People pick their friends based on several factors including proximity and similarity. Proximity is of importance because we prefer stimuli we see often as opposed to stimuli we’re not familiar with (mere-exposure effect). This principle also holds for friendships. Apart from starting friendships with people, we’re regularly exposed to, it is also easier to maintain relationships with those nearby us. Similarity is imperative for creating bonds, as it gives us topics to talk about and ideas to agree on. We also understand those who are similar to us a lot better. But could these similarities among you and your friends indicate that you might also be genetically similar?
Research has found that this seems to be the case by looking at a sample of adolescents in the United States. Guo (2006) found that there might be a genetic basis for trait-specific similarities between friends. To analyze genetic differences and similarities, identical (monozygotic) and non-identical (dizygotic) twins are always added to the sample. Interestingly, Guo also found that identical twins were more likely to list their twin as their best friend, compared to non-identical twins. Maybe we can speculate here that because identical twins share more genetic similarities, they might have more trait-specific similarities, which is an important factor in friendship.
Looking at more twin studies, we can see the same result for partner choice as well. Rushton and Bons (2005) compared identical twins to non-identical twins. They found that identical twins had friends and spouses similar to that of their co-twin. Again, we can speculate that there must be an underlying genetic effect for friend and partner choice.
Guo, G. (2006). Genetic similarity shared by best friends among adolescents. Twin Research and Human Genetics, 9(01), 113-121.
Philippe Rushton, J., & Ann Bons, T. (2005). Mate Choice and Friendship in Twins: Evidence for Genetic Similarity. Psychological Science, 16(7), 555-559.