Genetic similarity theory
Genetic similarity theory refers to an argued genetically caused preference for and altruism towards other persons that are genetically similar.
Giving support to a person with similar genetics is argued to increase the probability of survival of a person's genes. The theory can be seen as an extension of the genetically caused preference for and altruism towards relatives (kin altruism).
The genetic similarity, or not, of another person is argued to be detected through several indirect methods, such as by noting similarities in appearance, behaviors, and attitudes.
The concept of "ethnic nepotism" has many similarities to genetic similarity theory and the two theories are often discussed together. One difference is that genetic similarity theory usually refers to effects on the individual level, while ethnic nepotism refers to effects on the group level.
Animal studies on non-human species have found that individuals are able to detect genetic similarity in other individuals and modify behaviors based on this.
Studies have found that more genetically similar persons (such as monozygotic twins vs. dizygotic twins) are more altruistic towards one another, express greater appreciation towards one another, and have more similar friends.
Studies using blood groups have found that friends and partners are more genetically similar than more random pairs.
Studies using manipulated photos have found that persons express greater trust, and for the opposite sex greater attraction, when the faces were more similar to the face of person being questioned (and this despite without consciously realizing this similarity).
Some characteristics are more heritable than other characteristics. A prediction of the genetic similarity theory is that friends and partners should show greater similarity on such highly heritable characteristics than on other characteristics. Studies have found support for this.
One study examined degree of grief among parents after the death of a child. They found that children perceived as more physically similar to their parents were grieved for more intensely than less similar children.
The theory (as well as the related concept of ethnic nepotism) has been criticized on the grounds that the genetic similarity between non-relatives is too small to cause evolutionary effects. This has been argued to be theoretically incorrect and the genetic similarity theory has as noted above been argued to be supported by many empirical studies.
- J. Philippe Rushton. Ethnic nationalism, evolutionary psychology and Genetic Similarity Nations and Nationalism 11 (4), 2005, 489–507. http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8129.2005.00216.x/abstract http://psychology.uwo.ca/faculty/rushtonpdfs/n&n%202005-1.pdf
- Littlefield, C. H.; Rushton, J. P. (1986). "When a child dies: The sociobiology of bereavement". Journal of Personality and Social Psychology 51 (4): 797–802. doi:10.1037/0022-35126.96.36.1997. https://www.ncbi.nlm.nih.gov/pubmed/3783426
- Frank Saltera, Henry Harpendin. J.P. Rushton’s theory of ethnic nepotism. Personality and Individual Differences. https://www.sciencedirect.com/science/article/pii/S0191886912005569