A study published in the current issue of Journal of Personality studied adult male monozygotic (MZ) and dizygotic (DZ) twins to find that difference in religiousness are influenced by both genes and environment. But during the transition from adolescence to adulthood, genetic factors increase in importance while shared environmental factors decrease. Environmental factors (i.e. parenting and family life) influence a child's religiousness, but their effects decline with the transition into adulthood. An analysis of self-reported religiousness showed that MZ twins maintained their religious similarity over time, while the DZ twins became more dissimilar. "These correlations suggest low genetic and high environmental influences when the twins were young but a larger genetic influence as the twins age" the authors state.
Participants for this study were 169 MZ and 104 DZ male twin pairs from Minnesota. Religiousness was tested using self-report of nine items that measured the centrality of religion in their lives. The twins graded the frequency in which they partook in religious activities such as reading scripture or other religious material and the importance of religious faith in daily life. They also reported on their mother's, their father's, and their own religiousness when they were growing up. They were also asked to report on the current and past religiousness of their brother. The factors were divided into subscales-- external aspects of religion, like observing religious holidays, that might be the most susceptible to environmental influence and internal aspects, like seeking help through prayer, that might be the most susceptible to heritable influence. The external items were found to be more environmentally and less genetically influenced during childhood, but more genetically influenced in adulthood. The internal scale showed a similar pattern, but the genetic influences seemed to be slightly larger in childhood compared to the external scale and so more consistent across the two ages. "Like other personality traits, adult religiousness is heritable, and though changes in religiousness occur during development, it is fairly stable," the authors conclude.
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