You Are Genetically Similar to Your Friend

Friends are as genetically related as fourth cousins.

Posted Nov 03, 2020

Choosing friends is not as random as it may appear. There are subconscious factors that induce liking for specific people who are not relatives. Many people are willing to sacrifice limited resources for close friends. Animals would find it bizarre that an organism would share resources with others who are not genetically related. In fact, many scientists believe that the explosion in the size and intricacy of the frontal cortex of humans is mainly because of our complex social ties. Why are we drawn to some people and repelled by others? Is it true that "birds of a feather flock together” and why?

The answers lie in data analyzed from the multi-generational Framingham Heart Study (FHS) that started in 1948 to investigate environmental and genetic factors influencing cardiovascular diseases. Fowler and Christakis examined the genome of participants and their friends. Specifically, they analyzed close to 1.5 million markers of gene variations to measure the degree of genetic similarity between a participant and his/her friend versus a stranger (1). The researchers ensured that the paired participant and his/her friend were not related. They also controlled for shared ancestry. They found that friends were more genetically “related” to each other than to strangers. In fact, the genetic similarity between a pair of friends was similar to fourth cousins! Because the researchers controlled for ancestry, the genetic similarity is unlikely due to shared ancestry (also most participants were white).

What genes are we likely to share with friends?

The researchers examined the topmost similar genes between friends and the results were surprising. Top candidates were genes implicated in olfaction (sense of smell) and in linoleic acid metabolism. These results indicate that friends smell things the same way and/or that certain environments with characteristic odors attract people making them more likely to become friends.

Just like infants can accurately identify their mothers’ odors, so can friends. In one study, participants were able to distinguish friends from strangers based on blind odor tests (2). Olfaction is crucial for survival. For example, rotten food smells bad, so we don’t eat it. In the same vein, dissimilarity in olfaction genes may compel us to avoid others with incompatible smells. Also, olfaction is a sensory dimension of many emotional experiences. Thus, odors can increase or decrease emotional connections.

Another genetic similarity in friend pairs was in genes related to linoleic acid metabolism. Linoleic acid is a polyunsaturated omega-6 fatty acid mostly in plant oils (3,4). It is prevalent in the Western diet and found in vegetable oils and nuts. It is also commonly used in cosmetic and personal care products. Linoleic acid is involved in many bodily processes. These genes are involved in the pathway related to the metabolism of cholesterol, steroid, and various ingested substances (1).

Perhaps, this genetic similarity may affect preference for food, similarity in metabolism of food or even risk for disease. A study conducted by researchers at the Harvard School of Public Health found that replacing either saturated fat or carbohydrate with vegetable oils rich in linoleic acid may reduce the risk for developing coronary heart disease (5). It is not surprising that a longitudinal study that closely followed participants for 32 years found that people were more likely to become obese when a friend became obese. Precisely, it increased the person’s chances of becoming obese by a whopping 57% (6). Perhaps, the genetic similarity for linoleic acid metabolism is one of many factors leading to similarity in obesity among friends.

How about “opposites attract?"

There is a survival value in selecting others who complement our shortages. In the gene wide search for genetic similarity among friends, researchers found a more limited number of genes that were negatively correlated among friends. Specifically, genes that are implicated in the immune system. In other words, we subconsciously select friends who are immunologically different than us.

This too makes sense. It is advantageous that you and your friend would be susceptible to different pathogens or diseases. If you are resistant to pathogen "X" but not "Y" and your friend is resistant to pathogen "Y" but not "X," then when you are both exposed to pathogen "Y," you are likely to get sick but not your friend. Thus, your friend can take care of you (instead of both of you being sick, if you were resistant to the same pathogens). This also prevents the spread of diseases in social circles. 

Friendships are most salient during adolescence. Are adolescents also more genetically similar to their friends than strangers?

Using data from the Adolescent to Adult Health (Add Health) study, researchers echoed the same findings in a sample of 5,500 adolescents. Friends were more genetically similar to one another than to randomly selected peers (7). Specifically, genetic scores were positively correlated for BMI and educational attainment among friends, but not for height. They also found that schoolmates were more genetically similar to one another compared with random pairs of individuals. These findings suggest that specific environments may attract individuals who are genetically similar (but not related). In fact, when genetic similarity among schoolmates is compared with genetic similarity among friends, similarity between friends was attenuated. Sharing the same environment (school) contributed significantly to the genetic similarity between friends. It is worth mentioning that friends were still more genetically similar than random schoolmates.

It seems that human beings have a sub-conscious that selects friends based on synergy in some genes and complementarity in others. Indeed, friends are the family we chose. A friend can be defined as a "functional kin" who was selected in accordance with this genetic pattern. Also, we are drawn to others we "conveniently" find in our social environment. Of course, genes increase or decrease our gravitation to or avoidance of certain social environments, respectively (chicken and egg problem). It seems like Randy Newman was quite correct when he sang “you’ve got a friend in me” — at least science agrees with him! Now, go hug your friend, or shall we say “cousin.”

References

(1) Christakis, N. A. & Fowler, J. H. (2014). Friendship and Natural Selection. PNAS (111), 10796-10801.

(2)  Weisfeld GE, Czilli T, Phillips KA, Gall JA, Lichtman CM (2003). Possible olfaction-based mechanisms in human kin recognition and inbreeding avoidance. J Exp Child Psychol, 85(3), 279–295.

(3)  Belury, M. A. (2002). Dietary conjugated linoleic acid in health: physiological effects and mechanisms of action. Annu Rev Nutr, 22,505–531.

(4)   Whelan, J. & Fritsche, K. (2013). Linoleic Acid. Advances in Nutrition, 4(3), 311-312.

(5)   https://www.hsph.harvard.edu/nutritionsource/2014/11/05/dietary-linoleic-acid-and-risk-of-coronary-heart-disease/

(6)  Cohen-Cole, E., & Fletcher, J. M. (2008). Is obesity contagious? Social networks vs. environmental factors in the obesity epidemic. Journal of health economics, 27(5), 1382–1387. https://doi.org/10.1016/j.jhealeco.2008.04.005

(7)   Domingue, B. W. et al. (2018). The Social genome of friends and schoolmates in the National Longitudinal Study of Adolescent to Adult Health. PNAS, 115(4), 702-707.