A few months ago, the Washington Post ran an article titled, "Stop assuming that families with lots of children are religious." The article supported its case with some data from Pew (2014 Religious Landscape Study), which surveyed 35,000 Americans, and anecdotes about the plight of large secular families that must bear the heavy burden of answering questions from strangers who assume that they're religious.
One of the maternal units was quoted as saying: "People assume. They’ll come up and start talking about God to us. Which is a little awkward at times."
Unfortunately for them, new research is about to make their awkward lives even more awkward. A study just published in the Proceedings of the Royal Society B concluded that religious people really are more likely to procreate.
The study used two different data sets. The first came from the IPUMS International census, and it included 3.6+ million women of various faiths in 32 countries. The authors discovered that women in marriages that were religiously homogamous (i.e., in which the husband and wife share the same denomination) were likelier to produce more children. Additionally, they were less likely to be childless.
The second data set included 3,146 women in Wisconsin, about 86% of whom were in religiously homogamous marriages. These women showed similar patterns as those in the first data set. Additionally, the authors found a positive (but non-linear) association between how frequently a woman attended religious services and the number of children she had.
Perhaps counterintuitively, the authors discovered that a higher level of education strengthened the relationship between homogamy and number of children.
What explains all this data? The authors speculate that factors such as: (1) greater marital happiness and stability; (2) a lower likelihood of using contraception (either due to personal preference or religious conviction); and (3) a greater likelihood of holding traditional views of marriage and childbearing may lead religiously homogamous couples who attend services frequently to procreate more often.
The authors point out, however, that this correlation does not exist everywhere. In Brazil, for instance, religious homogamy is linked to women having fewer children. For some reason, in this overwhelmingly Catholic country, Genesis 9:7 does not apply.
Source: Martin Fieder, Susanne Huber. "The association between religious homogamy and reproduction." Proc. R. Soc. B 283: 20160294. Published: 13-July-2016. DOI: 10.1098/rspb.2016.0294