The researchers used digital photography techniques to decipher pages that Anne had masked with glued brown paper some time after writing them on September 23, 1942.
Reflections of tourists and canal houses are seen in the window of the Anne Frank museum in Amsterdam April 24, 2013.
The two pages are not the only time Frank jotted down dirty jokes or wrote about sexuality, although in later passages she treats the subjects more maturely.
The Anne Frank House Museum said at a presentation that it, and several Dutch historical institutes, were able to reproduce the lost pages after years of study by shining a light through them and photographing them in high resolution. "They make it clear that Anne, with all her gifts, was above all also an ordinary girl".
Researchers using digital technology deciphered the writing on two pages of Anne Frank's diary that she had pasted over with brown masking paper, discovering four naughty jokes and a candid explanation of sex, contraception and prostitution. 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.
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.
"The only element that might be interesting from the point of view about her development as a writer and as a teenager is the fact that she's creating, kind of, fiction" he said. Those passages were censored by her father before the diary was first published in 1947 but became available in more recent unabridged editions.
The pages, dated to September 28, 1942, were contained in the red-and-white checkered diary Anne had received for her birthday in June of that year, shortly before they went into hiding. "Papa has been there".
She also related this joke: "A man had a very ugly wife and he didn't want to have relations with her".
Anne wrote her diary while she and her family hid for more than two years during World War II.
In 1944, the Frank family was discovered, arrested and sent to separate concentration camps. Anne and her sister died in Bergen-Belsen camp. Only Anne Frank's father, Otto, survived the Holocaust.
The Anne Frank House, a museum located in Frank's former hiding place, did not quote directly from the text it had recovered.