This blog is intended to go along with Population: An Introduction to Concepts and Issues, by John R. Weeks, published by Cengage Learning. The latest edition is the 12th (it came out in 2015), but this blog is meant to complement any edition of the book by showing the way in which demographic issues are regularly in the news.

If you are a user of my textbook and would like to suggest a blog post idea, please email me at: john.weeks@sdsu.edu

Tuesday, June 2, 2015

Is There a Genetic Component to Migration?

Over the past hundreds of thousands of years, humans have migrated out of Africa to literally every part of the planet that is habitable. And the impact of migrants on places to which they go is often wrought with pain and controversy, at least in the short term, as I have often and recently noted. The newsletter of the American Sociological Association's Section on International Migration is called "World on the Move," and while that describes important global dynamics, it is also true that according to UN data the 214 million people who are currently living in a country other than where they were born represent only 3 percent of the world's population. Even in the world's most famous nation of immigrants, only 4 percent of Americans were living in a different county in 2014 than they had lived in the year before (and remember that crossing a county line makes you a migrant according to U.S. Census definitions. Indeed, this is why we study migration rather than staying in place--we assume that most humans are inherently sedentary, and so it is only migration that requires explanation.

I mention in the book that while migration is not necessarily a biological characteristic in the same way that fertility and mortality are, it nonetheless seems as though some people have a greater propensity to move than do others. Only this week did I discover a potential biological explanation for at least some of this, thanks to my  older son, John. He was in Denmark recently when a dinner discussion turned to migration and one of his dinner companions put him onto a recent story about "The Genetic Reason Why Some People Are Born To Travel All Over The World." The answer: dopamine. Yes, it's brain chemistry. 
In 1999, four scientists from UC Irvine published a paper titled “Population Migration and the Variation of Dopamine D4 Receptor (DRD4) Allele Frequencies Around the Globe” that explored the migration patterns and gene pool distribution of pre-historic human beings. They were originally researching for links between dopamine receptor D4 (DRD4) and Attention Deficit Disorder. While conducting the study, they discovered another weird correlation: people with the DRD4 genes tend to be thrill-seeking and migratory. And almost all study participants with this gene had a long history of traveling. From the study’s conclusion:
“As previous research has shown, long alleles of the DRD4 gene have been linked to novelty-seeking personality, hyperactivity, and risk-taking behaviors … It can be argued reasonably that exploratory behaviors are adaptive in migratory societies…usually harsh, frequently changing, and always providing a multitude of novel stimuli and ongoing challenges to survival” 
“The findings revealed a very strong association between the proportion of long alleles of the DRD4 gene in a population and its prehistorical macro-migration histories.”
With a bit of Googling I found that two Harvard researchers have more recently (2011) confirmed most of this analysis. Now, to be sure, the studies only find that higher levels of a particular type of dopamine are associated with moving farther than those with lower levels, and of course correlation is not necessarily causation. Furthermore, it is unlikely that brain chemistry explains more than a fraction of migration, but it does at least move us in the direction of a biological possibility. Dopamine is known to be a powerful neurotransmitter involved in many aspects of human behavior, so we cannot easily dismiss its potential importance.


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