USA Today is reporting a three-year RAND Corp. study showing that virginity pledges do deter some teens from having sex. Of 1,517 adolescents ages 12 to 17 in 2001 when the research began, teenagers who vowed to remain virgins until they were married were less likely to be sexually active than others who didn’t make a pledge.
About one-quarter of the adolescents surveyed (23.8%) made a promise to wait until marriage to have sex; 34% had broken it by 2004, compared with 42% of those who didn’t make the pledge and had sex during that time.
My Take?

I agree with Valerie Huber, executive director of the National Abstinence Education Association, who said in USA Today, “A lot of people would say a virginity pledge is an equivalent to an abstinence education program. It’s certainly not.” 
She goes on to point out, “(A virginity pledge is) a single event with a personal commitment, but there’s often not follow-up. We think abstinence education is a natural reinforcement of this personal decision they have made.”
However, my take is that virginity pledges are not nearly as valuable as purity pledges. The former involves only vaginal intercourse. The latter involves not only all physical sexual activity, but also the brain, attitudes, and emotions. 
And, we know that teens and young adults do respond to the positive encouragement and expectations of their parents.
Although many falsely believe that the vast majority of teens are having sex, the truth is that the CDC reports that a majority (52%) of U.S. high school students say they have not had sex.
Waiting until marriage to have sex is good not only from a physical health standpoint, but also from an emotional, relational, and spiritual perspective. 
You can learn more about helping your teen make highly healthy decisions in my book God’s Design for the Highly Healthy Teen.