A provocative new study published in the journal of Behavioral and Brain Sciences by two biologists from the University of New Mexico suggests that religiosity and strong family ties are partly due to cultural evolution driven by environmental factors, namely high rates of parasite infection. What the authors are NOT suggesting is an answer to the question "Are Overly Religious People Affected by Stress from Biological Parasites?" as one website suggests. The adaptation they propose would not generalize to individual behavior.
Here is the abstract (the paper itself has critical responses from other scientists):
Abstract: Throughout the world people differ in the magnitude with which they value strong family ties or heightened religiosity. We propose that this cross-cultural variation is a result of a contingent psychological adaptation that facilitates in-group assortative sociality in the face of high levels of parasite-stress while devaluing in-group assortative sociality in areas with low levels of parasite-stress. This is because in-group assortative sociality is more important for the avoidance of infection from novel parasites and for the management of infection in regions with high levels of parasite-stress compared with regions of low infectious disease stress. We examined this hypothesis by testing the predictions that there would be a positive association between parasite-stress and strength of family ties or religiosity. We conducted this study by comparing among nations and among states in the United States of America. We found for both the international and the interstate analyses that in-group assortative sociality was positively associated with parasite-stress. This was true when controlling for potentially confounding factors such as human freedom and economic development. The ﬁndings support the parasite-stress theory of sociality, that is, the proposal that parasite-stress is central to the evolution of social life in humans and other animals.