The acceptance of premarital sex is at an all-time high, according to a recent study.

Researchers at San Diego State University found substantial generational shifts in attitudes toward non-marital sex and number of sexual partners.

"The changes are primarily due to generation -- suggesting people develop their sexual attitudes while young, rather than everyone of all ages changing at the same time," Jean Twenge, who led the study, said in a statement. "This has caused a large generation gap in both attitudes toward premarital sex and number of sexual partners."

For the study, researchers analyzed data from the General Social Survey, a nationally representative survey of more than 33,000 U.S. adults taken between 1972 and 2012. They found substantial generational shifts in attitudes toward non-marital sex and number of sexual partners. The results were published today in the Archives of Sexual Behavior.

Their findings suggests that the biggest change was between the Greatest Generation born in the early 1900s and the Boomers born in the 1940s-1950s, though 1980s-1990s born Millennials are more accepting of premarital sex than their 1960s-born GenX parents. After barely changing at all during the 1980s and 1990s, acceptance of premarital sex increased from 42 percent in 2000 to 58 percent in 2012.

Acceptance of same-sex sexual relations more than tripled from 13 percent in 1990 to 44 percent in 2012. Number of sexual partners (controlled for age) also shifted substantially, from 2.16 for the Greatest Generation to 11.68 for 1950s-born Boomers and 8.26 for Millennials.

"Millennials are more accepting of premarital sex than any previous generation, yet have had fewer sexual partners than GenX'ers. This is consistent with their image as a tolerant, individualistic generation accepting others' choices and making their own," Twenge said.

Twenge theorizes that these shifts in sexual attitudes and behavior are linked to growing cultural individualism in the U.S.

"When the culture places more emphasis on the needs of the self and less on social rules, more relaxed attitudes toward sexuality are the almost inevitable result," Twenge said.

The findings are detailed in the Archives of Sexual Behavior website.