It is well known that human behavior and individual psychological traits are moderately to substantially heritable. Over the past decade, an increasing number of studies have explored the genetic and environmental influence on religiousness. These studies originate predominantly from countries generally considered more religious than the very secular northern European countries. Comparisons of the results are complicated by diverse definitions of religiousness, but several studies indicate that the influence of the family environment is most predominant in early life, whereas genetic influences increase with age. We performed a population-based twin study of religiousness in a secular society using data from a Web-based survey sent to 6,707 Danish twins born 1970-1989, who were identified in the Danish Twin Registry. We applied Fishman's three conceptual dimensions of religiousness: cognition, practice, and importance. In all polygenic models and biometric analyses, we controlled for gender and age. The study sample comprised 2,237 same sex twins, a response rate of 45%. We found high correlations within both monozygotic and dizygotic twin pairs in most items of religiousness, indicating a large influence from shared environmental factors. Personal religiousness such as praying to God, believing in God, and finding strength and comfort in religion were more influenced by genetic factors than were social forms of religiousness such as church attendance. We found a small tendency for increasing genetic influence with increasing age for some religious items, but not for all.
Keywords: religiousness, secular societyThere are many elements in our lives that form the lives we end up living and the values (beliefs, existential concerns, atheism) that are important to us. But where do these choices and fundamental values come from? And how are they formed and how do they gradually transform our lives? It is well known that human behavior and individual psychological traits are moderately to substantially heritable (Bouchard & McGue, 2003). In other words, to some degree we are as we are in our behavior and personality because we have inherited genes that make us that way. Over the past decade, an increasing number of studies have explored the genetic and environmental influences on a rather new and perhaps provocative realm: religiousness, often using the twin study design to facilitate assessment of the impact of genetic, shared environmental and unique environmental factors (Kendler et al., 1997;Kendler & Myers, 2009;Kirk et al., 1999aKirk et al., , 1999bKoenig et al., 2005Koenig et al., , 2008Tsuang et al., 2002;Vance et al., 2010). The twin study design takes advantage of the fact that twins are reared in the same environment, but while monozygotic (MZ) twins share all of their genes, dizygotic (DZ) twins share on average half of their genes and hence differences in intrapair similarity between the two may indicate the presence of a positive heritability. Accordingly, in twin studies we can assess how alike twin cou...