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Even More Reasons Not to Marry Your Cousin

“How do smart people breed stupid people?”

It’s a question from one population geneticist to another.

The second geneticist can tell from the goofy grin on his colleague’s face that the answer is going to be a joke.  He thinks about it and finally shrugs.

“By repeatedly screwing their sisters!” the first one squeals.

(Population geneticists aren’t selected for their senses of humor.)

Today, kids, we’re going to be talking about incest.  Now I’ll grant you up-front that incest is pretty much universally reviled.  But here’s the thing: Incest is defined culturally.  (More on this below.)

The less morally-culpable term is “inbreeding,” defined as follows:

“…to breed by the continued mating of closely related individuals.”

Animal breeders have used inbreeding as a technique from time immemorial to achieve many useful results.  Everything from creating cows that give more milk, to dogs that snuggle with kids instead of eating them.

Despite its long pedigree, inbreeding brings with it well-known dangers: the root cause of which is the accumulation of genetic goofs.  Such goofs would normally be masked by a working spare in the second set of chromosomes received from the animal’s mom or dad.  But this assumes a respectable genetic distance between mom and dad.

In animal breeding, it’s often “worth it” (from our perspective) to risk a genetic misfire by breeding close relatives — because the worst-case scenario is just an underperforming cow.

Do I care so much if the milk on my breakfast cereal comes from a less-than-stellar bovine?

Nope, not really.

We humans get a lot more persnickety when it comes to the reproductive strategies of our own species.  And for good reason.  For one thing, the “worst-case” scenario I mentioned above isn’t really the biggest biological misfire possible.

The true disaster scenario from inbreeding is an irretrievably rotten stretch of genetic material, with no spare, and a fatal flaw thus literally written into the resulting embryo or fetus.  The likely result is a “spontaneous abortion” (miscarriage).  This doesn’t cause much emotional strain if you’re a rancher and it’s happening to a cow; it does if you’re human and it’s happening in your immediate family.

Note:  Severe genetic maladies also occur for dozens of reasons that have nothing at all to do with inbreeding — but inbreeding significantly boosts the likelihood.

Human cultures worldwide — along with most animals — share an automatic revulsion to close inbreeding.  And interesting studies have shown that our degree of revulsion tracks closely with our genetic “distance” from the relative under consideration.

(i.e. Sociologist asks student: “Would you rather screw your first-cousin or your half-sister?” “Um, can I choose ‘None of the above’?” “Nope.  You’ve gotta pick one.” “Your study sucks.”)

You share half your genetic material with your mom, your dad, and any full siblings.

With half-siblings and grandparents, this overlap drops to 25%.

Corollary:  If you’re ever playing “Would You Rather…?” and some sick bastard asks if you’d rather sleep with your mom or your grandma — from a genetic perspective, the “correct answer” is your grandma.

For first cousins, the percentage of full genetic overlap drops to “only” 12.5%.  That’s just one gene out of eight.

And that’s where things get messy.

So what was incest, again?

As described above, inbreeding is simply a strategy for managed reproduction, with no judgment implied.

Incest, by contrast, is inbreeding plus moral editorializing.

Incest is inbreeding when we think it’s gross.

Nowadays, in most of the world, our definitional umbrella for incest extends pretty far.  The majority of you reading this will not have given serious romantic consideration to your first cousins.  I contend that this is a good thing.

This broad definition of incest is far from a universal norm, though.  Until modern times, cousin-marriages — known to sociologists as “consanguineous” marriages — were widely practiced in much of the world.

 In many places, they still are.

The reasons for consanguinity’s popularity in the olden days were pragmatic.  Until recently, most people eked out a rural existence with few potential mates living within a day’s walk.  Of those available nearby, many were often blood relatives.

The other reasons had to do with family loyalty.  Marriages inside a kin-group keep wealth and property consolidated.  And while divorce was less common in the past, the early death of a spouse was much more common.  Consanguineous marriages reduce the number of competing interests when settling estate claims.

So — this still happens?

It doesn’t just happen.  In some parts of the world, consanguinity is more popular than mini-skirts.

(Okay, that’s a bad joke — because consanguinity is most prevalent in the Arab world, which never really embraced mini-skirts.)

According to a report from the 2009 Reproductive Health Journal, Pakistan holds the dubious honor of being home to the world’s most consanguineous marriages — with a whopping 70% of marriages between blood-relative brides and grooms.  In Saudi Arabia, the number is 66.7%.  In Iraq, it’s 60%.

All in all, it is estimated that 1.1 billion people are either married to cousins, or the children of consanguineous unions.

Maybe it’s not so bad?

Albert Einstein married his first cousin.

So did Charles Darwin.

Consanguinity is not a practice limited to the Arab world, or the Amish, or people who shipwreck on desert islands while vacationing with their cousins.

In 2003, Discover Magazine published an article offering up a defense of close-but-not-too-close levels of inbreeding.  The authors pointed out that while the odds of serious genetic disorders do rise, they may not rise enough to warrant the fact that, say, 31 out of 50 US states have outlawed first-cousin marriages.

One point the article emphasized was that inbreeding’s negative effects are inversely proportional to the genetic health of the original breeding population.

In other words, if your family has a healthy genetic makeup with comparatively few defects, you may be able to (choke down your disgust and) safely inbreed for a few generations without any bad results.  If your family is not so genetically well-endowed, you won’t have to wait multiple generations to see problems.

The point made by Discover was that although the chances of congenital defects increase, the increase is still to a comparatively small number.

“Tripling the risk” sounds bad.

“Becoming 2% more likely” sounds a lot more palatable.

But if the base rate of a certain problem is 1%, then “tripling the risk” and “becoming 2% more likely” are the same thing — both get you to 3%.  Savvy statisticians and science writers can spin facts like this to suit their own agendas.

(Was it the hidden agenda of the Discover writer to seduce his own cousin? I can only say that the evidence does not discount such a possibility.)

I admit it.  I’m biased.

But I’ll admit my bias up-front: I’m pro-smarts.

I think our global society will succeed or fail based on the careful marshaling and increase of our collective cognitive resources.

I agree with Einstein (the cousin-f**ker mentioned earlier), who famously said:

“We cannot solve our problems with the same level of thinking that created them.”

If one accepts this idea, and adds the follow-on premise that smart people will inevitably create new problems, the implication is an ongoing intellectual arms race, with humanity continually bailing itself out of some dodgy jam it just recently got itself into.

Note:  This is exactly like every single TV sitcom’s plot, only with global catastrophe hanging in the balance.

So let’s grant Discover its point that serious congenital defects — while more likely in consanguineous marriages — are still not thaaat likely.

But now, let’s leave aside birth defects and official diseases with scary-sounding names.  Because…

Inbreeding makes people dumb.

Yes, exceptions exist.  Dumb is relative.  And everything I’m about to say is based on averages and likelihoods and “Normal Distribution Curves.”

A Normal Distribution, you may recall from science class, is the famous chart of a “bell-shaped curve” that can be used to predict everything from household power bills to the distance of the spitball you just threw at your teacher from the previous spitball you threw at your teacher.

In 1965, a Japanese study of cousin-marriages showed an average IQ deficit of 7 points in the resulting children.  (See more here.)

A 1993 Indian study showed an even further drop: 11.2 points of IQ.  (Among India’s 140 million Muslims, it’s estimated that 22% of marriages are consanguineous—meaning tens of millions of people.)

Intelligence, like other multi-variable traits, exhibits a “normal distribution” in any reasonably large population.  If consanguineous marriages reduce children’s IQ by somewhere between 7 and 11.2 points (we’ll round to 10 to keep the math easy), we can visually imagine taking the IQ-curve — with its peak normally at 100 — and sliding it to the left by 10 points, so its hump sits centered at 90.

To be fair, a 90 IQ isn’t very dumb.  Someone with 90 IQ is smarter than one out of every four people he meets.  This is not someone who wears a drool-bucket and is baffled by door handles.

But the problem isn’t what consanguinity does to the middle of the IQ curve; it’s what it does at the edges.

Amputating our allotment of geniuses.

Genius, if you go by the numbers, is defined as IQ 160 and above.

Normally, you’ll get six folks this smart in every 100,000 people.  That’s the straight Vegas odds if you’re betting on genius.

If you slice 10 points off the average IQ to accommodate consanguineous marriages, then to find how many geniuses you’re left with, you have to look at the number of people who would normally have had a 170+ IQ.  These are the only ones who will still be left at genius-level after the 10-point decrement.

The bell-curve has tapered down to super-skinny at this point.  As likely as not, you’ll have nobody with a 170+ IQ in a 100,000-person population.  Just 0.38 people per 100,000, to be exact.

So if you’re really hell-bent on doing it, consanguinity will cost your society almost 95% of the geniuses that random-ass luck would have given you for free.

Numbers Geeks:  You can see my calculations here.

Meanwhile, by sliding the bell-shaped curve left, you’ve pushed a much fatter slice into the dangerously low IQ territory.  An IQ of 70 used to be the cut-off for “borderline mental retardation” (back when that term was in vogue).  The term is no longer used — and the numbers-only designation wasn’t a good one — but this remains a level of measured intelligence at which teachers and social workers start making additional assessments to see, “Can this person really take care of himself?”

At straight Vegas odds, 70-and-below IQ “should” be just about 2.5% of the population.

But applying the 10 point penalty drops the entire 80-IQ-and-below population into the 70-and-below range.  Doing so quadruples the size of this group.  (A full 10% of the population on the standard IQ curve sits at 80 and below.)

Sometimes “tradition” is just plain dumb.

Can anything justify defoliating our limited supply of geniuses and simultaneously quadrupling the number of cognitive hard-luck cases?


But whatever it is would have to be pretty damn compelling.

Consanguinity doesn’t cut it.  Trading away all those IQ points for easier probate law and a convenient reduction in the number of in-laws… That’s just a bad bargain.

These days, over 60% of the world’s population lives in cities — cities complete with dance halls, Internet dating sites, and busybody spinsters with nothing better to do than help you hook up.  There’s no longer any geographical imperative for us to boink our relatives.

Of course, I realize the main motive force behind consanguineous marriages is not rational decision-making; it’s cultural inertia.  (Yes, that self-same bugbear who mandates European lawyers wear powdered wigs, and afixes Confederate Flag bumper stickers to the occasional pick-up.)

But culture, tradition… Are these really sufficient excuses for people to make their next generation dumb?

I am not trying to denigrate the Arab world, or the Amish.*

* Actually, nix that.  I’ve started with the letter “A” and will be denigrating all sociopolitical groups in alphabetical order.  Baathists and Belgians — you’re next!

I’m saying that sometimes studies like those cited above give us a clear signal that such-and-such cultural norm is demonstrably wrong.  When these signals come, we should count them as lucky breaks — even when they require us to break with tradition.

By ditching consanguineous marriage, cultures get what biology owes them anyway: a fresh shuffle to the genetic deck.  And the implementation is as simple as encouraging people to date outside their immediate gene pool.

That shouldn’t be too tough a sell, right?

A Conciliatory PS: 

I don’t expect I’ve got too many readers in consanguineous relationships, or who are the children of such relationships.  But I could be wrong.  With the overall number being about one person in eight worldwide, I could be very wrong.

If what I’ve written here offends you, or has left you wistful for additional IQ points that you might have had under different parentage, let me offer the following:

According to family lore, when I was a baby, my dad dropped me square on my head from a not-insignificant height.  Who knows how many IQ points this might have cost me, but if they ever do a large-scale study on head-dropping babies (they won’t), the results won’t be good.

But still, if they did — and if my family had a long tradition of head-dropping babies (we don’t) — I would still be eager to be among the first generation to formally shit-can that tradition.


  1. john says:

    Where did this come from? Do you have a photo of your female first cousins?

    Inbreeding, when referring to humans, also has moral editorial content. Like calling the Pocono 500 “The Inbreeders Cup”. How would science separate the effect on IQ of baby bottles of diet Pepsi and genetic non-diversity? They never seem to occur independently.

  2. john says:

    While you’re at it, how about a look at the genetic make-up and breeding lineage of the world’s great geniuses? Maybe more interesting than a lazy look at something everyone already agrees on? A possible, though maybe unlikely corollary to this story. If you want a dog that’s red coated, friendly, or very large what do you do? Breed closely or at least selectively for those traits. Kind or scary, but same with people. Since intelligence is probably a combination of genetic traits it is probably not like height or hair color, but either way, it would be a very interesting story if carried beyond the specificity of just the average IQ of a race of 10 billion people or something like that.

    1. Jesse Lawler says:

      John — I don’t think this is something that everyone already agrees on (unfortunately). My guess is that with something like intelligence that is so multi-variable, we could learn a lot from studying, say… physical beauty. (Similar levels of inheritability.) Beautiful parents are likely to have beautiful kids, but the more beautiful the parents, the less likely that the kids will meet or exceed the previous generation’s level of beauty. “Regression to the Mean,” I believe this is called.

  3. Terra says:

    Hello I’m new to this site, and I will go by Terra when I post my replies.
    Okay, just going to point out, it’s not a friendly, or mindful thing to leave a link to Even More Reasons Not to Marry Your Cousin in my inbox, right after I introduce myself with a short explanation for interest in your website. Is it just me here???
    Also, charmed, and pleased to meet you too. -_-
    I noticed this article read a bit differently, almost like a well informed, and heavily researched rant. I would know, I’ve written plenty myself. Care to share the source of the fire?

    1. Jesse Lawler says:

      Hi Terra – I’ve been researching a lot of brain do’s/don’ts lately, and this one just sort of flabbergasted me. There’s a lot of ways to screw up human brains (no surprise), but most of the really nasty ones are either rare (e.g. concussive injuries from kickboxing) or involuntary (e.g. mom can’t afford high-quality food, thus gives baby low-quality milk). But this seems like a totally “voluntary” way to undercut a baby’s intelligence, practiced tens of millions of times annually. (Admittedly, the cultures that practice consanguinity are probably not giving women the ability to marry whomever they want, so “voluntary” is meant more at the cultural level than the individual level.) So yes, a rant.

  4. Jesse, what if the cousin-marriages agree to adopt? What if they agree only to have children via IVF complete thorough genetic screenings (in a world with greater knowledge of such things)? What if CRISPR becomes accurate, cheap, and ubiquitous ensuring the best possible genetics for all parents. If we can avoid a less than ideal genetic situation do we even care about this at all? If you could ensure a healthy, top-tier smarts child, no matter the parents, would we condemn cousin marriages? Would we even have a basis for condemning sibling marriages? I’m more or less stealing this thought experiment from Jonathan Haidt, should anyone be interested.

    1. Jesse Lawler says:

      Zach — Yeah, technically it’s cousin-procreation, not cousin-marriages that create the problem. I think a weaker argument could be made that the social mixing that comes from marrying outside your family is beneficial — but this would be a far less slam-dunk case, based on sociology and stuff not easily quantified. Anyway Zach, next time you screw your cousin, just wear a condom. 😀

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