Sex may be on everybody's mind, but a new Kinsey Institute survey finds that most Americans don't know much about it.

Fifty-five percent of those surveyed flunked a test of basic sexual knowledge, Kinsey director June Reinisch said last week. Only about 25 percent knew that the typical American has intercourse for the first time at age 16 to 17 or that an estimated 30 to 40 percent of married men have had an affair, the report said. Only 21 percent knew that more than a quarter of men have had a sexual experience with another male as an adolescent or adult.

And half incorrectly thought that anal intercourse itself can cause AIDS, even if neither partner is infected with the AIDS virus.

The poll, conducted by the Roper Organization last year, is described in a new book, "The Kinsey Institute New Report on Sex." At a news conference, Reinisch said the poll was financed by the book's publisher, St. Martin's Press, and the Kinsey Institute for Research in Sex, Gender and Reproduction.

The poll involved face-to-face interviews with a nationally representative sample; 12 questions were answered on a form that was slipped into an envelope before it was returned to the interviewer. The questions in the poll were drawn from those most often asked of the Indiana-based Kinsey Institute and dealt with a variety of topics, including circumstances under which pregnancy is possible, erection problems and masturbation.

Those from the Midwest generally scored best, with 55 percent passing, followed by 47 percent in the West and 40 percent in the South and Northeast. "We never would have predicted this," Reinisch said.

Overall, there were no differences in accuracy by gender, but men did better in questions about sexual behavior and women in questions about sexual health and contraception, Reinisch said.