A new study shows that many teenagers and young adults tend to overestimate how often they use a condom during sex, Reuters reports.

According to the article, the study asked 715 African-American women between the ages of 15 and 21 how many times they'd had sex in the past two weeks and how many times they used protection. The women also provided vaginal-fluid samples to be tested for evidence of sperm.

Of the women who had been sexually active in the past two weeks, 186 said they'd used a condom every time. However, 34 percent of these women had Y-chromosome DNA—evidence of sperm—in their vaginal-fluid samples.

Researchers are still trying to determine the reasons for the discrepancy—whether the women were mistaken, misused the condoms, or had given the perceived more socially desirable answer when asked about condom use. “Regardless of whether the problem is condom user error or misreporting, the unfortunate result—in terms of risk for [sexually transmitted infections] and HIV—is the same,” said led researcher, Eve Rose of Emory University in Atlanta.

Researchers believe studies that use both self-reports and objective measures of condom use may provide better data on young people's sexual behavior and risks for STIs.

The women who participated in the study were enrolled in an HIV-prevention program.