Overall, 84 per cent of the world's inhabitants, which it estimated at 6.9 billion, identify with a religion, according to the study entitled 'The Global Religious Landscape' issued by the Pew Forum on Religion and Public Life on Tuesday.
Atheists now are the third largest group of people after Christians and Muslims.
Pew Forum demographer Conrad Hackett said the 2,500 censuses, surveys and population registers used to compile the report did not allow a further breakdown to estimate the world population of atheists and agnostics.
'It's not the kind of data that's available for every country,' he said. 'A census will typically ask what your religion is and you can identify a number of particular affiliations or no religion.
People not affiliated with any particular religion stand out for their relative youth compared with other religious traditions. Among the unaffiliated, 31% are under age 30 and 71% are under age 50. Comparable numbers for the overall adult population are 20% and 59%, respectively.
The study estimated Christianity was the largest faith at 2.2 billion adherents or 31.5 percent of the world's population.
The Roman Catholic Church makes up 50 per cent of that total, with Protestants - including Anglicans and non-denominational churches - at 37 percent and Orthodox at 12 percent.
Notable points of interest of the report include:
- Men are significantly more likely than women to claim no religious affiliation. Nearly one-in-five men say they have no formal religious affiliation, compared with roughly 13% of women.
- Among people who are married, nearly four-in-ten (37%) are married to a spouse with a different religious affiliation. (This figure includes Protestants who are married to another Protestant from a different denominational family, such as a Baptist who is married to a Methodist.) Hindus and Mormons are the most likely to be married (78% and 71%, respectively) and to be married to someone of the same religion (90% and 83%, respectively).
- Mormons and Muslims are the groups with the largest families; more than one-in-five Mormon adults and 15% of Muslim adults in the U.S. have three or more children living at home.
- The Midwest most closely resembles the religious makeup of the overall population. The South, by a wide margin, has the heaviest concentration of members of evangelical Protestant churches. The Northeast has the greatest concentration of Catholics, and the West has the largest proportion of unaffiliated people, including the largest proportion of atheists and agnostics.
- Of all the major racial and ethnic groups in the United States, black Americans are the most likely to report a formal religious affiliation. Even among those blacks who are unaffiliated, three-in-four belong to the "religious unaffiliated" category (that is, they say that religion is either somewhat or very important in their lives), compared with slightly more than one-third of the unaffiliated population overall.
- Nearly half of Hindus in the U.S., one-third of Jews and a quarter of Buddhists have obtained post-graduate education, compared with only about one-in-ten of the adult population overall. Hindus and Jews are also much more likely than other groups to report high income levels.
- Three-in-four Buddhists say they are converts to Buddhism.
- Jehovah's Witnesses have the lowest retention rate of any religious tradition. Only 37% of all those who say they were raised as Jehovah's Witnesses still identify themselves as Jehovah's Witnesses.
- Members of Baptist churches account for one-third of all Protestants and close to one-fifth of the total U.S. adult population. Baptists also account for nearly two-thirds of members of historically black Protestant churches.