When Dr. Ruth Westheimer walks by a bookstore and sees a book with “sex” in the title, she has but one option: she must buy it.

“God forbid there’s anything I don’t know about sex,” she said in a lecture to professors Naomi Rogers and Janet Henrich’s “Women’s Health” class, shuddering at the thought.

The obsession makes sense for one of the few people who can actually pull off the title of “sexpert” without a hint of irony.

Westheimer, known commonly as Dr. Ruth, the seemingly omnipresent 75-year-old “psychosexual therapist” and an expert in media psychology, is a star of a children’s television show; a former sniper; a spokeswoman for a variety of products ranging from Clairol’s Herbal Essences shampoo and, yes, body wash, to the Eroscillator, a dildo with the Dr. Ruth stamp of approval; and, this semester, a Yale professor.

Her Calhoun College seminar, “Personal Fulfillment and Intimacy in the Contemporary American Family,” which she co-teaches with Calhoun Master William Sledge and Calhoun Dean Stephen Lassonde, received scores of student shoppers on the first day of class. All 18 students who were admitted enrolled in the class, Sledge said.

Why the interest? Surely Yalies have a great desire to explore the function of the family in modern American culture. But there is, of course, that other reason.

“Come on, Dr. Ruth is teaching it,” Kirsti Potter ’04, who is taking the seminar, said.

But surprisingly, it was Westheimer’s Jewish identity, and not her fame as a sexual guru, that first brought her to Yale. Following the 1995 assassination of Yitzhak Rabin, the former prime minister of Israel, Dr. Ruth attended a memorial concert for him in New York. One of the performers was Inbal Megiddo ’98, a student in Calhoun and an accomplished cellist. Megiddo played a kaddish — a prayer for the dead — so moving that Dr. Ruth approached her after the service. Megiddo, who recognized Dr. Ruth, asked her to give a Master’s Tea at Calhoun. Dr. Ruth agreed. After numerous logistical difficulties that delayed her tea, she finally arrived.

The tea was a success, and Dr. Ruth soon agreed to help organize a college seminar with Sledge, a psychologist, and Lassonde, a historian. The entire class was premised on the desire to give Ruth a forum to let her personality and extensive experience shine.

“She had a real ability to communicate and it didn’t take a genius to figure that out,” Sledge said.

Potter said Dr. Ruth is not content to act as a color commentator.

“She’s so willing to stick her two cents in even when the academics try to run the show,” Potter said. “She’s interested in all of us learning important stuff she thinks we need to live happier lives.”

Meeting the sultan of sexual literacy herself is a bizarre experience. When Dr. Ruth arrived at the Calhoun Master’s Office for an interview, the room became a cauldron of noise and frantic activity as a master’s aid maneuvered Dr. Ruth’s large suitcase full of stuff (“I travel like the Queen of Sheba,” Dr. Ruth said later). Hugs and handshakes were distributed like candy on Halloween.

She bounded off to Sledge, disappearing into his office. After a brief moment, a buoyant, accented “You’re looking fabulous!” pierced the air.

Dr. Ruth has two sayings drawn “from the Jewish tradition” (she was raised Orthodox but is now a Reform Jew) that guide her career. One is “when you stand on the shoulders of giants, you can see farther,” which is especially fitting, considering that one of America’s most widely recognized authority figures on media psychology and human sexuality stands a mere 4 feet 7 inches tall.

The other saying is equally apropos. She asserts firmly that “a lesson taught with humor is one that is retained.” During lectures on serious topics such as the current state of American sexual education, she delivers one-liners with gusto.

“There are some women who like cunnilingus, for him to go down on her,” she said during her “Women’s Health” lecture. “And there are some men who think its dark down there.”

As a preeminent sex educator, Dr. Ruth seems unorthodox. Her short stature is coupled with a sharp German accent that lend her words a guttural harshness. She is unwaveringly positive and upbeat, but, as a consequence of her having been orphaned at the age of 10, she is continually haunted by the Holocaust.

Dr. Ruth owns a clock that plays a tune by Haydn which she loves. It is a song written in 1797 that was appropriated by Hitler and the Third Reich as the national anthem of Nazi Germany (the words were changed). She said she keeps the clock on principle, declaring that she would not allow Hitler to steal anything else from her. Still, she said, she is often reluctant to hear the piece.

“The Nazis are not going to take away that melody of Haydn from me,” she said. “But it is a problem. I’m sad at the same time — For me the Holocaust will never recede.”

The path from orphan to celebrity was not a smooth or quick one. At the age of 16, the diminutive Westheimer joined the Haganah, the Jewish freedom fighters who battled to help establish Israel’s independence. She was wounded, badly, in the legs.

After studying in Paris at the Sorbonne and later teaching kindergarten, she arrived in the United States in 1956, where she began her investigations of family dynamics. She received a doctorate of education in the Interdisciplinary Study of the Family from Columbia University Teachers College in 1970, when she was 43.

The Dr. Ruth that America — and much of the world — recognizes did not begin her career until she was in her fifties. In 1980, she was hired to host a taped, 15-minute radio program in which she answered listeners’ questions about sex and the family. The Dr. Ruth phenomenon exploded from there.

Though Web sites offer sound clips of Dr. Ruth saying certain irresistible words in her trademark accent, such as “pimp” and “scrotum,” she maintains that being provocative is wholly different from being scandalous. If her status as a short Jewish grandmother with a German accent allows more people to hear what she has to say, then so much the better.

At 27 books and 450 television shows and counting, Dr. Ruth is nowhere near retirement (“God forbid”). She has been working on a manuscript about herpes, which she hopes to publish soon. And though she decided recently to give up water skiing, Dr. Ruth says she has a new task — finding a companion (she has been married three times and is a widow).

And of course, she said, she has tried the body wash.

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