Monday, May 7, 2018

Outbreeding: not what you may think

Mean number of children as a function of geographic distance between Danish marriage partners (Labouriau and Amorim 2008).

Most of us know about the genetics costs of inbreeding. If you do a Google search for "inbreeding is bad," you get 35,900 hits.  "Outbreeding is bad" yields only 2.

Yet outbreeding does incur genetic costs. It can reduce fitness either by introducing alleles that are unsuited to the local environment or by disrupting co-adapted gene complexes. When a native trout species was hybridized with non-native trout, fertility fell by half with as little as 20% admixture (Muhlfeld et al. 2009).

Fertility is the canary in the coal mine. A measurable decline is a sign that some genes are malfunctioning, either at the time of fertilization or during embryonic development. A malfunction can occur because the genes from the mother and father are too similar—the risk is higher that both copies of a gene will be defective. It can also occur because one copy is too different—incompatibilities may develop with other genes.

That's what we know from data on fish and other animals. But what about our species? At what degree of relatedness do the costs of human outbreeding start to exceed the benefits? When you marry a Neanderthal? The answer may surprise you. An Icelandic study found that fertility peaks at marriages between third or fourth cousins. Fertility is lower when the prospective parents are more closely related ... or less.

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. (Helgason et al. 2008)

The data come from a time when birth control was not widely practiced. Nonetheless, there may have been something different about Icelanders who married beyond their fourth cousins. Perhaps they were more likely to go to university, meet someone from the other side of the country, and eventually settle down and have children late in life. 

These socioeconomic factors were controlled in a Danish study that measured geographic distance between marriage partners: 

The Danish study was based on the cohort of all women born in Denmark in 1954 who were alive and living in Denmark in 1969, totaling 42,165 women. This cohort was followed up to the end of 1999. The number of children born to each mother between the ages of 15 and 45 years old was determined and is referred to as fertility. The mean marital radius (MR) associated with each mother in the cohort was estimated using the distance between the centroids of the parish where she was born and the parishes where the partners with which she had children were born. (Labouriau and Amorim 2008)

Fertility peaked at around 75 km. This relationship between fertility and marital radius was not explained by education, family income, urbanicity, or mother's age at first birth. The authors concluded that their findings were consistent with those of the Icelandic study, the cause being the same in both cases: fertility rises with decreasing relatedness up to a peak level and then starts to fall. Inbreeding depression then gives way to outbreeding depression.

How exactly does outbreeding reduce fertility? Joffe (2010) points to the steady decline in sperm quality since the early 20th century, suggesting it may be due to an increase in outbreeding. He rejects the usually cited cause: the rising level of estrogenic compounds in the environment, e.g., dioxin, DDT, PCBs, PBBs, phthalates, etc. This proposed cause fails to explain why the sperm quality decline has varied so much spatially, even within the same country. Why, for instance, has it been steep in Paris and nonexistent in Toulouse? Why is it nonexistent in domestic animals that are just as exposed to estrogenic compounds? Finally, the decline seems to have begun before most of these compounds began to be commercially produced. 

Joffe (2010) also suggests that there may be a parallel decline in egg quality. We don't really know because sperm is much easier to collect than eggs for large-scale study.

Do we now have outbreeding depression?

Today, inbreeding depression has largely disappeared throughout the Western world. For a long time the beneficial effects of outbreeding were shown by a steady increase in height and a steady decrease in the age of menarche. Both trends have now ground to a halt:

In Northern Europe, adult height has largely stabilised, and the age of menarche has also settled at around 13 years, while weight continues to increase due to obesity. (Cole 2003)

The steady rise in IQ, known as the Flynn Effect, has sometimes been attributed to outbreeding, although this explanation has been challenged (Flynn 2007, pp. 101-102; Woodley 2011). In any case, the Flynn Effect, too, is slowing throughout the West (Flynn 2007, p. 143). In Scandinavia, mean IQ peaked during the late 1990s and has since declined (Teasdale and Owen 2005).

Has outbreeding become more problematic than inbreeding? That's what the latest findings suggest, yet that doesn't at all seem to be the current wisdom.


Cole, T.J. (2003). The secular trend in human physical growth: a biological view. Economics & Human Biology 1(2): 161-168.

Flynn, J.R. (2007). What is Intelligence? Beyond the Flynn Effect. Cambridge University Press.

Helgason, A., S. Pálsson, D.F. Guðbjartsson, þ. Kristjánsson, K. Stefánsson. (2008). An association between the kinship and fertility of human couples. Science 319(5864): 813-816.

Joffe, M. (2010). What has happened to human fertility? Human Reproduction 25(2): 295-307.

Labouriau, R., and A. Amorim. (2008). Comment on "An Association Between the Kinship and Fertility of Human Couples" Science 322(5908): 1634

Muhlfeld, C.C.,  S.T Kalinowski, T.E. McMahon, M.L. Taper, S. Painter, R.F. Leary, F.W. Allendorf. (2009). Hybridization rapidly reduces fitness of a native trout in the wild. Biology Letters, March 18  

Teasdale, T.W., and D.R. Owen. (2005). A long-term rise and recent decline in intelligence test performance: The Flynn Effect in reverse. Personality and Individual Differences 39(4): 837-843.

Woodley, M.A. (2011). Heterosis doesn't cause the Flynn effect: A critical examination of Mingroni (2007). Psychological Review 118(4): 689-693. 


Luke Lea said...

Interesting as always.

Anonymous said...

The most fertile groups in the US tend to be cloistered communities whose members don't move away from their birthplaces - religious groups like the Amish, Orthodox Jews, fundamentalist Mormons, etc. Although in their cases there are possible confounding factors like religion, natalist culture, patriarchy, etc.

Outbreeding events always happen in the context of external changes that reduce stability and disrupt the prior equilibrium. This disruption and reduction in stability may also be a factor in reducing fertility. These changes include war, invasion, conquest, migration, economic growth, technological developments like transport technology, etc. They continue until these changes let up and some new equilibrium and mixed population is developed. This was actually the case with the Icelanders as well. The Icelanders were founded by Viking men who raided Scotland and Ireland for women. Following this outbreeding event, a new stable state and equilibrium were reached.

epoche said...

I cant find the source of the quote, but I read awhile back that miscegenation is more likely to produce miscarriage. Has anyone read that anywhere?

Sean said...

Dawkins has something along the lines of the post in the Extended Phenotype, but the scientific studies have not really born it out (although they probably did not want to get such a result). Assuming miscegenation is deleterious, one would think the pregnancies would tend to end at the earliest stage to avoid wasting resources (from the mother's reproductive fitness point of view) on less viable offspring. Unless his reproductive fitness is dependent on care and provisioning of the mother and child, the DNA of the male has every reason to hack the woman's reproductive system to prevent miscarries of non optimal embryos as he contributes little and can go on to a succession of women. Women are surprisingly likely to get pregnant from sex with a stranger, or rape. I know that the man being excited makes his semen more potent, but I suppose the males of Africa and Europe have evolved with somewhat different constraints on their DNA's strategy. With the assumption that it is deleterious, |I would expect a black female/white male couple to have less reproductive fitness due to to miscarriages than all white or all black couples. There is said to be an IQ advantage for black male/ white female over the reverse.

Peter Frost said...

The widespread availability of birth control since the 1960s makes this kind of study harder nowadays. The only option is to study groups, like the Amish, that refuse to use birth control.

The Icelandic study covers the period from 1800 to 1965. It was a pretty quiet period: no invasions, no war, (and no Vikings). The Danish study was likewise unaffected by external events. It concerned women born in 1954 and who were still alive in 1969.


"Dawkins has something ..." "There is said to be ..." You can do better than that, Sean. Precision in discourse is a virtue.

Sean said...

Sorry Peter. Anyway, I always thought that outbreeding shelters the various (usually different in unrelated people) deleterious mutations for higher fitness, thus storing up trouble for the future because outbred mating can result in a person with lot of deleterious mutations not obvious to a prospective mate. So I thought if you are of mixed ancestry (eg biracial) it is better to backcross to one of the parental stocks than to mate with someone of the same ancestral mix, because the same deleterious mutations will be much more likely to come into contact with each other if two hybrid vigour people of the same racial (or maybe even subracial) outcrossing have a child.

I don't know how much DNA 3rd cousins would have in common. However, the studies in the post seem to indicate, as far as I can see it, that deleterious mutations are not in themselves that much of a problem, because in third or fourth cousins an appreciable amount of flawed DNA would certainly have common origins and thus surely line up in the offspring to affect function. I also wonder why it is that, except for a few hundred years in West during the middle ages, it was never forbidden to marry a second cousin. Perhaps traditional wisdom had determined outbreeding to be a bigger threat?

Anonymous said...

War and invasion are only the most obvious factors that facilitate outbreeding. Others include economic, technological, and social developments. Iceland may have been behind the rest of Europe and isolated from war and invasion, but it still would've undergone various changes from 1800 to 1965. There were also changes in Denmark in the 50s and 60s, such as a shift in employment from agriculture to industry, which would've facilitated outbreeding:

Anonymous said...

Unless his reproductive fitness is dependent on care and provisioning of the mother and child, the DNA of the male has every reason to hack the woman's reproductive system to prevent miscarries of non optimal embryos as he contributes little and can go on to a succession of women. Women are surprisingly likely to get pregnant from sex with a stranger, or rape.

Outbreeding requires physical contact and proximity. A woman's reproductive system doesn't need to be hacked because her limbic system has already been hacked by the proximity of foreign males. Regardless of how or why the foreign males got there, her limbic system is saying, "Why haven't these foreign male strangers been killed or repelled by the native males? Maybe it's because the foreigners are "better" and thus I need to acquire those foreign genes." The foreign males also suggest to her limbic system that new selective pressures are at work, like pathogens introduced by the foreigners that they're more coevolved with, and that therefore she has to acquire those genes to better adapt to the new pressures.

Sean said...

Merely arriving seems to be a dubious way to show that one has genes worth having. Bowery seems to overlook that the foreign males could literally hack the indigenous males braincase with copper axes. Y chromosomes do not spread because their bearers just showed up like a black swan and all the ladies swooned over their exotic immune system. Anyway, the evidence is that out-breeding is harmful and that presumably goes for the major histocompatibility complex too.

Anonymous said...

The major histocompatibility complex genes do influence human sexual behavior.

The group of genes that most differentiates human populations also happens to be the group that is most involved with your body's ability to differentiate between self and nonself -- friend or foe identification. This is because different populations tend to have different diseases -- and some populations -- particularly those in old habitats or those in high traffic trade centers, will have more diseases. These are the major histocompatability complex genes.

Experiments conducted on college age students have shown that females are unconsciously sexually stimulated by the mere presence of men with MHC genes different from their own.

The best theory to explain this instinctive sexual arousal by women is the fact that if a foreign male is allowed into the physical presence of fertile females of her tribe, it is likely that the diseases of his tribe will not be far behind and will, without the foreign male's immunities, possibly exterminate the female's tribe. The further removed the male's tribe is from the female's the less likely the female's tribe will have immunity to the foreign diseases.

Further, if the foreign male is from an older population, particularly with high population density, the need to acquire the foreign male's immunity may be quite urgent since both the variety and sophistication of his tribe's pathogens may be greater than her tribe's.

In the modern environment, where we have medical treatments for many forms of disease, the female's instincts may not be as adaptive as they once were, but there are examples of diseases, such as TB, where multi-drug resistant strains are evolving and to which certain blood groups, such as type B, are known to have greater immunity than others.

Sean said...

"Experiments conducted on college age students have shown that females are unconsciously sexually stimulated by the mere presence of men with MHC genes different from their own."
MCH genes much the same as their own would be from relatives. Avoiding blood relations is a very different thing from outbreeding. High testosterone (which women can smell) is bad for immunity but good for fathering a sexy son.

Santoculto said...

Outbreeding may be the one possible creation a new "breed", but some breeds are more genetically convergent than others. I don't attributed overweight with outbreeding depression because it's due behavior and genetic incompatibility and "epigenetic" transmission.

Peter Frost said...

Until the 20th century most people found their mates within a 75 km radius of home. So there would have been strong selection for behavioral mechanisms that discourage inbreeding and much weaker (or nonexistent) selection for mechanisms that discourage outbreeding.

Santoculto said...

Urban tribes may be a factor here in outbreeding but inbreeding look like the final path of every outbreeding, look like a intermediate period.

Santoculto said...

Another ''think'' is:

pseudo-imbreeding or phenotypical imbreeding

when people become more ''free'' to choice their partners they started to choice their similar sub-group ones. This re-creates a mating by similarity, but via phenotypes and not genotypes [ direct genetic family]. Example, ''stem and engineering'' people marrying more one each other = possibly/partly related with autism increasing.


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Anonymous said...

It strikes me as unlikely that there is a very significant difference between finding a mate 50 miles away as opposed to 300 miles, but there is no very noticeable effect if partners are from different continents. Maybe it's a real biological effect, but I can easily see it being confounded.

Anonymous said...

What does this suggest about hbdchick's hypothesis that outbreeding was selected for in Europe?