The study also found a wide disparity in the median age of religious populations, with Muslims and Hindus the youngest, and Buddhists and Jews the oldest. The median age of the youngest group, Muslims, was 23, while the median for Jews was 36.

Over all, Christians (including Catholics) are the largest religious group, with 2.2 billion people, about 32 percent of the world’s population. They are followed by Muslims, with 1.6 billion, about 23 percent. There are about one billion Hindus, about 15 percent of the global population, and nearly half a billion Buddhists, about 7 percent.

The study, “The Global Religious Landscape,” is a snapshot of the size and distribution of religious groups as of 2010, and does not show trends over time.

“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.