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. For illustration, I chose the five strongest correlations from among the female twins.
All the seven religiosity factors were found to be largely influenced by additive genetic and unique environmental effects, with surprisingly little influence from common environmental effects. At first, this seemed to conflict with other studies which found a strong role of family environment in the passing on of religious values. But the sample used in the present study had been comprised entirely of adults (with an average age of 43.06 years), leaving open the possibility that the common environment could be important in the development of religiosity in early life, whereas individual genetic and environmental effects are gaining importance later on.