The Importance of Inbreeding

Anthropologist / polymath John Hawks reviews two recent news stories about genetic genealogy, the first being Steve Olson's familiar claim that all living humans shared a common ancestor just a few thousand years ago, the other being the claim that lots of people are related to medieval royalty.

Hawks points out that the functional genetic impact of two people having a single common ancestor back before medieval times is negligible:

This means that the vast majority of your ancestral lines more than around 17 generations ago [1150 AD at 25 years per generation] have left no DNA to you whatsoever.

For instance, let's suppose that all humans include a single ancestor-descendant link in their genealogies that happened around 40 generations ago. This number will, of course, vary among people -- for some it might be 35 generations, for others 43, etc. But on expectation, the probability is rather less than one in a million that any single person inherited any DNA from this single link.

So from the point of view of genes, this kind of shared genealogical relationship is fairly trivial.

Then Hawks goes on to make a point that I yammer on about all the time: what matters is the degree of inbreeding (as in "A racial group is a partly inbred extended family"):

What is important for genetics is the extent to which these genealogical lines are replicated by inbreeding. If a single ancestor-descendant link occurs only a single time in anyone's genealogy, then it is likely to be genetically unimportant. But a single link may occur hundreds or thousands -- or even millions -- of times, because many lines of genealogy may trace back to the same few individuals.

This is why what matters is a matter of interpretation. In case we miss it, the AP article bludgeons us with it:

It also means that all of us have ancestors of every color and creed. Every Palestinian suicide bomber has Jews in his past. Every Sunni Muslim in Iraq is descended from at least one Shiite. And every Klansman's family has African roots.

As much as I would enjoy giving Klansmen the shivers, this line of logic doesn't appeal that much to me. You see, these one-quadrillionth portion genealogical links really can be important only under some kind of extreme one-drop-rule. Ha ha, Klansman! We can prove with mathematical certainty that 10^-15 of your ancestry is African! Never mind that you have fewer than 10¹⁴ cells in your body...one of those cells is one tenth African! You have the curse of Ham on you!

I don't like it. It doesn't offer much in the way of explanation. Does it matter that one of your ancestors helped build the pyramids? Does it matter that you share a long common ancestry with people halfway around the world? Does it even matter that all of us share all of our ancestry before some recent time?

From a genetic point of view, the answer is that it depends. Sure, all people share some recent ancestors, but how many? What proportion of ancestry is shared between different populations? And since their common origins, how have those populations changed?

We don't know these answers, but they are clearly testable in terms of allele frequencies. More on that later, but in a statistical sense, our ability to assess inbreeding using genes is a lot better than our ability to examine genealogies.

I've been talking about inbreeding as a key concept for understanding how the human world is arranged for about 8 years now, but the concept has failed to penetrate the intellectual world. A few supersmart guys like Steven Pinker and John Tierney grasped the implications of the concept in the extreme case of cousin marriage. But the general case continues to be ignored. Perhaps, Americans can only think of inbreeding in the context of that retarded hillbilly kid in "Deliverance," and thus are too emotionally creeped out to think about the general concept.

Perhaps there is a better term? Genealogical redundancy? Maybe, but another problem I've noticed is that "genealogy" in considered in intellectual discourse today merely as a pastime for eccentric hobbyists.

From a cultural point of view, there are two interesting points. The first is that a lot of people clearly do find a cultural utility to a one-drop-rule. That is, after all, why the AP articles are written the way they are. People do care whether they have one genealogical link to medieval royalty, regardless of the likely genetic insignificance of any single medieval ancestor. There is some logical sense to this -- since it is easier to make decisions based on simple yes-or-no facts than on fractional quanta. If the full facts aren't easily processed, people find it easier to assume that a royal link confers some status.

The other interesting point is that maybe culture is designed to provide a certain kind of solution to this genealogical problem. After all, the mathematical realities of genealogy and inbreeding didn't just arise in the Middle Ages; they have been with us forever...

Consider, for instance, that some archaeologists have assumed that hunter-gatherer bands persist with a half-life on the order of a few hundred years -- maybe 10 or 20 generations of time. This value might be an important element in the evolution of modern human behavior -- for instance, if it marks some kind of limit on the effective time period of oral traditions. Now, the potential number of ancestors of people after 10 or 20 generations is anywhere from a thousand to a million. It may make a great deal of difference how those million genealogical ties are organized -- if most of them come back by inbreeding to the same couple of hundred individuals, then the dynamics of groups will be quite a bit different than if they outbreed to several thousand or even hundreds of thousands of people. And if a large proportion of those links ultimately come back to a few group founders, then what those founders do in cultural terms might have a large material effect on their distant descendants.

Is it possible that cultures are mechanisms to facilitate an adaptive level of inbreeding? That they once functioned as cooperative blocs to promote the survival and proliferation of the genes of a few founders? That they may still do so in many circumstances (like the medieval aristocracy, for instance)? People do adopt cultures in ways that tend to reinforce this function -- integrating in ways that promote the chance that they or their offspring will have (in their view) good marriages, material comforts, and social success.

It's more than possible, it's a near certainty. The Old Testament, for example, is obsessed with, in effect, the reproduction with minimal diffusion of Abraham's DNA. The Middle East is full of heavily inbreeding groups, like the Samaritans. I don't think you'll find the same focus among the Chinese, which is why there are 1.2 billion people who think of themselves as Han Chinese, even if there are lots of clinal differences in their gene frequencies across the vast sweep of China. That's a big reason why there are 100 times as many Chinese as Jews and two million times as many Chinese as Samaritans.

One important question why some people behave in ways that will distribute their genes broadly through subsequent generations while other behave to distribute their genes narrowly. My guess would be that in purely Dawkinsian terms of maximizing copies of ones genes in future generations, the outbreeding strategy beats the inbreeding strategy, but I could be wrong.

Perhaps the inbreeding strategy works better in terms of the Darwinian survival and perpetuation of namable cultures than of genes: a survival of the most stand-offish. Abraham's descendents still have a name 3,800 years later, while the Hivites and Hittites of the Old Testament have been reshuffled out of cultural existence. It could well be, however that there was another man alive in 1800 BC who has more copies of his genes in existence today than does Abraham, but because his descendents tended to follow more out-breeding strategies, they have no cultural identity today.

People are agents in this process, acting in what they perceive to be their own interests, but coordinating actions culturally with others. Cultural preferences are not only retrospective (the people who look like this receive special treatment) but also prospective (if you do these things you -- or your children -- will receive special treatment). And a high proportion of doing these things involves placing your own progeny into certain privileged classes (marry the right kind of girl, pay the right brideprice, etc.

This explains a lot of modern American social behavior. Upper middle class Americans spend a fortune to get their children into the right social circles so they can marry well by buying expensive houses in exclusive school districts and sending them to expensive colleges. But we lack a vocabulary for discussing these phenomena, obvious though they are.

For example, I want American immigration policy to allow in people who would make good potential parents for my grandchildren and great-grandchildren and keep out people who would tend to make out lousy potential parents for my descendents. Yet, this is almost unthinkable in current political discourse, even though people think about it all the time in choosing to spend $45,000 per year to send their kid to an expensive college or subsidize a single daughter's rent so she can live in a fashionable neighborhood and snag a good husband.

My published articles are archived at iSteve.com -- Steve Sailer