April 16, 2013
The Incest Taboo And Genetic Load

Some day we'll have biotechnology that will enable parents to prevent their children from getting harmful recessive genetic variants. The genetic harm from making babies with close relatives will vanish once the harmful recessives aren't passed along. At that point would you still favor laws against incestuous relationships? One reason to remain opposed: the resulting high level of genetic similarity across generations will likely increase loyalty to family at the expense of loyalty to the rest of society.

On a somewhat related note: We all have lots of mildly harmful genetic mutations, only some of which are recessives. The non-recessives (i.e. the ones that harm us when we have just one copy of them) are reducing your level of functioning and my level of functioning every day. The term to refer to them is "genetic load". One way to identify all these harmful mutations is to compare the DNA of people (or Neanderthals as that post discusses) and eliminate the less frequent genetic variants. Most of the less frequent genetic variants are either harmful or neutral in effect. Eliminating them would make superior humans.

What is interesting about this idea of doing the comparison to basically vote on what to set as the genetic letter for each location in the genome: how do you choose the group to sample the DNA to do the voting?

Bringing the subject back to incest and harmful recessives: Would the voting approach do as good a job of identifying rare harmful recessives as it would in identifying harmful non-recessives? Keep in mind that populations that are highly inbred have genetic diseases first discovered in these populations. So the Mennonites and Amish accidentally have made it very easy to discover many harmful recessives. But how many of those recessives show up outside of those communities? How many other harmful recessives are so rare that they haven't been identified clinically?

Share |      Randall Parker, 2013 April 16 12:04 AM 

Bob said at April 16, 2013 6:23 AM:

"One reason to remain opposed: the resulting high level of genetic similarity across generations will likely increase loyalty to family at the expense of loyalty to the rest of society."

Why do you assume that "non-inbreeding" will increase "loyalty to the rest of society"?

Bob said at April 16, 2013 6:30 AM:

You might be overstating the dangers of incest:


"In Paris in 1876 a 31-year-old banker named Albert took an 18-year-old named Bettina as his wife. Both were Rothschilds, and they were cousins. According to conventional notions about inbreeding, their marriage ought to have been a prescription for infertility and enfeeblement.

In fact, Albert and Bettina went on to produce seven children, and six of them lived to be adults. Moreover, for generations the Rothschildfamily had been inbreeding almost as intensively as European royalty, without apparent ill effect. Despite his own limited gene pool, Albert, for instance, was an outdoorsman and the seventh person ever to climb the Matterhorn. The American du Ponts practiced the same strategy of cousin marriage for a century. Charles Darwin, the grandchild of first cousins, married a first cousin. So did Albert Einstein."

"So when a team of scientists led by Robin L. Bennett, a genetic counselor at the University of Washington and the president of the National Society of Genetic Counselors, announced that cousin marriages are not significantly riskier than any other marriage, it made the front page of The New York Times. The study, published in the Journal of Genetic Counseling last year, determined that children of first cousins face about a 2 to 3 percent higher risk of birth defects than the population at large. To put it another way, first-cousin marriages entail roughly the same increased risk of abnormality that a woman undertakes when she gives birth at 41 rather than at 30. Banning cousin marriages makes about as much sense, critics argue, as trying to ban childbearing by older women.

But the nature of cousin marriage is far more surprising than recent publicity has suggested. A closer look reveals that moderate inbreeding has always been the rule, not the exception, for humans. Inbreeding is also commonplace in the natural world, and contrary to our expectations, some biologists argue that this can be a very good thing. It depends in part on the degree of inbreeding."

"The traditional view of human inbreeding was that we did it, in essence, because we could not get the car on Saturday night. Until the past century, families tended to remain in the same area for generations, and men typically went courting no more than about five miles from home—the distance they could walk out and back on their day off from work. As a result, according to Robin Fox, a professor of anthropology at Rutgers University, it's likely that 80 percent of all marriages in history have been between second cousins or closer."

"Moderate inbreeding may also produce biological benefits. Contrary to lore, cousin marriages may do even better than ordinary marriages by the standard Darwinian measure of success, which is reproduction. A 1960 study of first-cousin marriages in 19th-century England done by C. D. Darlington, a geneticist at Oxford University, found that inbred couples produced twice as many great-grandchildren as did their outbred counterparts."

"Researchers have observed that animals in the wild may also attain genetic benefits from inbreeding. Ten mouse colonies may set up housekeeping in a field but remain separate. The dominant male in each colony typically inbreeds with his kin. His genes rapidly spread through the colony—the founder effect again—and each colony thus becomes a little different from the others, with double recessives proliferating for both good and ill effects. When the weather changes or some deadly virus blows through, one colony may end up better adapted to the new circumstances than the other nine, which die out."

"In some cases, outbreeding can be the real hazard. A study conducted by E. L. Brannon, an ecologist at the University of Idaho, looked at two separate populations of sockeye salmon, one breeding where a river entered a lake, the other where it exited. Salmon fry at the inlet evolved to swim downstream to the lake. The ones at the outlet evolved to swim upstream. When researchers crossed the populations, they ended up with salmon young too confused to know which way to go. In the wild, such a hybrid population might lose half or more of its fry and soon vanish."

KevinM said at April 16, 2013 7:39 AM:

"Most of the less frequent genetic variants are either harmful or neutral in effect.
Eliminating them would make superior humans."

But an inferior human population. Perhaps even an extinct one.

The gene for sickle cell anemia is harmful in Minneapolis, but a worthwhile trade-off in Kinshasha.

All the neutrals are no-cost potential speedbumps or barriers to novel pathogens, if nothing else.

The harmfuls - some of the most common must have a countervailing beneficial effect.

That we can't figure out what they are just means we are not as smart as we think we are.

Ben said at April 17, 2013 4:05 AM:

People aren't opposed to incest because of birth defects. They're opposed to incest because it's fucking gross.

The fact that we only think it's fucking gross because we've evolved to (in order to prevent birth defects) is irrelevant. You can can make it as safe as you want and it won't make any difference to how we feel. For as long as the mere thought of incest instinctively repulses us it will be taboo in polite society. It's the same reason people won't accept recycled sewage as a water source. The fact that it's healthy will never be more persuasive than the fact that it's disgusting.

Brett Bellmore said at April 17, 2013 4:36 AM:

I think we have to distinguish between sibling or cousin incest, and parent-child incest. The rational reasons for opposing the former would vanish if you got rid of nasty recessives, though sibling incest would still be infrequent due to instinct.

But the rational reason for opposing the latter is that parents have too much control over their children, we don't want to give them any incentive at all to mold them into sex partners instead of mature adults. And that won't go away just because of genetic engineering.

JayMan said at April 17, 2013 5:05 AM:


The fact that we only think it's fucking gross because we've evolved to (in order to prevent birth defects) is irrelevant. You can can make it as safe as you want and it won't make any difference to how we feel.

If it was no longer a problem in terms of genetic defects, eventually the instinctive repulsion against it will start to become less prevalent. Indeed, it might be actively selective out thanks to the pull of genetic similarity. That would take generations to make a meaningful difference, however.

destructure said at April 17, 2013 6:47 AM:

You've repeatedly said to just eliminate the rare variants. I have no objection to genetic engineering. But I strongly oppose making changes to genes for which we don't completely understand the consequences. That's more than jumping the gun. It's reckless.

Also, life has been around for billions of years. Strange that the "genetic load" hasn't made life infeasible eh? Yeah, well, there's a reason for that. It's called "incest". Far from being the boogeyman everyone thinks, it's the reason populations have remained healthy. People have historically lived in small family groups. So who do you think they screwed for the last few million years? What does that do? Well, offspring from cousins are much more likely to be homozygous for deleterious alleles. Being homozygous exposes these deleterious alleles in the phenotype. When this happens the offspring is much more likely to have the disease and die. And that removes the allele from the population.

Consider 2 copies of the same allele. The parents are cousins and are therefore more likely to carry both alleles. G=good copy & B=bad copy. So both parents are GB. Now, suppose they have 4 children. The four possibilities are GG, GB, BG and BB. Since the parents are just as likely to pass on one copy as another each combination is just as likely as any another. So, statistically, those would be representative.

As you can see, there is one offspring that doesn't carry any negative allele, 2 offspring that carry 1 negative allele and 1 offspring that carries 2 negative allele. The last one is likely to die and be removed from the gene pool. So this coupling provided a net improvement over the previous generation of one offspring with NO negative allele.

faruq said at April 17, 2013 9:12 AM:

maybe i'm not getting enough action.hence my over pre-occupation with science and the future...

Bluegra said at April 17, 2013 1:14 PM:

This is an interesting topic. My hunch is that incest would still be discouraged as I think that incest also gives the appearance of impropriety in terms of power imbalance. I think people would be worried about women in families being essentially coerced to have children for their families. If they had any doubts or hesitance they would be subject to familial pressure and scorn.

Personally, I think some sibling combos would make good nuclear families, but I would prefer if the woman would just get artificially inseminated. It's a good way to prevent inbred health issues, and besides, any resulting children would be the nieces and nephews of the man in the situation. I think that provides a pretty good incentive to take care of them.

Ronald Brak said at April 20, 2013 1:48 AM:

Genetic load is the chance of dying before reproducing as a result of not having the optimum gene alleles. Which means that in our current environment a person with minimal genetic load might be a psychopath who looks like Justin Bieber. (On the bright side, while he'll spread the genes for psychopathy, it should result in many people in the next generation being very pretty.)

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