The demographic transition -- the tendency for richer societies to have fewer rather than more children -- is, I think, most often attributed to social causes. For a variety of reasons -- because each child costs more, because they are more likely to survive and take of parents in old age, because of social stigma associated with large families, because of birth control, etc. -- couples in richer countries often choose to have 2 children rather than 10. This demographic transition accounts for the increasing age of the population in Western countries, as I discussed in an earlier post.

I never really considered the idea that there was a biological explanation for it though. A study in Iceland suggests that fertility might be related to the degree of inbreeding and outbreeding. By increasing outbreeding in our increasingly mobile Western societies, we have actually lowered fertility.

Helgason et al. publishing in the journal Science examined the fertility of families in Iceland from 1800 to 1965. Iceland has become a big laboratory for such studies and large genetic studies because they have super-accurate kinship records going back into the Middle Ages. (Many of these studies are performed by a company called deCODE genetics, and this is one of them.)

They found two important things in their study.

First, the average relatedness of couples in Iceland has decreased over the study period:

Our data indicated that there has been a decrease by a factor of 10 in mean kinship between Icelandic couples during the past two centuries, from 0.005 for couples with females born 1800 to 1824 to 0.0005 for those born 1950 to 1965. This is equivalent to a change from couples being related on average between the level of third and fourth cousins to couples being related on average at the level of fifth cousins. The primary cause is probably a demographic transition from a poor agricultural society to an affluent industrial society, involving extensive migration from rural regions to urban centers, accompanied by a rapid expansion in population size. (Emphasis mine.)

Second, the reproductive success of couples -- as defined by the number of children they have who later reproduce -- has an inverted U that peaks at a relatedness of about 3rd or 4th cousins. In other words, the most fertile and reproductively-successful couples are slightly inbred.

This is illustrated in the figure below (Figure 1B). The x-axis shows a measure of relatedness with 32 being the most inbred. You can see that reproductive success peaks at a moderate value of relatedness which the authors indicate is about 3rd or 4th cousins.

What are possible mechanisms for decreased fertility for more distant relatives? Well, I would guess that close inbreeding has a deleterious effect because of the possibility of inheriting two copies of a deleterious gene. On the other side, though, high genetic dissimilarity between the parents might render one more likely to get autoimmune diseases because your MHC genes might recognize the genes of the other parent as foreign. (I don't really have any evidence for that, but it is the reason you try and get a bone marrow transplant from a genetically-similar match. It limits the chance of graft vs. host disease.) Low relatedness might also lessen the ability of sperm to recognize eggs or limit egg implantation. I am just speculating here, but I can think of a variety of biological causes to explain this.

On the other hand, there could also be licit social causes. Maybe 3rd and 4th cousin marriages receive more social support by families than more distant marriages. The authors discount this hypothesis on two grounds. One, there is a statistically significant difference between say 6th and 7th cousin couples. Is there a good reason to believe that relations that distant would be treated different socially? Also, there is little or no change in the reproductive success curve as a function of kinship during the study period. It would probably be fair to assume that social structures varied widely during that time, but the effect remains largely unchanged.

So I am prepared to buy that this is a biological effect. What remains to be seen -- the authors must be still working on this -- is how much of the demographic transition can be explained by this change in fertility. Also, I wonder if there are alternative examples of societies that are wealthy but not mobile or societies that are mobile and not wealthy. That would allow you to disentangle the relatedness effect from the wealth effect.

All in all, very interesting stuff.

Helgason, A., Palsson, S., Guthbjartsson, D.F., Kristjansson, t., Stefansson, K. (2008). An Association Between the Kinship and Fertility of Human Couples. Science, 319(5864), 813-816. DOI: 10.1126/science.1150232

Hat-tip: Economist