“Something that may surprise a lot of people,” said Conrad Hackett, a primary researcher on the report, “is that the third-largest religious group, after Christians and Muslims, is the religiously unaffiliated. There may have been some guesses floating out there before, but this is the first time there are numbers based on survey data analyzed in a rigorous and scientific way.”

More than three-quarters of the religiously unaffiliated live in Asia, the majority in China. Many of the people in this group do hold some religious or spiritual beliefs and may even believe in a deity, but they do not identify with a particular faith.

People who practice folk or tribal religions, like African, Chinese, American Indian and Australian aboriginal traditional religions, make up another 6 percent of the world’s population. Smaller faith groups — like Bahais, Jains, Sikhs, Shintoists, Taoists, Wiccans and Zoroastrians — combined make up less than one percent of the global population. Jews, with about 14 million, make up only 0.2 percent of the global population.

The study is based on analysis of 2,500 different data sources, including censuses and demographic surveys of children and adults in 232 countries. It relies on self-identification, so it includes people who are not regular practitioners or orthodox believers of the religion they claim.

The religious groups with the youngest median ages have the highest levels of fertility and poverty and the lowest levels of female education, Jack A. Goldstone, director of the Center for Global Policy at George Mason University, in Virginia, said in an interview by e-mail. He noted that in predominantly Muslim countries where the government supports the education of women and girls, like Tunisia, Iran and Indonesia, the median age is higher than for Muslims in Africa and the Middle East.