"This is reality-check research," said the study's author, Lawrence Finer. "Premarital sex is normal behavior for the vast majority of Americans, and has been for decades."

The study, which used statistics from the 1982, 1988, 1995 and 2002 National Survey of Family Growth, asked about 40,000 people ages 15-44 about their sexual behavior and traced the trends in premarital sex back to the 1950s.

Of those interviewed in 2002, 95 percent reported they had premarital sex; 93 percent said they did so by age 30. At the same time, people are waiting longer to marry; 2005 data show median age at first marriage is just over 25 for women and 27 for men.

The study found women virtually as likely as men to engage in premarital sex, even those born decades ago. Among women born between 1950 and 1978, at least 91 percent had had premarital sex by age 30, he said, while among those born in the 1940s, 88 percent had done so by age 44.

"The data clearly show that the majority of older teens and adults have already had sex before marriage, which calls into question the federal government's funding of abstinence-only-until-marriage programs for 12- to 29-year-olds," Finer said.

However, Janice Crouse of Concerned Women for America, a conservative group which strongly supports abstinence-only education, said she was skeptical of the findings.

"Any time I see numbers that high, I'm a little suspicious," Crouse said. "The numbers are too pat."