Reliance on abstinence-only sex education as the primary tool to reduce teenage pregnancies and sexually transmitted diseases — as favored by the Bush administration and conservatives in Congress — looks increasingly foolish and indefensible.

The abstinence-only campaign has always been driven more by ideology than by sound public health policy. The program’s tight rules, governing states that accept federal matching funds and community organizations that accept federal grants, forbid the promotion of contraceptive use and require teaching that sex outside marriage is likely to have harmful psychological and physical effects.

At least nine states, by one count, have decided to give up the federal matching funds rather than submit to dictates that undermine sensible sex education. Now there is growing evidence that the programs have no effect on children’s sexual behavior.

A Congressionally mandated report issued this month by the Mathematica Policy Research firm found that elementary and middle school students in four communities who received abstinence instruction — sometimes on a daily basis — were just as likely to have sex in the following years as students who did not get such instruction. Those who became sexually active — about half of each group — started at the same age (14.9 years on average) and had the same number of sexual partners. The chief caveat is that none of the four programs studied continued the abstinence instruction into high school, the most sexually active period for most teenagers, so it is not known whether more sustained abstinence education would show more effectiveness.