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San Diego Researcher Finds DNA Similarities Between Friends

San Diego Researcher Finds DNA Similarities Between Friends
San Diego Researcher Finds DNA Similarities Between Friends
San Diego Researcher Finds DNA Similarities Between Friends GUEST:James Fowler, Professor of Medical Genetics and Political Science at the University of California, San Diego.

MAUREEN CAVANAUGH: This is KPBS Midday Edition, I am Maureen Cavanaugh. We all know that we tend to gravitate towards people who share our likes and dislikes. We usually find friends who share an interest in our activities and passions. A new study suggest that what seems like a lucky coincidence in finding the right friends may have a genetic explanation. A newly published study finds that nonrelated friends tend to have as much DNA in common as fourth cousins. Joining me is the study's Co-Author, James Fowler, Professor of Medical Genetics and Political Science at the University of California, San Diego. Welcome back to the program. JAMES FOWLER: Thank you for having me. MAUREEN CAVANAUGH: How did the study come about? What we do trying to find out? JAMES FOWLER: Everybody understands that we tend to choose friends that are like us. We have sayings about this, birds of a feather flock together. But very few people question were that comes from. My colleagues, Doctor Nicholas Christakis and I were really interested in how social networks evolve, how do we make friends, where did all of this come from? We were really interested in the evolutionary basis of this tendency to choose friends who are like us. That is really what drove our investigation of looking at genetic factors that might be similar between friends. MAUREEN CAVANAUGH: What did you find the data to conduct this research? JAMES FOWLER: We went to an old friend, The Framingham Heart Study, a fantastic study that started in the 1940s, and started following the children in the 1970s and the grandchildren in the early 2000's. It is a really long-running study that comes about every two to four years and answers a bunch of social questions, and seeing a doctor and so on. We got from the study information both about social networks, close friends who also happened to be in the study, because it was a close group of people, and also genetic information. MAUREEN CAVANAUGH: So it all comes from people who took part in this study. You actually have information that these people were in a social network and they were friends with one another. JAMES FOWLER: That is absolutely right, we have done a lot of work in the social network data before. We use that to show for example how obesity spreads, how happiness spreads, and so on. MAUREEN CAVANAUGH: You say this research doesn't reflect people's tendency to be friends with people of similar ethnic backgrounds, or similar ancestry. Why not, why don't we see that in this research? JAMES FOWLER: We were very careful to control for that. That is a trivial explanation. Clearly, white people choosing friends that are white people and so on, you would come to expect there would be some small differences within friends. We did not want to really have that be what we're looking for. We're looking over and above ancestry that we find similarities. The way we do that, we look at a group of people who are all very ethnically similar. The people who are in the Framingham Heart Study are all white, mostly from Irish and Italian backgrounds. We also to trick where we compared the people who are actually friends to people who are strangers in the same group. So I would compare you and your friend to someone in this group that you did not know. So we could say over and above the tendency for you to make friends with people from the same ethnic background, is there extra similarity on top of that. MAUREEN CAVANAUGH: When you say in this study that friends tend to be genetically similar, what does that mean? How much DNA do friends actually share? JAMES FOWLER: Depending on how you look at it, it is either a lot or a very little bit. We are so similar to one another, it is like we are fourth cousins, it is like we shared a great-great-great grandparent in common. This is people who don't actually share an ancestor in common. It is less than 1% of all of your genes. That is a really large number in the evolutionary time. What this means now is that our friends are literally like our family. If I am doing things to help you as a friend, I am not just helping you, and helping someone who has similar genes to me. This can unlock some of the mysteries of how altruism evolved, and how it was able to be sustained over evolutionary time. MAUREEN CAVANAUGH: We were talking about perhaps 1% of similar DNA to people who are friends. How does that compare with a closer family relationship? How much DNA would I share with my sister cousin? JAMES FOWLER: Siblings share about one quarter. Between your parents, it's about one half. Every time you go a step, you have to cut it in half. MAUREEN CAVANAUGH: How does it compare to random strangers who are not your friends? JAMES FOWLER: Random strangers in principle should not have anything in common except for the tendency for us to live in groups that are ethnically similar to each other. That is why it is so important to control for that. That is something that you would expect, it is called background relatedness. MAUREEN CAVANAUGH: You are seeing this, be DNA, and we know how that looks, there are shaded areas and so forth. You're saying that the DNA profile comes out unexpectedly similar between friends. Not by a huge amount, but unexpected similarities are coming out. How does that similar DNA result in similar attributes? JAMES FOWLER: We know from a wide variety of studies that there are genes that underlying important human behaviors. We have identified over twenty genetic markers that associated with obesity. There are twin studies that suggest that a large portion of variation in how open or friendly we are, or personality traits that can be attributed to differences in our genes. This is part of a broader said of research that shows that genes have an influence on our traits. Some of those traits are the ones that tend to choose other friends that are like us. So, happy people choose other happy people to be friends with, for example, if genes influence happiness, we would expect the similarity to go down to the DNA. MAUREEN CAVANAUGH: How is that beneficial in the evolutionary sense? JAMES FOWLER: Think about how genes work. Usually, for complex traits like happiness, there will be hundreds or thousands of genes that have an impact on this outcome. Any one of them will have a tiny effect, but is not the case that you have a certain gene that makes you slightly more happy, but your friends all have it as well. It is a reinforcement effect. It is a combination of genes in the environment that explain how small differences come to large outcomes. MAUREEN CAVANAUGH: In other words, I think one of the examples that you give, if you and your friends have a similar predisposition to feeling cold, you might all get together and start fires more often back in the day, so to speak, for the evolutionary progress to take place. Or if you all shared a certain capacity to smell a certain thing, that might have an advantage. Is that the sort of thing we're talking about? JAMES FOWLER: It is, and it points to another aspect of the study. We found that the genes that were most likely to share in common with our friends are also the genes that have been evolving the fastest. This is truly extraordinary, it means that these genes are the ones that are driving human evolution. Other researchers have puzzled over the fact that human evolution appears to be speeding up. This research is just a possible explanation. It is research that having friends unlock the potential for us to increase the rate at which you and I seek fire because we are cold, it increases the rate of that and explains why evolution has picked up of the last tens of thousands of years in humans. MAUREEN CAVANAUGH: Are we picking our friends because we biologically to those similarities? JAMES FOWLER: That is an open question. It is possible that there is some mechanism where we detect how we are related, and that goes into our choice of friendship. For example, there is research on incest aversion that suggest that we are good at figuring out that our close family members are actually closely related to us so that we avoid reproducing with them. The question is whether or not we can detect something this subtle. Fourth cousins way more than something I expected to find, but it is possible that we have developed a kinship detector that allows us to choose friends. There are other explanations that are simpler. One is that we have a certain tendency to be drawn to the same environment. We will meet other people drawn to that environment. You may love the smell of coffee, and I happen to love it myself. Here we are in the studio becoming friends. That is a possibility, it is not that I chose you because you like coffee, it is because we both ended up in this place. Another possibility is that there are other things in the world doing selection for us. Universities select students, right? With certain attributes. If genes influence those attributes, you may end up in a pool of people who all share the same genetic attributes in common. There are a lot of different ways that you might explain this besides me choosing you specifically because you and I are similar. MAUREEN CAVANAUGH: From what I know of this study, when it comes to smell, there is more of a tendency for friends to have similar DNA that would be dispose them to have an acute sense of smell. However, when it comes to immunity from disease, that is least likely to be shared among friends, why is that? JAMES FOWLER: In fact, it is the opposite. This is a really remarkable finding. Hidden within the average similarity across the whole genome are certain parts of the genome that are actually different between friends. I actually love this because I have always puzzled over these two sayings, birds of a feather flock together and opposites attract. I always thought those can't both possibly be true. Our study suggests that the reason that they are both true is that some traits are opposites attract, and some are birds of a feather. The immune system genes are the ones that we found that are actually opposites attract. It is similar to what other researchers have found in spouses. If you are susceptible to a deadly disease, you do not want your spouse to get it, because your spouse will give it to you. Your spouse is an extra wall of protection. You want them to have a different immune system, to keep you from getting whatever you are susceptible to. I suspect that something similar is going on with friends. MAUREEN CAVANAUGH: All of this is going on such a subtle level that none of us would be aware of this, is that right? JAMES FOWLER: I think that is right, I doubt there will ever be one attribute or one set of genes that is so overwhelming in this process that it is something that we are consciously aware of. I think basically this is a culmination of hundreds of evolutionary nudges, pushing us in this direction. MAUREEN CAVANAUGH: Using the results of the study you developed a friendship score, what is that? JAMES FOWLER: We took our model where we look at friends and compared them to strangers and we found the genes where friends tend to be similar, let's say genes A, B, and C. the genes in which friends tend to be different are genes X, Y, and Z. If we know which genes are which, we can take that information to a new set of people, for whom we do not know whether or not they are friends. What we expect if they line up, and they are sharing all of genes that friends share and are opposite on the genes that friends tend to be opposite on, that suggests that those two people are supposed to be friends even if they do not know it. So we can compare the friendship score that shows how much they line up with this research that we have done. We try to use that friendship score for different pairs with a different group of people to try to predict which people were friends with one another, and we were actually able to do better than sheer chance. In fact, if you have a very high score, you are about 24% more likely to be friends then if you have a very low friendship score. It potentially explains a lot of differences between people we become friends with and people we don't. MAUREEN CAVANAUGH: And there's also an indication that having similar DNA with a friend can actually change or enhance your behavior in some way. JAMES FOWLER: When we compared the genes we have in common to those of that we don't, we found that the genes that we have in common are the ones that appear to be evolving the fastest. Really, the only way to explain that relationship is that there is something about surviving that is really important. Think about language, for example. The very first person who had a mutant gene that allowed them to speak language better, that will be great for them but only if they have a friend who has it as well. Will be no good to them if they are the only mutants, you need to get that synergy between you and other people in your social network for that to be beneficial. The fact that we find these similar genes under growing more rapid evolution is suggestive that friends have turbocharged evolution in humans. MAUREEN CAVANAUGH: That is a fascinating study co-authored by Nicholas Christakis. It is published in the proceedings of the National Academy of Sciences, and it came out today, just about an hour ago. I've been speaking with James Fowler, thank you very much. JAMES FOWLER: Thank you Maureen.

People often talk about how their friends feel like family. Well, there's some new research out that suggests there's more to that than just a feeling. People appear to be more like their friends genetically than they are to strangers, the research found.

"The striking thing here is that friends are actually significantly more similar to one another than we were expecting," says James Fowler, a professor of medical genetics at the University of California, Davis, who conducted the study with Dr. Nicholas Christakis, a social scientist at Yale University.

In fact, the study in Monday's issue of the Proceedings of the National Academy of Sciences found that friends are as genetically similar as fourth cousins.


"It's as if they shared a great- great- great-grandparent in common," Fowler told Shots.

Some of the genes that friends were most likely to have in common involve smell. "We tend to smell things the same way that our friends do," Fowler says. The study involved nearly 2,000 adults.

This suggests that as humans evolved, the ability to tolerate and be drawn to certain smells may have influenced where people hung out. Today we might call this the Starbucks effect.

"You may really love the smell of coffee. And you're drawn to a place where other people have been drawn to who also love the smell of coffee," Fowler says. "And so that might be the opportunity space for you to make friends. You're all there together because you love coffee and you make friends because you all love coffee."

They also found some interesting differences among friends: They tend to have very different genes for their immune systems. Other researchers have reported similar findings among spouses.


"One of the reasons why we think this is true is because it gives us extra protection. If our spouses have an immune system that fights off a disease that we're susceptible to, they'll never get it, and then we'll never get it," Fowler says. "And so it gives us an extra layer of protection."

"It's obvious that humans tend to associate with other people who are very similar to themselves," says Matthew Jackson, a professor of economics at Stanford University who studies social networks. "This gives us evidence that it's operating not just at a level of very obvious characteristics but also ones that might be more subtle — things that that we hadn't really anticipated."

Taken together, Fowler says the findings could help explain all sorts of things, including how relationships are driven by genetics and how that, in turn, may be influencing human evolution.

"I think the biggest implication is that evolution can't be studied as a Robinson Crusoe phenomenon. We didn't evolve isolated — separate from others. We evolved in communities. We evolved with our friends."

On a more personal level, it could help explain that cozy feeling we get with our friends.

"It's as if we were surrounding ourselves with a new family," Fowler says. "It's the family we chose, rather than the family we're born with."

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Corrected: April 16, 2024 at 3:37 PM PDT
KPBS Midday Edition's Maureen Cavanaugh and Patty Lane; and KPBS Evening Edition host Peggy Pico contributed to this story.