The positively correlated associations between religiosity (of a Christian-centric nature) and low levels of promiscuity and high levels of abstinence and marriage-centricity are generally quite clear enough. We often see that religion seeks to suppress sexual promiscuity through its doctrines that promote a monogamous sexual reproductive strategy characterized by low promiscuity, exclusive heterosexuality and high premiums on marriage and fertility.
Intuitively, we might guess that our sexual inclinations owe to how much exposure we have towards our adopted religion. Thus, we might, quite reasonably, suppose that someone who regularly attends Sunday school and comes from a religiously devoted family with staunch practices in the home would more likely be shaped into a long-term mating, marriage-inclined and sexually abstinent person, than someone who does not observe those traditions and customs. Indeed, it was found that treating premarital sex as sinful creates incentives to marry earlier, and condemning abortion and birth control as sin makes people have children.
However, when we look at the US, which is considered the most religious nation compared to its other western counterparts, what is fascinating is that it is remarkably evenly divided - approximately 42% of adults never attend religious services, 18% attend intermittently, and 40% attend services regularly (information from the 2006 US General Social Survey).
This suggests that, while a country may adopt non-secular values to predominantly guide its affairs and inform its citizens, not everyone may agree or be inclined to go along with those values. In the case of the US, this divide is exemplified by the emergence of the Religious Right and the Liberal Left.
As evolutionary psychologist Kenrick (2011) colloquially and aptly states it, "the prototypical member of the Liberal Left ... may wait until at least the end of college before marrying and beginning to have children and then may delay even a few years longer to go to graduate school, law school or medical school. Because the human ability to resist sexual urges has a hard time outlasting all that postponement, these folks do not like the Religious Right's attempts to impose rules against premarital sex [or] tools of family planning. ... [The Liberal Left pose a problem for the Religious Right] because a large number of sexually loose young people playing the field threatens to disrupt the strict system that religious folks have set up to enforce and reinforce family bonds."
Working on that insight, Weeden, Cohen and Kenrick (2008) proposed the reproductive religiosity model - instead of religiosity affecting our mating strategy (whether we can be promiscuous short term maters, or should be committed, abstinent long term maters), it is instead our mating strategy that makes us calculate the costs and benefits of adopting a religion, or remaining devoted to our current religion. If I am unable to bear the cost of abstinence from premarital sex and I do not want to marry early, my exit strategy is to drop my impeding religion.
By analyzing data from two large sources - 21,131 respondents in the 2006 US General Social Survey and 902 undergraduate students who were probed about their family plans, sexual attitudes, religious attendance, and moral feelings about issues ranging from lying to stealing - it was found that the strongest predictors of religiosity were factors related to sexual and family values. While there were other typical variables that predicted for religiosity, such as being female, older, or a non-drinker, and being high in conscientiousness and low in sensation-seeking, statistically controlling for sexual and family value items made the links between these other typical variables with religiosity disappear. In other words, everything we might have assumed to be associated with religiosity can be boiled down to sexual and family values. The study by Weeden, Cohen and Kenrick (2008) thus provide evidence that, on average, whether we are religious or not in the first place depends on how promiscuous we want to be.
If that causal link is true, could it be possible to manipulate people's mating strategy and thus alter their religiosity, in the psychology laboratory no less?
A study by Li, Cohen, Weeden and Kenrick (2010) sought to test that idea. A cleverly deceptive cover story and elaborate experimental design was used, but in brief, participants were ultimately made to look at either desirable members of their own sex or desirable opposite sex members (such a priming method has been found to be effective in conjuring either a mating motivation state - when we check out attractive opposite sex persons - or a mating threat state - when we are made to look at attractive same sex persons). Participants were also made to fill out a survey on the pretext of finding out their attitudes; embedded in the survey were questions pertaining to religiosity.
The results showed that when the men looked at attractive ladies and when the women looked at attractive guys, there was no discernable effect of mating motivation on religiosity. Interestingly, the laboratory setting was unable to capture any desire to give up religion when participants were made to feel more motivated to mate. However, what was more interesting was that, instead, participants who looked at attractive members of one's own sex expressed greater belief in religion. Being primed with mating insecurity leads people to become more religious.
We see support here for the classic antagonism played out between the Religious Right and Liberal Left. Once again, Kenrick (2011) states it best, so he will be quoted here: "When you become aware that there are a lot of highly attractive mating competitors out there, it reduces the perceived benefits of playing a fast and loose mating strategy ... For women, a lot of attractive competitors means less attention from the attractive men who might provide good genes, and fewer fellows vying to support your offspring. For men, on the other hand, an abundance of especially handsome and available guys means that if you are playing the field, you may be playing with yourself for most of the game. Under circumstances of limited opportunities, any normal person - who does not look like a fashion model - could benefit from the religion-based supports for monogamy."
This is not to say that religious practices do not reduce sexual promiscuity - all other things equal between two people who are subjected to different levels of religious piety, we would expect the one who has been told that things like premarital sex are sinful would be less inclined to do the deed. However, these studies highlight another crucial direction in the causation, that sometimes people may choose how religious they want to be based on the perceived cost of carrying out sexual "transgressions" under the religion they are affiliated to. And at the heart of the differing values the Religious Right and the Liberal Left promote, each camp is sustainable because they encourage and reinforce different mating patterns; there is antagonism only because a clash of these value systems is highly disruptive to each side's foundations for their own reproductive status quo.
Weeden, J., Cohen, A., & Kenrick, D. (2008). Religious attendance as reproductive support Evolution and Human Behavior, 29 (5), 327-334 DOI: 10.1016/j.evolhumbehav.2008.03.004
Li YJ, Cohen AB, Weeden J, & Kenrick DT (2010). Mating Competitors Increase Religious Beliefs. Journal of experimental social psychology, 46 (2), 428-431 PMID: 20368752