Category Archives: survival

Our genes influence our social networks

Chromosomes magnified - photo by BlueSunFlower

Chromosomes magnified - photo by BlueSunFlower

If you don’t have enough friends or aren’t the social butterfly of your class, now you can blame your genes.

Nick Christakis (Harvard Medical School) and James Fowler (UCSD political scientist) are back with more controversial findings suggesting some genetic determination in our social networks (both in forming friendships and determining where we are in social networks).  Christakis: “the beautiful and complicated pattern of human connection depends on our genes to a significant measure.”  Previous work by Christakis looked at how our social networks and who is in them shape our likelihood of obesity, happiness, and smoking, among other outcomes.

They researched 1,100 same-sex twins in the National Longitudinal Study of Adolescent Health (colloquially called “Add Health”). Add Health examined high school students in 1994-1995 and asked questions regarding economics, physical health and social involvement. Christakis and Fowler compared the social networks and patterns of identical same-sex twins against fraternal ones to separate nature (genes) from nurture (upbringing).

Their findings go far beyond what people might think about the genetic influence on personality traits (being outgoing, shy, etc.). For example, how often the subject was named as a friend and the likelihood that the subject’s friends knew one another were strongly genetically influenced, but interestingly not the number of friends that the subject listed. This suggests a genetic determinant of being popular (beyond a simple disposition toward being outgoing); further buttressing this interpretation, whether the subject was more the center of attention (central to these networks) or more of a social outcast (peripheral to these networks) was also heritable.

Christakis admits that some of the findings are puzzling, like the fact that the likelihood that my friends Bill and John know each other is attributable to my genes; what this likely means is that some people are genetically disposed to introduce their friends to each other more or to host or arrange social events where these friends would have chances to meet each other.

‘Given that social networks play important roles in determining a wide variety of things ranging from employment and wages to the spread of disease, it is important to understand why networks exhibit the patterns that they do,’  Matthew Jackson, a Stanford University economist, wrote in a commentary accompanying the study called “Do We Inherit Our Positions in Life?”.

James Fowler… said its implications go beyond the theoretical. For some time, scientists have suspected a genetic role in certain conditions, such as obesity. Now, Mr. Fowler wants to investigate whether the dynamics of social networks might affect public-health outcomes, for instance, by exposing people to certain behaviors, such as smoking.”

“Our work shows how humans, like ants, may assemble themselves into a ‘super-organism’ with rules governing the assembly, rules that we carry with us deep in our genes,” says Nicholas Christakis.  Christakis et al. also believe that there may be an evolutionary explanation for their findings since one’s position in social networks had costs or benefits to the survival of one’s genes. Being central to a group likely contributed to survival during periods of food scarcity since one could learn where food supplies were, while being peripheral to groups helped genes survive in periods where deadly germs were being transmitted by social contact. Christakis: “It may be that natural selection is acting on not just things like whether or not we can resist the common cold, but also who it is that we are going to come into contact with.”  The paper notes: “There may be many reasons for genetic variation in the ability to attract or the desire to introduce friends.  More friends may mean greater social support in some settings or greater conflict in others.  Having denser social connections may improve groupsolidarity, but it might also insulate a group from beneficial influence or information from individuals outside the group.”  The authors note that more work is required to understand what specific genes are at work and what possible mediating mechanisms might be.

The authors acknowledge some controversy in studies comparing identical twin studies to fraternal twins, with critics noting that identical twins may have a stronger affiliation with  each other that causes them to be more influenced by each other than fraternal twins.  The authors note that twin studies have been validated by comparing identical twins raised apart versus together (suggesting that it is not the shared environment).  The authors further note that personality and cognitive differences between identical and fraternal twins persist even among twins mistakenly believed to be identical by their parents (indicating that parental patterns in raising these ‘identical twins’ can’t explain the outcome).  Finally, they note that that once twins reach adulthood, identical twins living apart tend to become more similar with age, which doesn’t fit with a notion of the importance of their shared environment.

The study appeared online in James Fowler, Christopher Dawes and Nicholas Christakis,  “Model of Genetic Variation in Human Social Networks” in Proceedings of the National Academy of Sciences journal (January 26, 2009).

“More specifically, the results show that genetic factors account for 46% [95% confidence interval 23%, 69%] of the variation in in-degree (how many times a person is named as a friend), but heritability of out-degree (how many friends a person names) is not significant (22%, CI 0%, 47%). In addition, node transitivity [the likelihood that two of a person’s contacts are connected to each other] is significantly heritable, with 47% (CI 13%, 65%) of the variation explained by differences in genes. We also find that genetic variation contributes to variation in other network characteristics; for example, bertween-ness centrality [the fraction of paths through the networks that pass through a given node] is significantly heritable (29%, CI 5%, 39%).”

See also “Genes and the Friends You Make” (Wall Street Journal, 1/27/09 by Philip Shishkin)

See other articles by Christakis et. al on social networks.


Heroes and Cowards

heroes-and-cowardsMatt Kahn and Dora Costa have just recently published Heroes and Cowards : The Social Face of War. I haven’t read the book, but I have read the terrific underlying articles that formed the genesis of this book.

One of them was a fascinating look at why more soldiers didn’t desert their companies in the Civil War than the 200,000 who did — after all there was almost a 50% chance that they would die or be seriously wounded if they stayed with their troop and a negligible chance they would be caught and punished if they deserted.

See this excerpt from Chapter 1:

James Monroe Rich left his wife and his trade for the low and irregular pay of a Union army soldier in the Civil War. He marched through heat and dust, through torrential thunderstorms and deep mud. He marched with gear weighing 45 to 50 pounds—guns, cartridges and cartridges boxes, woolen and rubber blankets, two shirts and two pairs of drawers, canteens full of water, rations, and trinkets from home. He marched with his comrades even when they “were falling on every side” in a failed frontal assault where “the lead and iron filled the air as the snowflakes in an angry driving storm. James was lucky. He survived the war. Over one-quarter of the men in his company did not.

Unlike James, George Farrell was well paid to enlist and take the place of another man who had been called up. He joined a company that had been re-formed with new men and saw no comrades die. Unlike James, he deserted twice, the second time successfully. Why did James stand up for his comrades while George did not?

While their story is told through the eyes of 9 men serving in the Civil War, Costa and Kahn do extensive statistical analyses and controls to verify their conclusions.  Digitally tracking the involvement of 41,000 soldiers from 1861-1865, Costa and Kahn found that “social capital” (the degree of connections they had with others in their troop — e.g., profession, age, hometown, extended blood ties) that predicted troop cohesion. For example, Union soldiers who served alongside men from the same occupations deserted at one-third the rate of counterparts in more diverse companies (where 1 in 10 deserted).

Costa and Kahn quote military strategist Ardant du Picq: “Four brave men who do not know each other will not dare to attack a lion. Four less brave, but knowing each other well, sure of their reliability and consequently of mutual aid, will attack resolutely.”

They also found that survival rates in Andersonville — a despicable Confederate war camp where nearly 40%  of captured soldiers died– was significantly higher if one had close ethnic ties to other prisoners.  Survival rates for Union soldiers born in Ireland, increased dramatically from 60% to 90% if there were 15 other Irish immigrant prisoners but only edged up to 64% if there were 15 other comrades from their original company.  Kahn hypothesizes that “[Y]our comrades would help you get healthy if you got sick and share their food rations… “So in P.O.W. camps, diversity actually turned out to be a bad thing. It hindered survival rates.”

But Costa & Kahn found long-term benefits from diversity of African-Americans in Civil War companies: increased literacy; increased changing of their slave name after service; and increased moving away from their hometown after the War.  Companies with both African American former slaves and freemen had higher desertion rates than units that didn’t mix these groups, but over 30% of former slaves learned to write in these mixed groups versus only 16% in former slave-only units.  Kahn calls them quasi-job-training programs. “[F]or every 10 percentage-point increase in comrades who hailed from a particular state different than the home state of an illiterate solider, the likelihood of that illiterate soldier ultimately relocating to that state jumped by more than 30 percent. ”  Kahn calls this “the Zagat Guide effect…So if we’re in the same company, and I’m from New Jersey, you are more likely to move to New Jersey after the war. We believe that I taught you about the benefits of New Jersey. Serving in a diverse unit helped open horizons for men who had previously enjoyed no mobility whatsoever.”

We’ve long preached about the importance of social capital, but Costa & Kahn show how these social ties help not only in a daily “business as usual” climate but in times of greatest adversity where our lives are on the line. They show where notions of altruism, group identity, and willingness to sacrifice come from, and how they are informed by those around us.  And how we act on our loyalty toward others, even when it holds great cost to us, by increasing the chance that we “go down with the ship.”

To see a WSJ review of Heroes and Cowards, see “Why Soldiers Fight“.

Read Chapter 1 of Heroes and Cowards here.