Archives for category: inbreeding

i’ve been trying to think through polygamy and if there’s any potential there for the selection for clannishness like i think there is with long-term cousin marriage. (i think i might have sprained a parietal lobe while doing so. (*^_^*) ) i very much have subsaharan african societies in mind here, but, of course, polygamy occurs elsewhere, too.

on the surface it seems obvious that long-term polygamy ought to set the stage for the possible selection for clannish behaviors like cousin marriage (imho) does. like repeated cousin marriage, strict polygamy ought to narrow the relatedness within a population — the result of strict polygamy should be a greater number of half-siblings in a population than in a randomly-mating population, and, of course, half-siblings are more closely related to one another than non-siblings, so a society full of half-siblings could potentially lead to an accelerated selection for nepotistic altruism in a way similar to cousin marrying societies.

however, one big difference is that in polygamous societies generally — even in subsaharan african societies (where there’s a lot of polygamy) — people do not marry/mate with their half-siblings. (it does occasionally happen in some subsaharan societies, but only occasionally.) so, unlike in cousin-marriage societies, “genes for nepostistic altruism” (whatever they might be) might *not* become concentrated in family lineages. yes, there are a lot of half-siblings in polygamous societies, but any particular nepostistic altruism (“clannishness”) genes they might have (gotten from their fathers) will get diluted as they move out into the general population and marry non-relatives. if polygamy isn’t a driver of accelerated selection for nepotistic altruism (and i’ve rather persuaded myself that it isn’t), that could explain why subsaharan africans are generally pretty civic-minded, comparatively speaking. (the poor outcomes seen in african nations are perhaps more the result of other factors like low iq, high disease rates, etc., rather than clannishness. dunno. Further Research is RequiredTM.)

i should note here that polygamy in subsaharan africa is extremely variegated — in some societies, it’s typical for the first wife to actually be a cousin, and then the rest not. so there can be a layer of cousin marriage in amongst the polygamy. in other societies, cousin marriage is completely avoided. in yet other societies, the series of wives might be sisters (sororal polygyny), which makes all the offspring not only half-siblings (because they have the same father) but also cousins (because their mothers are sisters). here you would think that any selection for nepotistic altruism should very much be amplified. of course, in many subsaharan african societies — especially the polygamous ones — there’s often a lot of hanky-panky going on, so not all of the siblings will truly be half-siblings, etc. that’ll dilute your genes for nepotistic altruism right there.

another thing i also thought of regarding subsaharan and/or polygamous societies is the fact that all of the half-siblings don’t always grow up together. in patrifocal polygamous societies, yes — there you’ll have one man living with all of his wives (poor fellow!) and all of his kids, so all the half-siblings will be raised in the same place and interact with one another — and, presumably, continue to do so as adults. in matrifocal societies, a mother and her children reside with the mother’s family, not her husband and his family. this occurs in some polygamous societies, too.

it seems to me that, even if polygamy was a driver of accelerated selection for nepotistic altruism, such selection couldn’t possibly happen if the carriers of the clannishness genes don’t interact. if the half-siblings from polygamous unions don’t grow up together, or don’t interact much as adults, but rather with their (ordinary, i.e. not inbred) cousins, how would clannishness be selected for? it wouldn’t, i don’t think. or it wouldn’t be selected for in an amplified, accelerated way (which is what i think happens in the long-term cousin marriage scenario).

and that’s as far as i got with thinking through polygamy (i shall return to this topic, i’m sure). but thinking about the patrifocal vs. matrifocal family types got me to thinking about something else.

thought experiment: let’s say you eliminate cousin marriage from a population, but don’t eliminate the extended family. say you get rid of the inbreeding, but individuals continue to interact mostly with their close (extended) family members — more so than with the other members of society who are unrelated to them. you would think that it would take longer for clannishness to disappear — for “genes for nepostistic altruism” to get diluted in the population — than in a society where both cousin marriage AND the extended family were simultaneously eliminated.

i am, of course, talking about medieval western versus eastern europe here. the extended family was eliminated quite early in the middle ages in western europe via manorialism along with cousin marriage (serious changes to both were well underway in western europe by the 800s). in eastern europe, the cousin marriage bans appeared later simply because christianity had arrived later. and, especially the further east one goes (like into russia), the fewer pressures there were to eliminate the extended family. quite the opposite, really. for example, this was the situation in the baltic regions, including belorussia, in ca. the fifteenth century [pg. 440]:

a “…’kinship holding’, was collectively held by the extended family. Rural settlements often contained more than one kinship holding, and each holding was in turn subdivided among smaller households within the extended family….”

and in russia as late as the late seventeenth and early eighteenth centuries [pg. 444]:

“Russian manorialism was distinctive in several important ways…. In Russia…it was the peasant commune that allocated these taxes and obligations among the households. The village commune in Russia had emerged in the late seventeenth or early eighteenth centuries in response to increasing demands from the state and the landowning elite; peasant communes not only allocated obligations, but also chose their officials, held court, selected recruits for conscription levies, and kept written records of their activities. The communal clerk was sometimes the only member of his commune who could read and write….

“[O]n Russian manors, where hired labour was often not available, the peasant family had to personally perform labour obligations at the same time that it worked its own farm. This required large, often multi-generational, households with enough labour capacity to serve the simultaneous needs of both the manorial economy and the family farm…. As Steven Hoch has shown, however, life in the large household was hardly a rural idyll; household patriarchs formed a communal elite that ruled with despotic brutality, ruthlessly exploiting their families and denying any autonomy to the adults under them. At the same time, however, the large household also protected the peasant family from ruin.”

(hmmm. ever wonder where the russian love for [left-wing] authoritarianism comes from?)

even if eastern europeans/russians began to avoid cousin marriage around, say, 1000 (conversion to christianity), they didn’t quit residing in extended families and mostly interacting with their extended family members until, like, yesterday. (again, this pattern appears to be more pronounced the further east one travels.) so the dilution of nepotistic altruism genes in eastern european populations — via nepotistic behaviors being misapplied to individuals not sharing the same altruism genes (i.e. unrelated individuals) — didn’t happen as quickly as it did in western europe where people began regularly interacting with non-kin much earlier in the middle ages.

family types matter.

that’s all i’ve got for you for now. more soon! (^_^)

previously: start here and cousin marriage in sub-saharan africa and fulani, hausa, igbo, and yoruba mating patterns

(note: comments do not require an email. russian peasants.)

in his post A pathway to pro-social behavior, peter frost references this article Second-to-Fourth Digit Ratio Has a Non-Monotonic Impact on Altruism in which the authors say:

“We find an inverted U-shaped relation for left and right hands, which is very consistent for men and less systematic for women. Subjects with both high and low digit ratios give less than individuals with intermediate digit ratios.

the subjects were w.e.i.r.d.-ish, btw — university students from granada, spain.

peter comments:

“From one population to the next, digit ratios tend to cluster around different means, perhaps because altruism has been favored or disfavored to different degrees.”

i’m not familiar with those differences (will have to investigate), but i did post this from turkey in a recent linkfest:

“Inbreeding is associated with lower 2D:4D digit ratio”

“We compared the 2D:4D ratios of 122 male and 108 female consanguineous (children of first cousin marriages) high school and university students to those of 142 male and 122 females controls. Across hands and sex, consanguineous parentage was consistently associated with lower, more masculine-typical, digit ratios. Digit ratios were 1.3–1.9 times more variable among the consanguineous group than the control group. While socio-economic status cannot explain the effects seen in our data, we found that lower, more masculinized, digit ratios were associated with lower family income.”

inbreeding leads to…whatever genotypic/phenotypic package that results in lower 2d:4d digit ratios *and* lower rates of altruistic behaviors? dunno. Further Research is RequiredTM.

ftr — i come from a population that has been inbreeding up until fairly recently, and i’ve got a low 2d:4d digit ratio. uh oh.

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i know you’ve been wondering.

well, obviously the gypsies are a highly endogamous group — they mostly marry other gypsies. the actual cousin marriage rates vary though from (as you’ll see below) ca. 10-30% first cousin only marriages amongst gypsies in slovakia to 29% first+second cousin marriages amongst gypsies in spain [pdf] to 36% first+second cousin marriages amongst gypsies in wales [pdf]. these rates are comparable to those found in places like turkey (esp. eastern turkey) or north africa…or southern india. tells us that the rates of first cousin marriage (that includes double-first cousin marriage) amongst slovakian gypsies ranges from 10.1% to 14.7% [pdf – pg. 10 – i think the reference is to this 1994 paper].

another study of gypsies in slovakia (in svinia) found that the cousin marriage rates have actually increased over time since the early twentieth century, the researcher guesses due to the increasing population size (the more cousins around to marry, the more cousin marriage – maybe?). from Svinia in Black and White: Slovak Roma and Their Neighbours (2005) [pgs. 84-85]:

“Analysis of the marital choices made by local Roma shows that 75 per cent of the children born here between the 1930s and the early 1970s had a least one parent who hailed from elsewhere, whereas that ratio fell to 25 per cent during subsequent years. This dramatic shift doesn’t mean that young people no longer leave Svinia for spouses in other settlements — some continue to do so — but it does mean that of those who remain behind, which is the vast majority, far fewer end up with spouses from outside the community than used to be the case with their parents and grandparents.

“I don’t know how to explain this remarkable shift toward settlement endogamy. The people who are responsible for it don’t indicate any significant changes in their preferences, and the most plausible conclusion one can draw in the absence of evidence pointing in a different direction is to correlate the shift with the dramatic increase in Svinia’s population size, which translates into a corresponding growth of the local marriage universe. Unlike their parents and grandparents who lived in a small community that imposed strict limits on their choice of partners, the people who have reached maturity in more recent years have faced a much expanded pool of potential local partners, enabling them to make a selection within their own settlement.

“The result of this shift has been a decrease in the amount of traffic between Svinia and other Romani communities. Every marriage with a resident of another settlement brings about increased contact between the two communities. Relatives and friends travel to and fro as they attend baptisms, birthday parties, funerals, and other important events…. The shift from choosing mostly spouses from other settlements to marrying predominantly one’s own neighbours has gone hand in hand with a remarkable increase in the rate of unions between relatives. It is certain that common-law marriage involving close relatives did exist among Svinia’s first- and second-generation Roma. Indeed, there is strong evidence that the common-law spouse of Bartolomej (1912-73), one of Juraj and Hania’s sons and the founder of one of the lineages of *jarkovci*, was his biological niece. And of Hania and Juraj’s 27 grandchildren who remained in Svinia and found partners there, seven chose first cousins or first cousins once removed. But this rate of roughly 25 per cent pales in comparison with the situation among the third- and fourth-generation: people who have reached maturity during the last 30 years or so and who have remained in their ancestral settlement. Of the 159 persons in this category, 101 (or close to 65 per cent) opted for a biologically related partner. Roughly one half of these unions involves close cousins — first and once removed — while the other half consists of more distant degrees of consanguinity….

“Interestingly, local Roma profess avoidance of cousin marriage, and few of those who have broken this norm will disclose it voluntarily. Confronted with genealogical evidence, most will shrug their shoulders and declare pragmatically that affection overrides conventions. On the other hand, many people have such a vague grasp of their own ancestry that they cannot establish the identities of all four grandparents. This means that more distantly linked spouses, such as second or third cousins, are often not even aware of their relationship.”

and here about gypsies in albania from Roma and Egyptians in Albania: From Social Exclusion to Social Inclusion (2005) (egyptians??) [pg. 18]:

“Most marriages, especially among Roma, are intra-ethnic and arranged through match-makers. In fact, 95 percent of Roma and 74 percent of Egyptians preferred members of their own ethnic group as marriage partners.

“Traditional Marriage Partners. Most Roma and Egyptians still marry within their own ethnic group. More Egyptians than Roma would accept a marriage between a family member and an Albanian.

Among many Roma families, moreover, marriage partners must be members of their own primary *fis*. Many Cergar and Bamill Roma in Delvina, Gjirokastra, Levan, Fier, Fushe Kruja, and Korca arrange marriages between first and second cousins. Endogamy is practiced by some Roma in isolated localities, or by Roma that recently migrated from the country, and is explained by the limited access to available marriage patterns there. Roma explain endogamy through such metaphors as ‘The good horse should be sold within the village’ and ‘Why should the good apple get eaten by someone else?’

“The tradition of marrying members of one’s own primary *fis* is, however, undergoing change. One Roma leader in Tirana explained: ‘Many marriages happen nowadays between members of different Roma *fise*, whereas before they didn’t. Everyone wanted [to marry someone] from his own *fis*. My father and my wife’s father belong to the same *fis*; therefore my wife and I married…Today, however, the youth don’t care about this tradition, and they’ve even started marrying whites, Egyptians, or Roma from other *fise*.’

“Marriage with a member of another ethnic group is sometimes punished with ostracism….”

we’ve heard about these *fise* in the balkans before. here’s more about the gypsy *fise* (i think we can just call them clans) [pgs. 21-22]:

“Roma social organization is based on the *fis*. Members of one *fis* are usually persons who patrilineally descend from a common male *fis* name. The main branches of the *fis* — large families — serve as the bases for the creation of new *fise*.

“Arben, a Roma who enjoys a high standing within his *fis*, explained: ‘My *fis* is made up of all cousins [first, second] that have my last name, Demir.’ When a *fis* extends for several generations into a few dozen families, *fis* membership can reach into the hundreds. Now into its third generation, the Demir *fis* is composed of approximately 70 families and several hundred individuals.

“*Fis* structures can take several forms. Many Roma only consider persons with a common last name as *fis* members. But some Roma think that the children of female *fis* members can still be considered *fis* members although they have different last names. The concept of the *fis* is relative and dependent upon the outlook of *fis* members and the point in the family tree from which the *fis* begins. Patrilineage indicates an ongoing social change. But while patrilineage has, up to now, been a rather stable form of kinship social organization of the Roma *fis*, forms of matrilineage have become less common.”

and, finally, on some gypsies in romania — Exploring Gypsiness: Power, Exchange and Interdependence in a Transylvanian Village (2007) [pg. 79]:

“The only restrictions of marriage among the hamlet Roma are between members of the immediate family, between uncles and nieces, aunts and nephews and between Roma and gaže [that’s us]. These restrictions are, however, not expressed as rules, but as a self-evident question of morality and practicality. As the gaže ‘have no shame’ and do not speak Romanes, and as such mixed marriages are also rejected by gaže, they will not generally happen. As marriage to gaže does not create alliance, and thus kinship, it is not strategically interesting. When such marriages did occur they were the result of individual choice, but were not rejected if they were seen as prosperous for the family household or familia in general. The hamlet Roma preferred to marry Roma of their own subgroup, generally Roma to whom they were already related. Although most Roma told us that it was bad to marry too close, first- and second-cousin marriages were common, both between cross and parallel cousins (vero/verisoara). These were regarded as true Roma (Roma cace), people one knows and can trust because they already belong to one’s kin network and speak Romanes properly.

how long have gypsies been marrying close cousins at these rates? who knows. long time probably.

*edit 10/24: anonymous points out that the “gypsies” on My Big Fat Gypsy Wedding are actually irish travellers — or mostly irish travellers. thanks, anonymous! the irish travellers marry their cousins A LOT.


apparently, a couple of the principles on My Big Fat Gypsy Wedding who got married were first cousins. caused a bit of a stir among some non-gypsy viewers i guess. i must’ve missed that episode. (~_^) interesting to see here the restrictions on women — related to reproduction, of course — just like in other inbreeding groups — like the arabs, for instance:

“‘My Big Fat Gypsy Wedding’ stars believe in incest, not pre-marital sex”

“…The show’s stars add that they also embrace traditional values and distinct gender roles.

“‘We don’t believe in sleeping with men before we are married,’ Annie explained. ‘The woman’s role in a gypsy family is to stay home, take care of kids, clean, get your nails done, and take care of duties at home. The kids, especially the girls, are learning to clean the house and parents are very strict on them. Growing up I was not allowed to stay at friends’ houses even when I was 15 or 16 years old, I was not allowed to go to parties or have a boyfriend or do anything. I just cleaned the house.’

“Nettie continued that the ‘normal age for a girl to get married is between 16 and 18,’ and that females are allowed fewer freedoms than their male counterparts.

“‘It is just the way we were brought up,’ she insisted. ‘A girl has more at risk with her reputation than a boy does. A girl has to go a little further than a guy to protect her reputation. A girl is to be looked at as a decent young girl, where a boy can do whatever and nobody is going to look at him in a bad way….'”

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a very kind reader points me to a fairly newly published book (pub. last month) titled Our Political Nature: The Evolutionary Origins of What Divides Us by avi tuschman (thank you, very kind reader! (^_^) )

the very kind reader thought i might be interested in the book since it’s got sections on inbreeding and outbreeding … and altruism. yes, indeed! that’s right up my alley!

i’ve only glanced through the book, but this caught my eye … sounds vaguely familiar [from chapter 10]:

“Inbreeding Increases Altruism

“There’s another potential positive-feedback mechanism that promotes inbreeding in animal populations: inbreeding can increase altruism. Altruism is the force that causes an individual to reduce his or her own fitness while increasing the fitness of another. But what’s the connection to mate choice?

“Inbreeding raises the genetic relatedness of the members of the group. Since inbreeders share a higher percentage of their genes with one another, they act less selfishly toward one another. By helping closely related individuals, inbreeders are helping to propagate copies of their own genes. Chapter 19 explains in greater detail this particular phenomenon, which is called ‘kin selection.’

“If specific alleles associated with altruism exist in (or mutate into) a population, then inbreeding can increase the frequency of these ‘altruistic’ alleles until they are ‘fixed’ in the population; that is, they can become permanent, contingent on continued inbreeding.

“In a random mating system, in contrast, altruism is likely to be lower because individuals share fewer genes in common with each other. In an outbred population, two nonrelatives may reproduce. Half of their offspring’s DNA would come from the mother, and half from the unrelated father. Among inbreeders, however, parents already share a proportion of their DNA with one another. Therefore, the inbred offspring have *more* than half of each parent’s genes. The longer a population’s history of continuous inbreeding, the higher the genetic overlap becomes. Having a child that shares more than 50 percent of one’s DNA is an easy way for a parent to increase the propagation of its genes, and therefore to gain greater fitness — without even expending any additional reproductive effort.

“Although sustained inbreeding in a human population can increase altruism among the in-group, it can also *decrease* altruism toward out-groups. This hostility may particularly occur when colonies of inbreeders live side by side but do not outbreed. Separation by a reproductive boundary would entail a sharp drop in genetic relatedness between the two groups. So altruism would be high within the groups but very low between them.”


like i said, i’ve only glanced through the book, but i couldn’t see anywhere where mr. tuschman connects inbreeding or outbreeding with family types. or that long-term mating patterns and their downstream effects on social structures (like on family types) likely influence selection within populations.

still, looks like a very interesting book! i shall definitely be having a read of it. (^_^)

edit: from chapter 19:

“Yet regardless of political orientation, close family members tend both to behave more altruistically toward each other than toward distant kin and to treat distant kin better than non-kin. When we translate Hamilton’s Rule from biology into the language of politics, kin-selection altruism toward close kin becomes ‘nepotism’ and the preferential treatment of distant kin becomes ‘tribalism.'”


see also:

previously: practically everything on the blog and mating patterns, family types, social structures, and selection pressures

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just for a change of pace.

the consanguineous (second cousin or closer) marriage rates in afghanistan are high. tells us that the rate is between 40 and 49%. more details are to be had in Consanguineous Marriages in Afghanistan (2012) and Prevalence of Consanguienous Marriages in West and South of Afghanistan (2012), including consanguinity rates by province and ethnic group.

back in the 1970s, joseph westermeyer found that peoples in southeast asia had different mating patterns depending on what elevation they lived at — the higher up, the closer the mating patterns (see also here). this pattern appears to be holding true wherever i look (example) — and now we have afghanistan.

here’s a map of the mean inbreeding coefficients for the provinces studied in the two papers above — higher coefficients indicate greater inbreeding (click on map for LARGER view):

Afghanistan provinces - inbreeding coefficients - colored

aaaaand here’s a topographical map of afghanistan. elevation and inbreeding look to match pretty closely (would be nice to have data from the other provinces, too):


here’s a breakdown of consanguinity rates by ethnicity in the country. the numbers are also sorted here by region depending upon which paper they came from — the first paper dealt with the north and east of the country, the second with the south and west. remember that consanguineous marriages include: double-first cousin marriage, first cousin marriage, first cousin-once-removed marriage, and second cousin marriage:

– north & east –
Turkmen = 48%
Hazara = 47%
Uzbek = 44%
Pashtuns = 43%
Tajik (Shi’a) = 43%
Tajik (Sunni) = 38%

– south & west –
Turkmen = 64%
Hazara = 53%
Sadats = 51%
Tajik (Sunni) = 51%
Pashtun = 50%
Tajik (Shi’a) = 49%

the turkmen in the lead!

interestingly, while there is more consanguineous marriage in the south and west of afghanistan, the inbreeding coefficients are higher in the north and east of the country, indicating that there are greater amounts of closer marriages in those (high elevation) regions. and this does appear to be the case — the percentages of double-first cousin marriages are higher in the north and east:

– north & east –
Turkmen = 8.7%
Pashtun = 7.9%
Uzbek = 7.5%
Hazara = 6.4%
Tajik (Sunni) = 6.3%
Tajik (Shi’a) = 4.0%

– south & west –
Sadats – 3.0%
Pashtun – 2.3%
Tajik (Shi’a) – 1.8%
Hazara – 1.2%
Turkmen – 1.2%
Tajik (Sunni) – 1.1%

i’m going to guess that there’s more father’s brother’s daughter’s (fbd) marriage in the north and east of afghanistan rather than in the south and west, since fbd marriage tends to push towards greater amounts of double-first cousin marriage (and, therefore, greater inbreeding in general). i’m also going to guess that the tajiks really don’t practice much fbd marriage at all, either in the north or the south — except maybe for the sunni tajiks in the north.

how long have the various afghani populations been marrying their cousins? dunno. long time prolly. fbd marriage was most likely introduced to the region by the arabs, so the afghanis probably adopted that form of cousin marriage sometime after the mid-600s.

previously: this one’s for g.w. and the flatlanders vs. the mountain people and kandahar vs. levittown

(note: comments do not require an email. turkmen girl & baby in afghanistan.)

the walled family compounds of kandahar


…vs. the invisible boundaries of levittown


previously: there’s no place like home

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in addition to being concerned about too much inbreeding and how that might hinder the building a christian society here on earth, thomas aquinas also worried about the effects of too much outbreeding.

from his Summa Theologica [pg. 2749]:

“The degrees within which consanguinity has been an impediment to marriage have varied according to various times…. [T]he Old Law permitted other degrees of consanguinity, in fact to a certain extent it commanded them, to wit that each man should take a wife from his kindred, in order to avoid confusion of inheritances: because at that time the Divine worship was handed down as the inheritance of the race. But afterwards more degrees were forbidden by the New Law which is the law of the spirit and of love, because the worship of God is no longer handed down and spread abroad by a carnal birth but by a spiritual grace: wherefore it was necessary that men should be yet more withdrawn from carnal things by devoting themselves to things spiritual, and that love should have a yet wider play. Hence in olden time marriage was forbidden even within the more remote degrees of consanguinity, in order that consanguinity and affinity might be the sources of a wider friendship; and this was reasonably extended to the seventh degree, both because beyond this it was difficult to have any recollection of the common stock, and because this was in keeping with the sevenfold grace of the Holy Ghost. Afterwards, however, towards these latter times the prohibition of the Church has been restricted to the fourth degree, because it became useless and dangerous to extend the prohibition to more remote degrees of consanguinity. Useless, because charity waxed cold in many hearts so that they had scarcely a greater bond of friendship with their more remote kindred than with strangers: and it was dangerous because through the prevalence of concupiscence and neglect men took no account of so numerous a kindred, and thus the prohibition of the more remote degrees became for many a snare leading to damnation.”


previously: st. augustine and st. thomas aquinas

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via dienekes via jayman:

Genome-wide estimates of inbreeding in unrelated individuals and their association with cognitive ability


“Research on consanguineous marriages, and other forms of inbreeding, has long shown a reduction in cognitive abilities in the offspring of such unions. The presumed mechanism is that detrimental recessive mutations are more likely to be identical by descent in the offspring of such unions and so have a greater chance of being expressed. To date, research on the relationship between inbreeding and cognitive ability has largely been restricted to recent inbreeding events as determined by pedigree…. It has been suggested that intellectual disability is under negative selection, and that recent deleterious mutations have an important role in the underlying aetiology. The wealth of molecular genetic data currently available allows estimates of inbreeding on a genome-wide level and to examine the effects of long-term ancestral levels of inbreeding. Such an association with inbreeding, as measured by runs of homozygous polymorphisms (ROH), has previously been identified with several behavioural traits, such as schizophreniz, Parkinson’s disease and personality measures, as well as non-behavioural traits such as height.

“The relationship between inbreeding on a population level and cognitive ability is particularly interesting due to assortative mating, non-random mating, which is greater for cognitive ability than for other behavioural traits, as well as physical traits such as height and weight. Positive assortative mating has been reported for cognitive ability, particularly for verbal traits, with spousal correlations generally around 0.5. Assortative mating should lead to greater genetic similarity between mates at causal loci for cognitive ability and to a lesser extent across the genome, which in turn reduces heterozygosity at these local. In other words, in contrast to the genome-wide reduction of heterozygosity caused by inbreeding, the reduction of heterozygosity due to assortative mating for a trait is limited to loci associated with the trait…. Another difference between inbreeding and assortative mating is that the effects of inbreeding are expected to be negative, lowering cognitive ability, whereas the effects of assortative mating affect the high, as well as the low end of the ability distribution, thus increasing genetic bariance, that is, when high-ability parents mate assortatively, their children are more likely to be homozygous for variants for high ability, just as offspring of low-ability parents are more likely to be homozygous for variants for low ability….



“The Twins Early Development Study (TEDS) recruited over 11 000 families of twins born within England and Wales between 1994 and 1996…. In this analysis, individuals were excluded if they reported severe current medical problems, as well as children who had suffered severe problems at birth or whose mothers had suffered severe problems during pregnancy. Twins whose zygosity was unknown or uncertain or whose first language was not English were also excluded. Finally, analysis was restricted to twins whose parents reported their ethnicity as ‘white’….

“Cognitive measures

“Verbal and non-verbal tests were administered using web-based testing. The verbal tests consisted the Similarities subtest and the Vocabulary subtests from the Wechsler Intelligence Scale for children (WISC-III-UK). The non-verbal tests were the Picture Completion subtest from the WISC-III-UK and Conceptual Grouping from the McCarthy Scales of Children’s Abilities. A general score was derived from the test battery as the standardized sum of the standardized subtest scores, which correlates 0.99 with a score derived as the first principle component of the test battery score.

“Runs of homozygosity

“FROH was defined as the percentage of an individual’s genome consisted of runs of homozygosity (ROH)…. [O]nly ROH with a minimum of 65 consecutive SNPs covering 2.3Mb were used when calculating the total proportion of the genome covered by ROH. In addition, the required minimum density in a ROH was set at 200kb per SNP, and the maximum gap between two consecutive homozygous SNPs was set at 500kb….


“Table 1 includes descriptive statistics for FROH and the three measures of cognitive ability (general, verbal, and non-verbal). FROH is slightly positively skewed, as it represents the total percentage of the genome that includes runs of homozygosity (ROH). The average percentage of genome covered by ROH was 0.7% (95% CI 0.65-0.72%). Verbal and non-verbal abilities correlate 0.49; because general cognitive ability is the sum of the standardized verbal and non-verbal subtests, they correlate much more highly with general ability (0.87 and 0.86, respectively).

inbreeding and iq - table 01

“Table 2 presents the results of the linear regression analyses. No significant regression was found between FROH and the cognitive measures after correction for multiple testing, although the association with non-verbal cognitive ability was nominally significant (P=0.03). Although this association was not statistically significant, it is noteworthy that every regression in Table 2 is *positive*, indicating that increased homozygosity tends to be associated with *higher* cognitive scores across different measures of cognitive ability (general, verbal and non-verbal).

inbreeding and iq - table 02

“Our analysis identified 87 loci where ROH overlapped in 10 or more individuals. For these overlapping regions we tested for association with each of the cognitive measures and again showed no significant associations after correction for multiple testing (P-values of less than 5.7 x 10-4). A sign test of the direction of effect across all ROH showed a disproportionately large number of *positive* associations, indicating that ROH are associated with higher cognitive ability (P=0.002). The sign test was non-significant for verbal ability but highly significant for non-verbal ability (P<10-6). The sign test for non-verbal ability alone remained significant after correcting for an individual’s genome-wide FROH score (P<10-6).

“As explained earlier, positive assortative mating can also lead to genome-wide homozygosity for trait-specific loci, and, unlike inbreeding, assortative mating can affect the high as well as the low end of the ability distribution. One possible explanation for the trend suggesting a positive correlation between homozygosity and cognitive scores in our data is that positive assortative mating on intelligence might be greater for high cognitive ability individuals….


“Our results show that within a representative UK population sample there was a weak nominally significant association between burden of autosomal runs of homozygosity and higher non-verbal cognitive ability. This nominal association with *increased* cognitive ability is counterintuitive when compared with the results from more extreme inbreeding based on pedigree information. A potential explanation for this direction of effect is that individuals with higher cognitive ability might show greater positive assortative mating, which would lead to increased homozygosity at loci for higher cognitive ability in their offspring. However, in a separate sample we showed that greater positive assortative mating was not associated with higher cognitive ability. While these findings seem to provide clear evidence against this hypothesis, it is possible that the genome-wide genetic finding reflect historical mating habits that no longer exist today. It should also be noted that there was a reduction in the standard deviations for spousal correlations in the increased cognitive ability groups by an average of 6% compared with the decreased cognitive ability group (see Table 3), which could reflect less genetic variability in the high ability couples or a ceiling effect on the cognitive tests. This lesser phenotypic variability at the high ability end would have a small effect in reducing the spouse correlations and potentially confound our analysis….

“Overall, these results highlight the importance of understanding mating habits, such as inbreeding and assortative mating, when investigating the genetic architecture of complex traits such as cognitive ability. The results certainly suggest that there is no large effect of FROH on reduced cognitive ability, the expected direction of effect. The nominally significant associations found in this study may even suggest that in the case of non-verbal cognitive ability, beneficial associations with homozygosity at specific loci might outweigh the negative effects of genome-wide inbreeding and that the relationship between inbreeding and cognitive ability may be more complicated than previously thought.

so, although obviously Further Research is RequiredTM, these researchers have concluded that both the absence of reduced cognitive ability and the slight increase in cognitive ability which they found in individuals who had runs of homozygosity (roh) in their genomes (evidence of matings between genetically similar individuals) were probably NOT due to assortative mating (i.e. smart people mating with smart people).

furthermore, they suggest that the inbreeding-causes-reduced-cognitive-ability meme is incorrect — or at least that the situation is more complicated than the idea that it’s the accumulation of recent deleterious mutations which haven’t been selected away that is the (whole) problem. in fact, a little inbreeding seems to have a positive effect on some cognitive abilities!

i’ve suggested a couple of times one way in which inbreeding might result in a low average iq in a population, and that is if the inbreeding leads to clannish, altruistic behaviors between extended family members which then result in the deleterious mutations NOT being weeded out.

one real world example i’ve offered is how life works in egyptian villages and how the more successful and affluent (and, presumably, more intelligent) members of a clan are obliged to help out their less successful and poorer (and, presumably, less intelligent) clan members. so, apart from mentally retarded individuals not reproducing, where is the negative selection for deleterious mutations here? there is none. or it’s a lot weaker than in more individualistic societies (like gregory clarks’ medieval england) where it’s more every man for himself — in clannish societies, deleterious mutations might be able to hang around for a long time, riding on the coattails of those with fewer deleterious mutations.

(note: comments do not require an email. i’m my own grandpa! [no, I’M not! it’s just the song.])


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