Demographic Transition Surprise
As human societies grow richer, people have fewer children. In most species, such an increase in available resources leads in the opposite reproductive direction. What makes the “demographic transition” even more paradoxical is that in less developed times and places, the rich do not have smaller families than the poor.
Most explanations of the demographic transition are social, and none is really satisfactory.
A study by Agnar Helgason, of deCODE Genetics, has recently provided a new explanation: that the mixing-up of people caused by the urbanisation which normally accompanies development is, itself, partly responsible — because it breaks up optimal mating patterns.
Iceland’s records since its founding by a few Vikings are so good that the antecedents of today’s inhabitants (apart from a handful of recent immigrants) are known with precision. Its medical records are also good, and most Icelanders have given genetic samples to deCode.
The study’s principal finding is that the most fecund marriages are between distant cousins. The optimum degree of outbreeding (measured in terms of the number of children and grandchildren produced) lay somewhere between cousins of the third and fourth degrees.
(”Kissing cousins, missing children,” The Economist)
Icelandic women born between 1800 and 1824 who mated with a third cousin had significantly more children and grandchildren (4.0 and 9.2, respectively) than women who hooked up with someone no closer than an eighth cousin (3.3 and 7.3). Those proportions held up among women born more than a century later when couples were, on average, having fewer children.
Despite the general pattern for reproductive success favoring close kinship, couples that were second cousins or more closely related did not have as many children.
With close inbreeding — between first cousins — there is a significant increase in the probability that both partners will share one or more detrimental recessive genes, leading to a 25 percent chance that these genes will be expressed in each pregnancy.
Mating with a relative might reduce a woman’s chance of having a miscarriage caused by immunological incompatibility between a mother and her child. Some individuals have an antigen (a protein that can launch an immune response) on the surface of their red blood cells called a rhesus factor. In some cases — typically during a second pregnancy — when a woman gets pregnant, she and her fetus may have incompatible blood cells, which could trigger the mother’s immune system to treat the fetus as a foreign intruder, causing a miscarriage. This occurrence is less probable if the parents are closely related, because their blood makeup is more likely to match.
It may be that the enhanced reproductive success at the level of third and fourth cousins (who on average would be expected to have inherited 0.8 percent to 0.2 percent of their genes from a common ancestor) represents a point of balance between the competing advantages and disadvantages of inbreeding and outbreeding.
(”When Incest Is Best,” by Nikhil Swaminathan)