A new study sponsored by the United Nations lends credibility to faith leaders who have long argued that behavioral change is key to combating the spread of AIDS, says a Catholic expert on the disease. “Within the United Nations, there is more and more attention to focusing on abstinence and the reduction of the number of sexual partners as well as the strategy of promoting condoms,” said Msgr. Robert Vitillo, special adviser to Caritas Internationalis on H.I.V. and AIDS. “This is a validation of what we’ve done.” Released on July 13, the study from the Joint U.N. Program on H.I.V./AIDS indicates that the prevalence of H.I.V. among young people has declined by more than 25 percent in 15 of the 21 countries most affected by the disease. In eight countries, the declines in H.I.V. rates resulted, at least in part, from positive changes in sexual behavior among young people, including youth waiting longer before they become sexually active and having fewer partners.