Another article about cousin marriage in the UK. The issue here is simple; you have a National Health Service which covers everyone, and doctors are noticing that Pakistanis are overrepresented in many cases of recessive diseases. The culprit is probably cousin marriage. Here are two points which are both valid:
'In our local school for deaf children, half the pupils are of Asian origin though Asians only form about 20 per cent of the population,' said Ann Cryer, MP for Keighley. 'I also know of several sets of parents in my constituency who are cousins and whose children are severely disabled. I have no doubt that the mothers and fathers being closely related to each is a key factor.
...
This last claim is hotly disputed by genetic counsellors and Muslim doctors. They point out that the danger of a child having birth defects if the parents are cousins is double that of other children, which means the risk rises from about 2 per cent in the general population to about 4 per cent when the parents are closely related. A risk of 4 per cent therefore does not make it 'likely' there will a genetic problem, as Woolas claimed, say genetic counsellors.
What's going on here? First, it is correct that though first cousin marriages increase the likelihood of deleterious diseases in the offspring multiplicatively, the chances are still relatively low. On the other hand, in terms of absolute numbers the number of those with a given recessive disease may increase multiplicatively as well. In other words, though on the individual scale the expected risk is low, when it comes to population level outcomes ubiquitous cousin marriage is going to mean a society where a range of recessive diseases are far more common. You can predict the proportion of individuals with a recessive disease who are the products of first cousin marriage by the following formula:
K = c(1 + 15q)/(c + 16q - cq)
The proportion of first cousin matings being c within the population, and q being the frequency of the recessive allele in question. Here's some numbers for the United States....
Condition - % of affected children whose parents were first cousins
Total color blindness - 15
Albinism - 21
Xeroderma Pigmentosum - 23
Ichthyosis Congenita - 35
Tay Sachs - 40
There is one major caveat here: these assertions apply to first cousin marriages where the individuals are related through one line of descent. That is, in the United States when individuals marry their cousins, they are usually marrying someone with whom they share one set of grandparents. And that's it. In many societies you have large inbred clans where people are related along many lines of descent, and this tends to amplify the characteristics of inbreeding. I think this is the real long-term problem when you have culturally sanctioned cousin marriage; it will occur generation after generation so that pedigree collapse may be inevitable.
In any case, this portion of the article made me wonder:
'The danger posed by cousin marriage is highly exaggerated,' said Aamra Darr, a senior research fellow at Bradford University. As Darr pointed out, women in Britain are more likely now to have children when they are over 30, increasing the likelihood of them having babies with Down's Syndrome. But no one suggests that there should be a ban on over-30s having babies, Darr added. Medicine has adapted to improve screening services for these women.
'We should recognise that for British Pakistanis, cousin marriages represent significant cultural advantages. Recent advances mean we can pinpoint many of those at risk of having affected babies. That is where we should be placing our efforts.'
What exactly should individuals do when they find out that their future child is going to carry a recessive condition? Is this doctor suggesting that selective abortions should be the appropriate response to adapt to the ubiquity of first cousin marriages in the Pakistani community? Or pre-implantation screening?
Related: Other posts on cousin marriage.
I don't understand...is either measure likely to be shunned by the Pakistani community? If so, wouldn't people do it anyway and simply not talk about it?
I don't understand...is either measure likely to be shunned by the Pakistani community? If so, wouldn't people do it anyway and simply not talk about it?
religious muslims tend to be opposed to abortion (though there isn't a consensus i think). and it is usually religious muslims who are most likely to believe that cousin marriage is obligate.
In cultures where cousin marriage is common, is it also common for kids to have little contact with those cousins during childhood? I realize that such marriages are probably arranged anyway, but most of us (like most Americans) who are personally squicked out at the idea of cousin marriage grew up thinking of our cousins in much the same way as we would our own siblings. At least that was true during my own childhood, when cousins often lived in proximity, and shared a lot of childhood experiences if they were close in age.
I also recall reading about cultural distinctions between "cross cousins" and "parallel cousins", depending on whether they shared a set of grandparents via opposite-sex or same-sex siblings, and that cultures making this distinction often raise one type as virtual siblings and the other as virtual strangers, and that cousin marriages were generally arranged between the type not raised together (parallel, I think?)
Anyway, if we really have a general human tendency to avoid having sex or falling in love with someone with whom we were raised, it seems like a society that encouraged cousin marriages would have to have some taboos against rearing potential mates together -- otherwise, even in a society that valued male dominance and female obedience, there'd be a lot of marital strife. Is this fairly accurate, or is the situation more complicated than that?
A couple of points and a question:
Cousin marriage is actually practiced in a very large number of cultures, and it's widespread non-practice is a recent phenomenon.
There are probably kinship and alliance related advantages for cousin marriage (there is a huge literature on this).
Having said that, even where cousin marriage is practiced, this is something that generally benefits the ascending generation (dad, and granddad in particular) but not the kids getting married, so one finds it actually happening in large percentage mainly in societies with a lot of inter-generational control, and not so much in other cases.
I don't know if any of that is relevant, but i thought it might be nice to have some additional context.
My question: I know what the answer to this in various non-human animals (the answer being yes) but I'm not sure for humans. What do you think: If everyone faithfully marries their cousin, thus 'exposing' deleterious but also recessive mutant genes, would this not over a few generations reduce the frequency of those genes?
I know of a captive population of siamangs that has bred among siblings for five generations with no effect. That is just one of MANY examples. It would probably be true of humans, but what I don't know is how faithful the system has to be, and how small the population, for this to actually have an effect (of reducing the frequency of these alleles through selection)
What do you think: If everyone faithfully marries their cousin, thus 'exposing' deleterious but also recessive mutant genes, would this not over a few generations reduce the frequency of those genes?
there is some evidence of genetic load decreasing in populations which practice a lot of incest.* e.g., among south indian groups that practice uncle-niece marriage. also in japan there is some data. eventually in a situation like the one for pakistani britons you'd probably have an equilibrium, as load is being pumped out, but the very close genetic relationship means that the de novo mutations across the generations keep injecting new recessives into the population. also, i suspect there are non-lethal problems with lack of complementation; e.g., the fact that first cousin offspring are about 5 IQ points dumber than you would expect from their parental values for example.**
Having said that, even where cousin marriage is practiced, this is something that generally benefits the ascending generation (dad, and granddad in particular) but not the kids getting married, so one finds it actually happening in large percentage mainly in societies with a lot of inter-generational control, and not so much in other cases.
in the british case there's also the benefit toward immigration; you bring over relatives from the old country. it also solidifies ethnic identity as native born individuals marry an immigrant, so reinforce cultural authenticity across generations by continiously injecting first generation immigrants into the community....
julie, your point about familiarity is correct. but, i think it is one which applies ceteris paribus. in other words, there are just other factors which mitigate against the social conditioning of exposure. for example, an arab friend of mine explained that cousin marriage was so common because most of his relatives simply didn't ever encounter people of the opposite sex who weren't cousins. as for marital strife, sure that's an issue, but these are not often nuclear family societies where husband-wife dynamics are the overwhelming factor in family life. e.g., the mother-son connection, and daughter-in-law vs. mother hostility, is a major issue, and that might encourage cousin marriage since if a woman is related to her husband's family she is not as isolated.
* see Consanguinity, Inbreeding, and Genetic Drift in Italy and Population Genetics and Microevolutionary Theory.
** see hartl & clark.
greg, btw, re: sibling incest, etc. in ancient egypt this was apparently common among regular people (the romans banned it around 290 or so). and some data from modern times (think fritzl case) shows that not all these offspring have really noticeable deleterious recessives...but, we humans really put a value on the marginal quality of life and physiological fitness. i think there are issues even with these sibling matings in terms of stuff that isn't expunged by exposing deleterious recessives (there might be issues with lack of heterozygosity, for example, loss of heterozygosity on the molecular level, etc.).
The first question I'd ask is how many of those kids were actually born in the UK. If I had an infant who was deaf, and a choice between emigrating to England or staying in a country with minimal national health care and minimal public education for the disabled (or none), I'd be a whole lot more likely to emmigrate.
Also, last time I checked, Moslems treated cousin marriage very differently depending on the gender of the parents. Marriage between the children of two brothers was considered incest, and between the children of two sisters was considered a good match. How are these cousin marriage rates being figured?
The first question I'd ask is how many of those kids were actually born in the UK. If I had an infant who was deaf, and a choice between emigrating to England or staying in a country with minimal national health care and minimal public education for the disabled (or none), I'd be a whole lot more likely to emmigrate.
good point. but i don't think this is a major issue; i assume that if you are a british national who is living in pakistan you'd want to fly back to take advantage of national health service benefits already.
Also, last time I checked, Moslems treated cousin marriage very differently depending on the gender of the parents. Marriage between the children of two brothers was considered incest, and between the children of two sisters was considered a good match. How are these cousin marriage rates being figured?
there are many different opinions on this sort of thing. e.g., some muslims explicitly talk about emulating the prophet and his family and their marriage patterns. but, if i had to guess i would bet that you have it inverted in preference; my arab friends talk about cousin marriage as a way to cement relations between brothers, not between sisters. the cross-cousin stuff is more common among south indians, some of whose cultures only recently emerged from matrilineality.
I don't know that Islam as religion (or Muslims) so much advocates cousin marriage as it does simply allow it. It seems that a better correlation maybe tribal versus non-tribal cultures (as evidenced by higher rates in Pakistan, Saudi) although I'm sure there maybe better explanations/multiple factors.
Also, perhaps my understanding of this is rudimentary, but don't Jews also have high levels of consanguinity, and while they surely do have higher levels of recessive diseases, don't they also on average have higher IQ's? I guess my question is how does this jive with the five point drop vs anticipated in cousin marriage, and also, could there be some selective advantage in passing on some of these recessive genes?
I don't know that Islam as religion (or Muslims) so much advocates cousin marriage as it does simply allow it. It seems that a better correlation maybe tribal versus non-tribal cultures (as evidenced by higher rates in Pakistan, Saudi) although I'm sure there maybe better explanations/multiple factors.
this is basically correct i think. there is some evidence though that cousin marriage has increased in much of the islamic world recently in areas with a lot of wealth because of particular inheritance rules and what not.
I guess my question is how does this jive with the five point drop vs anticipated in cousin marriage, and also, could there be some selective advantage in passing on some of these recessive genes?
right, i said 5 points lower than would otherwise be predicted. you see what i'm saying? e.g., assume IQ is 50% heritable (half the variation in the pop is due to variation on genes) and you a population mean of 100, and two parents with IQs of 130. the expectation for any given offspring would be an IQ of 115 per the breeder's equation (r = h^2s). but if you have first cousins, then subtract 5 points. get it?
by analogy. say you have two siblings who are both 2 standard deviations above the norm in height. they're opposite sex, and they do the deed and have offspring. height is about 80% heritable, so you expect the offspring to be taller than average. but, since they're inbred they might have other issues which will make them shorter than you would expect from the mating of two people who are 2 standard devations above the norm. got it?
btw, first cousin assuming no other inbreeding is around 4 points depression according to this
http://books.google.com/books?id=bNfuX26mSsAC&pg=PA123&dq=IQ+cousin+mar…
it is important to note that double first cousins in israel among arabs showed greater effect of depressiong....
How does all that mesh with enduring high Jewish IQ despite relatively high consanguinity?
How does all that mesh with enduring high Jewish IQ despite relatively high consanguinity?
an inbred population can still be taller than average. if you don't understand that, you just won't ever get it and can trust me or not (i.e., you'll need to understand genetic architecture and how it relates to phenotypic expression to a level of sophistication you probably lack).
also, cousin marriage and uncle-niece marriage isn't common in western jewish communities today anyhow from what i know outside of small groups such as hasids. one generation of outbreeding usually masks deleterious effects.
How does all that mesh with enduring high Jewish IQ despite relatively high consanguinity?
Regression to the mean is only by a few points (as noted above).
Or, it could be that first cousin marriage is not that common, though more distant relative marriage is.
Or, it could be that some aspect of Jewish law with regard to mating behavior could be protective (different recommended sexual frequency by occupation, for example, or a prohibition on marriage for certain kinds of mental illness).
Or, it could actually be a different population, so regression the mean is to a higher mean.
Or, it could be that some kind of associated cultural behavior is enhancing IQ in general. I can't imagine what.
Regression to the mean is only by a few points (as noted above).
if you're looking at population averages you would weight by the proportion who are offspring of consangineous relatonships. i doubt among most american jews cousin marriage is more common today than among most americans, period, so it would probably have minimal effect (orthodox are about 10%, but of these only 1 out of 10 or so are hasidic communities where cousin marriage might be very prevalent because of smaller potential marriage pool*).
* many hasidic groups will only marry with their own sect, so the pool isn't the whole hasidic group.
quick consulation of google books says that first cousin marriage rate in the early 20th century among jews was about 1%. i doubt it would be much higher today, and probably lower.... (seeing as how 30-50% of american jews outmarry depending on how you define 'jew').
The upside of sibling marriage/mating is that not all siblings are full siblings. (That probably only helps a little).
Regarding rates of cousin marriage: Do remember that these are nominal categories. An entire village of Yanommamo shifted one day from marrying actual cousins to marrying much more distant relatives because an important guy needed to get a mate for a relative, and reclassified someone in the ancestry of one of the lineages. This invalidated several existing marriages (people ignored that) and allowed a lot of "cousin" marriages among people who were more randomly related to each other.
These aren't like European cousin marriages. They are the product of repeated cousin marriages over a few generations. That has got to up the odds of having a recessive genetic illness up if you share not just one set of grandparents, but also four great great grandparents or more.
Although, that "50% of our deaf children are Asian" quote isn't far off double the normal rate fro a 20% share of the population, which is what you'd expect from one off cousin marriages.
I've seen an IQ depression of 7 points from a Pakistani study somewhere...
Cousin marriage was considered completely acceptable in Jane Austen's time (well, at least in her books). Are there any data from that period as to what the effect was, if any?
Has anyone ever considered outbreeding depression between human populations? If this phenomenon exists, it would explain tendencies of racism, which is not just a white man's privilege, at least partially, as a taboo similar to the one towards incest.
Has anyone ever considered outbreeding depression between human populations? If this phenomenon exists, it would explain tendencies of racism, which is not just a white man's privilege, at least partially, as a taboo similar to the one towards incest.
1) yes. the data are mixed in terms of physiology, but there doesn't seem to be a decrease in fertility.
2) no, it wouldn't explain it. black people and white people, for example, never bred before the last 300 years. how exactly are you adapting to a situation which never occurred? most interbreeding occurred between adjacent groups, who would be of the same race and very genetically close.
a much more plausible explanation is that racism is just an extension of ingroup-outgroup psychology which developed in tribal societies. it's the same dynamic which leads to regional or religious conflicts as well.
razib said "one generation of outbreeding usually masks deleterious effects"
Could inbreeding followed by crossing the inbred lines go beyond masking the deleterious effects and produce a superior strain of human as with plants and "hybrid vigour"?
Re. Pakistan ethinic origin marrage consanguinity in England causing deafness. Deafness is the least of it.
Brain Size Is Linked To A Gene by Nicholas Wade gives the facts but underplays the devastating effects of microcephaly. Genes associated with microcephaly are not thought to be responsible by the educated in Pakistan, instead a strange conspiracy theory involving a shrine taking babies and restricting their had growth is subscribed to even by those with a knowledge of genetics.
Warning googling this subject may turn up some unsavoury sites. Photos of the afflicted may also be upsetting to some.