The Outbreeding Ratchet

Alan Bittles, in his summary of consanguinity research, notes that consanguinity prevalence is currently rising:

…as greater numbers of children survive to marriageable age, the traditional social preference for consanguineous unions can be more readily accommodated.

Looking into the future, he predicts a reversal (my boldface):

Irrespective of prevailing legislation, a future decline in the prevalence of consanguineous unions can be predicted, accompanying the expected reduction in family sizes. It seems probable that this decline will not be uniform in effect across populations but will be mainly observed in urbanized populations and among couples who share higher educational standards and later ages at marriage. The specific type of consanguineous union contracted may also prove to be an important determining factor.

Bittles also notes that consanguineous unions tend to increased fertility:

A partial explanation for these findings is the generally lower parental age at marriage and the age at the first birth of couples who are close relatives (Bittleset al. 1991, 1993).

When taken together, these effects would constitute a positive feedback loop that could push populations into a spiral of mutually reinforcing decreasing levels of consanguinity and fertility once critical thresholds are crossed.

The thought that such an outbreeding ratchet could actually be the hidden cause behind low western/urban fertility levels does not inspire confidence in the reversibility of fertility trends and future viability of these societies.


HT Steve Sailer for linking to Alan Bittles and Michael Black’s The site offers maps and tables of consanguinity/endogamy data from around the globe as well as a research summary by Alan Bittles from which the following quotes are excerpted.

Sociodemographic aspects of consanguinity

The reasons most commonly given for the popularity of consanguineous marriage can be summarized as: a strong family tradition of consanguineous unions; the maintenance of family structure and property, and the strengthening of family ties; financial advantages relating to dowry or bridewealth payments; the ease of marital arrangements and a closer relationship between the wife and her in-laws; and greater marriage stability and durability (Bittles 1994; Hussain 1999).

Although the usual account of consanguineous marriage is broadly negative and associated with low status—so much that “inbred” is usually understood as a term of derision—Bittles also notes that:

In some populations a high prevalence of marital unions between close relatives has however been reported among land-owning families, and in traditional ruling groups and the highest socioeconomic strata (Bittles 1994, 1995a).

Consanguinity and reproductive behaviour

As we have already seen in the Iceland study the price for outbreeding/exogamy may be reduced fertility. Bittles doesn’t disagree:

In general, higher total fertility rates are reported for consanguineous marriages (Bittles 1995b).

Consanguinity, morbidity, and mortality

The dark side. Bittles reports robust associations between first-cousin levels of consanguinity and increased child mortality, increased risk of rare genetic deseases and preliminary indications of increased risk of adult disorders such as cancer and cardiovascular disease.

Future prospects

Irrespective of prevailing legislation, a future decline in the prevalence of consanguineous unions can be predicted, accompanying the expected reduction in family sizes. It seems probable that this decline will not be uniform in effect across populations but will be mainly observed in urbanized populations and among couples who share higher educational standards and later ages at marriage.

Tribal Advantage (Pt. 1)

At least before industrialization hit, people probably experienced relatively high levels of cultural outbreeding depression. Even in our time, businesses that reward high levels of trust show clear tribal advantage:

“The Yiddish mensch is losing his bread”

A handful of Indian Jain families have largely taken over the Antwerp diamond business, taking the Jews’ centuries-old and seemingly unassailable position. The dethroning has taken less than 30 years. A large factor, outside of just working hard, was being more tribal than the Tribe:

A business largely controlled by your own family is always far superior to competitors, Jahwery says. The other diamond dealers from Gujarat would probably agree. They rely on their worldwide family networks to build and maintain headquarters on every continent. […] Ashwin Jahwery has branches in Taiwan, Thailand, China, Australia, Great Britain and Spain, all of them run by his nephews.


“We always have the possibility of global distribution because a cousin or nephew who can blindly be trusted can always be sent to any country to set up operations” […] That the Jews lacked similar extended families was a major disadvantage for them, in Mehta’s opinion.

Moreover, the Jews’ former dominant position in the trust-intensive diamond business was itself a good demonstration of tribal advantage, in this case working to the Hasids’ advantage compared to their more outbred gentile competitors.

“The friendliest place I have ever been”

Meanwhile, EvolutionistX indirectly illuminates the emotional side of tribal advantage by shining a light on its inverse:

The inverse of clannishness is atomization, and atomization is lonely and stressful. In the atomized society, you are stuck on your own, with no one to catch you if you fall. You might be a single mother or an only child, or a hikikomori. Either way, you’re alone–and most people don’t seem to cope well with loneliness.

Iceland and Fruit Flies

In their reaction to the Iceland study, which showed clearly reduced fertility associated with outbreeding above about the third or fourth cousin level, many commenters seemed to prefer a default explanation of cultural causation, discounting biological cause.

Razib Khan in Why cousin lookin’ fertile…(?) nicely contracts possible cultural explanations into a single abstract model:

[P]erhaps this dynamic is a function of the difference between genetic variance and cultural variance, and the reproductive sweep spot is the region where the former is maximized across the reproductive pair and the latter minimized. […] Gene flow across demes quickly equilibrates allele frequencies so that drift can’t fix differences. But culture may be different; whereas it is very difficult to maintain greater between group variance than within group variance with genetics for coterminous populations, it is not as difficult when it comes to culture.

Comparing with Figure 1B from the Iceland paper, this looks plausible, if only because it looks just like what any kind of additive interaction between simple inbreeding and outbreeding depression curves would look like, but still:

Figure 1B from Iceland paper

Yet. If cultural drift over a relatively short time in a relatively homogenous population has enough pull to cause this kind of clear reproductive disadvantage for outbreeders, can we really, as a default assumption, discount biological causation?

Let’s look at an artificial breeding experiment from the 1950’s using fruit flies. Quoting from the Wikipedia article on Reproductive Isolation:

In the experiment equal numbers of males and females of both species were placed in containers suitable for their survival and reproduction. The progeny of each generation were examined in order to determine if there were any interspecific hybrids. These hybrids were then eliminated. An equal number of males and females of the resulting progeny were then chosen to act as progenitors of the next generation.

The fly species chosen were sufficiently interfertile to breed into almost 50% of hybrid progeny at the beginning of the experiment, but that changed very quickly:

Generation Percentage of hybrids
1 49
2 17.6
3 3.3

Of course, the experimental condition of 100% hybrid mortality was far beyond what we’d expect from Razib’s cultural infertility effect, certainly in a place like Iceland, but then we don’t have to explain something like the near total cross-infertility of those fruit flies, either.

Helgason et al. (2008)

An Association Between the Kinship and Fertility of Human Couples

Previous studies have reported that related human couples tend to produce more children than unrelated couples but have been unable to determine whether this difference is biological or stems from socioeconomic variables. Our results, drawn from all known couples of the Icelandic population born between 1800 and 1965, show a significant positive association between kinship and fertility, with the greatest reproductive success observed for couples related at the level of third and fourth cousins. Owing to the relative socioeconomic homogeneity of Icelanders, and the observation of highly significant differences in the fertility of couples separated by very fine intervals of kinship, we conclude that this association is likely to have a biological basis.

I actually missed this back in 2008. It got attention from bloggers (John Hawks, Razib Khan) and the mainstream press. The Economist writes that, if outbreeding infertility is a thing, we should conclude that

The demographic transition is thus, in part, a pure accident.

This blog suspects that the story is more interesting.


Figure 1 from the Paper, click to enlarge.