Between 2006 and 2010, the number of unmarried teenagers between the ages of 15 and 19 who reported to the Centers for Disease Control and Prevention that they’ve had sex dropped below 50 percent. The C.D.C. described this as an actual drop in the number of teenagers having sex; the cynical addition of the word “reported” is mine: obviously the only way for the C.D.C. to determine whether those teenagers were sexually active (and how) was to ask them, and I’ve expressed my doubts about the resulting data.

Now the Guttmacher Institute is backing up those teenagers with hard facts: in 2008, the pregnancy rate among teenagers dropped to its lowest point in more than 30 years. In 1990, when the rate was at its highest, 116.9 out of every thousand girls between the ages of 15 and 19 became pregnant; in 2008, only 67.8 did. Among young women under 15, the pregnancy rate fell even more.

The Guttmacher Institute offers no conclusions about what lies behind the welcome change. For that, a return to those C.D.C. numbers might help: of the teenagers who admitted to having had sex, most reported that they used birth control, even for their “first time.”

A representative of the Guttmacher Institute just let me know that it did, in fact, draw some conclusions. “A large body of research has shown that the long-term decline in teen pregnancy, birth and abortion rates was driven primarily by improved use of contraception among teens.”