A spokeswoman for Dr. Birx declined to comment in an email.

Dr. Mark Dybul, who directed Pepfar during most of the Bush administration and now runs the Global Fund to Fight AIDS, Tuberculosis and Malaria, said by email that he did not wish to comment. He noted that previous studies by Dr. Bendavid had shown that Pepfar prevented infections and saved lives.

Michael Gerson, now a columnist for The Washington Post who was a Pepfar advocate and a close adviser to President Bush, also said he would have no comment on the study until the experts he trusted could read it.

Mr. Lo said he spent a year analyzing dozens of health surveys that the United States paid for in countries around the world.

Originally called the World Fertility Surveys, they were begun in the 1970s. They were later subsumed into the large Demographic and Health Surveys, now paid for by the United States Agency for International Development, that document health behaviors in dozens of countries. Spending on abstinence and fidelity peaked in 2005 and began to drop after the Obama administration took office in 2009.

Mr. Lo compared data from 1998 to the present in 22 African countries, 14 of which received Pepfar money and eight that did not. He looked at answers to three questions that are part of the extensive questionnaire given to people interviewed: What was your age when you had sex for the first time? At what age did you have your first child? How many people have you had sex with in the last year?

When answers about age at loss of virginity did not appear to be truthful, he said, he used a conservative form of adjustment, calculating backward from the birth of the first child.

Although the numbers changed over time, the differences between the Pepfar and non-Pepfar countries did not change after 2005. That indicated “no detectable effect” from the expenditure, he said.