three months after the family began their two-year stay at the hiding place.
In 1944, the Frank family was discovered, arrested and sent to separate concentration camps.
Her father Otto was the only member of the family to survive the war. Anne Frank and her sister Margot died at Bergen-Belsen in 1945.
The house where the Franks hid has been turned into museum, making it one of Amsterdam's most visited attractions.
Her diary was published two years later and has been read worldwide and translated into at least 60 languages.