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
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This biological effect may play an extremely small role in the demographic transition of which you speak; however, I would guess that this factor is dwarfed to insignificance by factors such as employing contraceptive strategies to delay pregnancy until education and career goals are achieved, which is subsequent to the social norm of waiting a few years after puberty to marry, in contrast to some less wealthy societies. I doubt very much that there are very many distantly related cousins trying to have 10 children, but only managing two because they are not related enough. It is an interesting study though, and while thinking about breeding cousins tickles my taboo reflex it does pose interesting questions about human reproductive biology. I would be curious to see that above curve extended to intermarriage of northern Europeans with Australian Aboriginals. Does the fertility curve continue to decline? Does it level off? Or is this effect specific to Icelandic people?
Doesn't Japan pretty much sink this hypothesis for the demographic transition? Low birth rate. Aging population. High development. And among the most homogeneous populations in the developed world. They are not exactly a nation of immigrants.
Of course one counterexample doesn't rule out ANY biological contribution, but it does raise doubt that biology is a MAJOR contribution to the demographic transistion.
there are possible incompatibilities:
http://scienceblogs.com/gnxp/2007/11/interracial_mating_spontaneous.php
or the rh- frequency in basques probably made matings between basques and non-basques less fertile in pre-modern times.
Doesn't Japan pretty much sink this hypothesis for the demographic transition? Low birth rate. Aging population. High development. And among the most homogeneous populations in the developed world. They are not exactly a nation of immigrants.
the same applies to iceland. this isn't about racial or ethnic group differences, but even more fine-grained variation. but it's just one data point. there are others to the contrary:
http://scienceblogs.com/gnxp/2008/02/fertility_is_complicated.php
Dear Colleagues:
As humanity's most luminous beacon of truth, science provides us with a last best hope for the survival of life as we know it on Earth. We must make certain that scientific evidence is never downplayed, distorted and denied by religious dogma, politics or ideological idiocy.
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Hopefully leading thinkers and researchers will not continue supressing scientific evidence of human population dynamics and instead heed the words of Nobel Laureate Sir John Sulston regarding the emerging and converging, human-driven global challenges that loom ominously before humankind in our time, âweâve got to make sure that population is recognized.... as a multiplier of many others. Weâve got to make sure that population really does peak out when we hope it will.â
Sir John goes on, âwhat we want to do is to see the issue of population in the open, dispassionately discussed.... and then weâll see where it goes.â
In what is admittedly a feeble effort to help John Sulston fulfill his charge to examine all available scientific evidence regarding human population dynamics, please give careful consideration to the following presentation and then take time to rigorously scrutinize the not yet overthrown science from Russell Hopfenberg and David Pimentel regarding human population dynamics and human overpopulation.
http://www.panearth.org/GPSO.htm
Please accept this invitation to discern the best available science of human population dynamics and human overpopulation; discover the facts; deliberate; draw logical conclusions; and disseminate the knowledge widely.
Thank you,
Steve Salmony