The pages contain four "dirty jokes" and an explanation of sex, contraception and prostitution, the ABC reports, which were covered with gummed pieces of brown paper when the diary was first found - presumably Anne's attempt to hide her risqué writing from her family.
Anne wrote her diary while she and her family spent two years in hiding in an Amsterdam canal-house, in an attempt to avoid Nazi occupiers during World War II. "I'll use this spoiled page to write down "dirty" jokes", she wrote-then listed four, along with an imagined lesson on sex education and some information on prostitutes.
In the passage on sex, Anne described how a young woman gets her period around age 14, saying that it is "a sign that she is ripe to have relations with a man but one doesn't do that of course before one is married".
Cooped up in an attic for months on end, Anne Frank jotted down some of her observations about life in her diary.
And about prostitution, she observed, "All men, if they are normal, go with women, women like that accost them on the street and then they go together".
"It's a very cautious start to her becoming a writer", he said.
The pages' content had remained unknown for decades because Frank had erased them. The passages show how she "creates a fictional situation that makes it easier for her to address the sensitive topics that she writes about", he said. "Papa has been there". That the pages existed was already known, but she had pasted the two original diary pages with brownish paper.
In August 1944, Nazi soldiers found them and the Frank family were immediately sent to Auschwitz.
After the war, Otto Frank had his daughter's diary published, and it went on to become a symbol of hope and resilience that has been translated into dozens of languages.
Anne, who died in a Nazi concentration camp before she turned 16, would have written the passages aged between 13 and 15.
The Anne Frank House, a museum located in Frank's former hiding place, did not quote directly from the text it had recovered.
Her only surviving family member, her father, published her diary in 1947.
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