Sunday, November 22, 2009

The dispersal of human offspring

(Human offspring disperse for the same reasons as these dandelion seeds, but the effects of dispersal are quite different.)

It is normal in birds and mammals for young to disperse from their birth area.  There are a variety of biological explanations as to why this is adaptive.  A common reason given is that dispersal reduces the chances of offspring mating with their parents (or competing with them for resources like food), which would result in a higher degree of inbreeding and a higher probability of recessive, deleterious genes manifesting themselves in the offspring of such a mating between close relatives.  In birds, females tend to disperse farther than males from their natal area, and in mammals, males tend to disperse farther than females.  Again, if male and female siblings disperse different distances from where they were born, they are less likely to encounter each other when they reach reproductive age and, therefore, siblings are less likely to mate with each other.  So, while birds and mammals have different sex-specific dispersal patterns, the effect is the same.

We are all familiar with the bad jokes told about human inbreeding (=incest) in communities where everyone stays near home, and there is little movement of new humans into this isolated community.  This is probably an extreme case for humans.  I have to figure that for most of human history (3-4 million years), young males probably dispersed to nearby villages, probably no more than miles or tens of miles away.  After all, they had to walk.  Once they got there and were accepted, they found young females to mate, settled down in their new digs, and had babies.  Young female humans probably stayed near home more often, although the details of all this varied with cultures around the world.  In some cultures, females are simply kidnapped from nearby villages and brought to the male's home.  Important as well is that in this ancestral system everyone knew everyone else within the home community, and they probably knew almost everyone in all the neighboring communities. 

But in recent times, meaning decades or a few centuries, this pattern of relatively short dispersal distance and everyone knowing everyone else changed dramatically.  Many young people still remain in close proximity to their parents and to where they were born; they retain close friendships with many of their peers from high school.  But many others disperse hundreds or thousands of miles from their family, their birthplace, their homeland.  This can be a somewhat painful experience for those of us who enjoy being with our adult children on a regular basis.

This diaspora-like phenomenon has consequences for society as well, I believe.  Human behavior seems to be influenced and tempered mostly by peer pressure.  We tend to be on our best behavior when we are being watched by people who know us and who know our family.  Our family, in turn, puts pressure on us to behave in a socially-acceptable manner.  When humans move to a community where literally no one knows them, human behavior has a tendency to change.  I am not familiar with studies that document this, but I am betting they exist.  In other words, when you are not directly accountable to a social system in which your status is known and familiar to others, I predict that, on average, humans will be somewhat more likely to engage in immoral or illegal behavior.  For this pattern to emerge in the data, we would need to examine a sample of thousands of individuals who dispersed and compare their behavior to thousands of similar individuals who did not disperse.  If entire extended families dispersed together, I think my prediction would be weaker.

Thus, there are good biological reasons why human offspring might disperse from their natal area, but this dispersal may also have effects, or unintended consequences, in societies where it is common.  I love playing these mind games with myself to see where it leads me.  Having just helped one of my sons disperse even further away from home than he already was has caused me to focus on this topic again.  (In the case of my son's recent move, I am more concerned about what his new community will do to him than what he will do to it.  This must be a common parental reaction.)  I was always fascinated with dispersal in the mammals and birds I studied, but there is nothing quite like thinking about human behavior to get the juices flowing.  Of course, human dispersal is another one of those book-length issues, but maybe this little essay will start you thinking about the movement of people in a new and creative way.