Behavioral and Brain Sciences

Parasite-stress promotes in-group assortative sociality: The cases of strong family ties and heightened religiosity

Corey L. Finchera1 and Randy Thornhilla2

a1 Department of Biology, University of New Mexico, Albuquerque, NM 87131.

a2 Department of Biology, University of New Mexico, Albuquerque, NM 87131


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.


  • assortative sociality;
  • collectivism;
  • family ties;
  • Homo sapiens ;
  • individualism;
  • infectious disease;
  • parasites;
  • religion;
  • religiosity;
  • sociality

Corey L. Fincher is a Research Assistant Professor in biology at the University of New Mexico. His research over the past few years has focused on questions regarding topics such as where cultural variation and other forms of biological diversity come from, and how the evolved response to infectious disease stress variation contributes to human value systems and national development. Ultimately, though, his interests include the evolved psychology of all organisms.

Randy Thornhill is a Distinguished Professor at The University of New Mexico. The evolution of human behavior and psychology are topics in the majority of his publications, including two books, one with Craig Palmer, A Natural History of Rape, and one very recently with Steve Gangestad, The Evolutionary Biology of Human Female Sexuality. Judging from the citation record of his papers and books, he has contributed to a range of disciplines including ecology, evolutionary ecology, behavioral ecology, evolutionary biology, entomology, ornithology, and human psychology and behavior. His main interest continues to be sexual selection processes, especially female choice.