The study in the latest issue of JAMA Pediatrics looked at the results of two surveys to determine how common it is for US boys to have sex before the age of 13.

A total of 7.6 percent of the nearly 20,000 male high school in one study reported having sexual intercourse before the age of 13.

In the second study -- of 7,700 males between the ages of 15 and 24 years old -- the figure was 3.6 percent.

The proportion varied widely by geography -- from five percent in San Francisco to 12 percent in Houston, Texas, to 25 percent in Memphis, Tennessee.

It also varied by race and ethnicity with non-Hispanic black and Hispanic male adolescents more likely than their white peers to have had sexual relations before becoming teenagers.

In the survey of 20,000 male high school students, 19.0 percent of the non-Hispanic blacks reported having sexual intercourse before the age of 13, 4.4 percent of the non-Hispanic whites, 9.0 percent of Hispanics and 7.8 of those classified as others.

In the survey of 7,000 males between the ages of 15 and 24, the numbers were 10.5 percent for non-Hispanic blacks, 2.2 percent for non-Hispanic whites, 3.4 percent for Hispanics and 1.6 percent for others.

The authors of the study noted that while the overall numbers differed in the two surveys they showed similar differentials by race and ethnicity.

"Broad cultural scripts about masculinity and sex hold that men should start having sex early and have sex often," the study's authors said.

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"For young men of color, particularly black males, racist stereotypes of hypermasculinity may also contribute to expectations of early sexual initiation."

The study found that boys whose mothers did not have a college degree were more likely to report having had sexual intercourse before the age of 13.

Among those aged between 18 and 24 who reported having sex before the age of 13, 8.5 percent characterized it as unwanted, 37 percent said they had mixed feelings about it and 54.6 percent described it as wanted.

"These findings underscore the need for providing comprehensive sex education that is culturally informed and inclusive before an individual's first sexual encounter and ensuring that health care practitioners discuss sex with their male patients starting during middle school years or earlier," the study's authors said.

The study did not look at girls' sexual behavior but cited other research as saying that boys were more than twice as likely as girls to experience sexual intercourse before the age of 13.

The study was conducted by Laura Lindberg of the Guttmacher Institute in New York and other researchers.