In my previous post on the gender differences in morality (and thus religiosity), I hypothesized prematurely that religiosity was probably minimally genetic and mainly sociological at a 9:1 ratio of sociological:biological. As fate would have it, I stumbled across some twin studies that conclude that religiosity, as well as other pro-social phenomena, are closer to 50% inherited.
The more generalized twin study focused on overall sociological behaviors being genetic:
Findings from twin studies yield heritability estimates of 0.50 for prosocial behaviours like empathy, cooperativeness and altruism. First molecular genetic studies underline the influence of polymorphisms located on genes coding for the receptors of the neuropeptides, oxytocin and vasopressin. However, the proportion of variance explained by these gene loci is rather low indicating that additional genetic variants must be involved. Pharmacological studies show that the dopaminergic system interacts with oxytocin and vasopressin… The present experimental study tests a dopaminergic candidate polymorphism for altruistic behaviour, […] Altruism was assessed by the amount of money donated to a poor child in a developing country, after having earned money by participating in two straining computer experiments. Construct validity of the experimental data was given: the highest correlation between the amount of donations and personality was observed for cooperativeness… Carriers of at least one Val allele donated about twice as much money as compared with those participants without a Val allele… Cooperativeness and the Val allele of COMT additively explained 14.6% of the variance in donation behaviour. Results indicate that the Val allele representing strong catabolism of dopamine is related to altruism.
As I wrote in that previous post, empathy is highly correlated with religiosity. I also seemed to have guessed correctly that empathy was genetic; I was just wrong about how much it is genetic.
Here is the more specific twin study, itself referring to previous twin studies, focused on religiosity:
For decades, religiosity (defined as beliefs or behaviors towards superempirical agents) has been explored like other traits such as musicality, intelligence or skin color by Twin Studies – which conclusively found it to be partially inherited by genes and partially dependend on environmental (cultural) clues. In fact, religion turns out to be fully comparable to other biocultural traits such as speech or music.
Now, Kenneth S. Kendler, Hermine H. Maes and Todd Vance from the Virginia Commonwealth University in Richmond, VA, presented another Twin Study with rather large sample of 1106 monozygotic twins and 1501 dizygotic twins on "Genetic and Environmental Influences on Multiple Dimensions of Religiosity" (J Nerv Ment Dis 2010; 198: 755-761), DOI: 10.1097/NMD.0b013e3181f4ao7c.
Building on lots of earlier Twin Studies, they selected 78 religion-related items for their questionnaire, which were organized (by way of a statistical VARIMAX rotation) into 7 factors: General Religiosity, Social Religiosity, Involved God, Forgiveness, God as Judge, Unvengefulness and Thankfulness.
And as those (many) earlier studies (e.g. Bouchard and Koenigs), they found the correlations among monozygotic twins to be far stronger than among dizygotic twins, strongly supporting the notion of genetic heritability of the trait.
So it might actually be that religiosity is closer to 50% biological and 50% social. Meaning that women being more religious than men might itself be more genetic and less social conditioning; with the sociological factors (like groupthink, etc.) themselves being a genetic predisposition affecting women more than men. Indeed, almost everyone knows that men show off when in the presence of women; romantic or sexual priming increases men’s proclivity for risk taking. But one study I read showed that when women are given a romantic prime, they volunteer more. There goes that groupthink again…