Some people are deeply religious and others not at all. Evolutionary psychologists are interested in determining the functions of religiosity in social life that lead to this diversity. Two camps within the discipline have conflicting hypotheses; one believes religion’s essence is in promoting behaviors related to cooperation, while another believes that the attraction of religion has to do with sex and reproduction.

In a new study published in the journal Evolution and Human Behavior, University of Pennsylvania psychologists have used a massive global survey to show that people’s attitudes towards sexual behaviors is far more predictive of their religiosity than their attitudes toward cooperation.

The study was conducted by senior researcher Jason Weeden and professor Robert Kurzban of the Department of the Psychology in Penn’s School of Arts and Sciences.

At the core of their research is data from the World Values Survey, which has been administered to nearly 300,000 people from more than 90 countries for the last two decades. The survey contained questions on a wide variety of subjects, including respondents’ views on culture, religion, politics and morality.

One section asked respondents to say how much they felt certain behaviors could be justified. The researchers categorized some of these behaviors, such as “lying in your own interest,” “stealing property” and “cheating on taxes if you have a chance” as having to do with “cooperative” morals. They selected others, including “abortion,” “sex before marriage” and “homosexuality” as representing “reproductive” morals.     

The researchers compared respondents’ answers in these two categories with answers to questions about their religiosity, such as belief in god and the frequency with which they attended religious services.  

“When you make this comparison, it turns out that reproductive morals are the big, clear winner,” Weeden said. “Once you know the difference between how people think about things like abortion, premarital sex and divorce, you don’t learn anything else about how religious they are by asking what they think about lying and stealing.”

“It’s clear that people in religious groups cooperate,” Kurzban said, “but it’s never been clear that religious groups are more cooperative than other kinds of groups. In fact, you can learn a whole lot more about who is religious by asking them about their reproductive morals than about their cooperative morals.”

While this relationship was consistent the world over, the researchers found differences in the strength of the correlation between regions.

“In wealthy regions, such as parts of Asia, Western Europe, North America, Australia and New Zealand, the relationship between religiosity and reproductive morals is really big,” Weeden said. “But for poorer regions, the correlations are smaller. Outside of countries that have suppressed religion, people in poorer countries tend to be highly religious and overwhelmingly conservative when it comes to reproductive morals but not necessarily cooperative morals.”

This new study builds upon research Weeden and Kurzban have been conducting during the last several years on the way people make decisions about religion.   

“The usual story is that being raised in a religion causes you to have certain beliefs about abortion or premarital sex,” Kurzban said. “We think the causality works both ways. The major story most people miss is that the way people want to live can have a big impact on whether they want to be religious or not. People want to live their lives a certain way, and then look around to see if being religious helps them do that.”

“We think of being involved in a religious group as a social tool; it’s either useful or not useful to you,” Weeden said. “What churches do, in developed countries in particular, is provide an environment that helps out people who want committed relationships and more children. People in religions monitor each other, put social costs on casual sex and related areas that make those behaviors less likely to happen. They also provide services, like daycare and babysitting, social safety nets that are particularly useful to people who have lots of young children.

“If you live a lifestyle where a stable marriage and lots of children is important to you, belonging to a church mitigates some of the risks that go along with that lifestyle, making religion an attractive tool. But if you’re, say, a college student who likes to party and isn’t planning to get married or have kids for a long time, all you’re getting from a religion is a bunch of hassles. So in the real world what happens is that those people stop going to church, even if they were raised religious.”