The numbers of people without a religious affiliation are increasing in some countries. But some demographers project that the global population will be more religious, not less, for one simple reason: religious believers tend to have more babies.

The growth in the religiously affiliated population is “largely about having relatively high fertility rates,” Conrad Hackett, a demographer at the Pew Research Center, told CNA Dec. 15.

“There is such rapid growth expected in parts of the world that currently really have little to no unaffiliated presence, Africa in particular. India is another place where a lot of population growth is expected,” he said.

The religiously unaffiliated population is growing in some countries such as the U.S., where they have been nicknamed the “nones.” However, the largest populations of the unaffiliated are in Japan and other countries with aging populations that are not replacing themselves.

“Therefore the global story is that the unaffiliated are expected to decrease as a share of the world's overall population,” Hackett said.

Hackett is one of the authors of the study, “The future size of religiously affiliated and unaffiliated populations,” published April 2 in the journal “Demographic Research.” Researchers' projections took into account factors such as a population's religious composition, differences in fertility rates, age structure, and patterns in changing to and from religious belief.

The researchers projected that in the year 2050 the religiously unaffiliated will make up 13.2 percent of the world population, a decline from 16.4 percent in 2010.

“The religiously unaffiliated are projected to decline as a share of the world's population in the decades ahead because their net growth through religious switching will be more than offset by higher childbearing among the younger affiliated population,” the paper said.

Religiously unaffiliated have both low fertility and an old age structure. While this population segment is projected to grow in North America and Europe, it will decline in the populous Asia-Pacific region.

The researchers projected that in the year 2050 the religiously unaffiliated will make up 13.2 percent of the world population, a decline from 16.4 percent in 2010. They noted that the median age of women with a religious affiliation is six years younger than that of unaffiliated women. The 2010-2015 total fertility rate for those with a religious affiliation is 2.59 children per woman, compared with 1.65 children per woman without a religious affiliation.

Hackett told CNA that in the West younger women, who are more likely to have children, are also more likely to be unaffiliated. At the same time they are likely to have fewer children than women who affiliate with a religion.

The religiously unaffiliated percentage of a population can keep pace with the religious in certain countries also because they benefit from switching between religious affiliation and non-affiliation. In countries like France and the U.S., their numbers benefit because people have been changing their self-identification from Christian to non-religious.

Hackett and his researcher colleagues also published a related report for the Pew Research Center, “The Future of World Religions: Population Growth Projections 2010-2050.”

He said one surprising finding of the report, which received significant media coverage earlier this year, was “the degree to which Muslims are expected to outpace the world’s overall population growth.”

The researchers projected Muslims to grow from 23.2 percent of the population in 2010 to 29.7 percent in 2050, an estimated increase in population size of 73 percent. By comparison, the Christian percentage of the world population is projected to remain stable at 31.4 percent, with an estimated increase of 35 percent in population size.

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