外网版知乎：如果你和你的异性兄弟姐妹是地球上仅存的人类， 你会为了拯救人类而繁衍后代吗? 这在道德上是错误的吗?（上）

Ray Friesenhahn
The question of morality here is really not in regards to the violation of the incest taboo, which becomes somewhat irrelevant in the complete absence of any other humans. Consider the perspective of the male, whose sister is now his sole partner for life and survival. Would you put your sole partner at risk (maternal death rate during childbirth was historically around 1%, but would likely be several times that in such conditions, without any experienced helpers) for an outcome that, as so many others have pointed out, has such low potential for success because of the severe genetic bottleneck? Also, from the perspective of survival, your partner would be dependent upon you and unable to contribute much to your mutual survival for several weeks to several months around childbirth, greatly increasing the risk to the survival of all three of you - if the child survives much beyond birth.But consider that the two of you just survived some unprecedented catastrophe - be it man-made, such as nuclear war combined with biological warfare, or natural, such as an asteroid strike or gamma ray burst, followed by disease and climate disruption. You’ve come out of your deep underground bunker after several years surviving on your stockpiles (probably meant to keep a small community alive, but they didn’t make it), and you find you both have immunity to whatever diseases followed the other catastrophes. What you need to keep in mind is that the world is really a very vast place. Even if you wander for years, and use radio transmitters to try to contact others without success, you cannot possibly be certain that you actually ARE the last people on Earth. If you feel an obligation to repopulate the human species on Earth, then it really becomes your duty to continue seeking any other surviving enclaves. With the size and current population of Earth, and great diversity of geographic and environmental conditions, the fact that you two survived means there is a very high probability that some others did as well. Even if you wander for decades in search of them, beyond the time of reproductive viability for yourselves, you will at least have had a mission to give your existence a purpose. Then, if you do finally find others, you could at least help in the rearing of THEIR children, and help to recreate a society, rather than just a species.

这里的道德问题实际上不是关于违反乱伦禁忌的问题，
因为在完全没有其他人类的情况下，
乱伦禁忌就变得有些无关紧要了。
想想，从男性的视角看，
他的姐妹现在是他唯一的生活和生存伴侣了，
你会让你的伴侣陷入危险的境地之中吗？
（从历史上看，孕妇分娩时，死亡率为1%左右，在题主设置的这种情况下，没有任何有经验的助产婆，那么
孕妇的死亡率可能还提升好几倍的）.
正如许多人所指出的那样，这么高的分娩风险，是因为遗传瓶颈严重吧？
此外，从生存的角度来看，你的伴侣也依赖于你的.
在分娩前后的几周到几个月里，你的伴侣也无法为你们的共同生产做出什么贡献的.
如果你的孩子生下来后，还存活下来的话，那么，你们3个人的生存风险还变得更大.
不过，想一想，你和你的姐妹刚刚经历了前所未有的灾难——无论是人为的，比如核战争与生物战的结合;还
是自然的，比如小行星撞击或伽马射线爆发，然后是疾病和气候骤变，
你已经从地下深处的地堡里活着出来了，
几年来你一直依靠你的储备食物生存
(那些食物可能是维持一个小社区生存的食物，但社区里的人都死光了)，
你会发现你们对其他灾难之后的瘟疫都有免疫力。
你需要记住的是，这个地球真的是一个非常广阔的地方。
即使你外出乱逛几年，使用无线电发射机试图联系他人却没有成功，你也不可能确定你实际上就是这个地球
上最后一个人.
如果你觉得有义务让地球上的人类重新繁衍生息，
那么继续寻找任何其他现存的人类，就真的成了你的责任。
鉴于地球的大小和目前的人口数量，
以及地理和环境条件的多样性，你们两个幸存下来的事实意味着，其他一些人也幸存下来的可能性非常高的.
即使你花了几十年的时间去寻找它们，
在你自己的生育能力之外，你至少还有一个使命，
那就是给你的存在赋予一个目的。
然后，如果你最终找到了其他人，
你至少可以帮助他们抚养孩子，重建一个社会，
而不仅仅是一个物种。

Patrick Rannou
Well, how could we be sure we’d the last ones? Let’s assume it is the case.In the first few years, we’d be all focused on BASIC survival. Having enough food on the table, so to speak. Starting a family where instead of 2 persons working their ass off just to survive, suddenly only the male now has to provide for 3 persons, that seems quite dangerous until you reach a setup where you are safe and have plenty of stored food and resources to last you a couple bad crops / bad winters years.We’re not medical experts, and me and my sister would be well over 50 years old. by then. So, the risk of miscarriage or of death on childbirth would be wayyyy huge! Sorry but I guess that’s “it” for humankind, then.Even if a new humanity is born, it would eventually be saddled with the weight of several very bad genetic diseases. Not so good a legacy. It’s not as if me or my sister are the best human specimens possible in the first place.I suppose there would be some weak odds of repopulating, but they’d be very low. More like a crazy gamble, really. We’re just a bit too old to have children safely EVEN with all the modern current tech.As for the moral aspects. Well, morality is defined by social convention. With every else “gone”, the only social conventions would be those that we 2 would mutually decide to agree upon. From, today’s point of view, yes we’d both say it’s morally wrong. But as the 2 only survivors? I guess such a traumatic experience could mean that anything becomes possible.Especially after a few years. Don’t underestimate the human need for love and companionship. Feeling alone can be really THOUGH for some people. Oh, at first everything might look and be the same. But a few years down the line, when the only way to survive a harsh winter would be to share a single bed in order to share body heat as much as possible? Yeah, very possible that eventually the brother/sister dynamics get slowly dropped in favor of a “life partners” one. Who knows? Trying to predict how you will change to adapt to a stressful environment, is kind of hard to do.I’d think it would go more along with the survivors state of sex drive and “carefulness”. i.e. take 2 youngsters brother/sister let’s say 10 years old, after a few years they start having high hormones/libido, they each are the only emotional relief in a very dead world, and they are still learning social conventions, making their own path as they go, and liquor stores can hold “good” alcohol for years and years. Contraceptives wouldn’t even be carried, because after a few years, they would be way too out of date. So, 5 years after the weird cataclysm, when they are 15 years old and hormones REALLY kick-in, they get one random night of way too much drinking … and whoops, “things might happen”. Depends on the people.i.e. I’d say it would come more as an accident, after years of loneliness, than on purpose. And not to everybody. If the siblings eventually start having sex, and are of reproductive age, and aren’t super-careful, then yeah, they’d probably eventually have a tons of more-or-less little genetically defective kids, most of whom will die on childbirth or before reaching a few years old.But at my and my sister’s age, the libido might still be there, but lower than before. And we have much more strongly ingrained social virtues and moral compasses. So, yeah, we’d just let humanity “rest” forever.You can’t live without food or air or water, but living without sex is pretty easy.

如何确定我们是地球上最后的两个人呢？
让我们假设一下，
在最初的几年里，我们都专注于基本的生存而努力，
努力保证桌上有足够的食物.
然后组成一个家庭，以前为了生存，2个人都努力干活，
现在，建立家庭后，突然就变成一个男人干活，提供3个人的生存物资. 这似乎是很危险的.
除非你们两个先存储够用几年的食物.
而且，我不是医学专家，而我和我妹妹都已经50多岁了。
到那时。因流产或分娩时死亡的风险将是非常巨大的!
抱歉，但我想这就是人类的生活.
即使一个新的人类诞生了，
它最终也会带有一些非常严重的遗传疾病.
我和我妹妹并不是最好的人类标本.
我想，我和我妹妹会有一些微弱的重新繁殖的可能性，
但是，这种可能性非常的低.
我和妹妹要生出一个新人类，那就是一场豪赌，真的，
因为我们太老了，就是有现代这么先进的医疗技术，
我和妹妹也保证不了能安全地生出一个孩子来.
至于道德方面，道德是由社会习俗定义的.
随着其他一切都“消失”，
唯一的社会惯例将是我们两人共同决定同意的那些.
从今天的观点来看，是的，我们都认为兄妹结婚在道德上是错误的。
但作为唯一的两个幸存者呢?
我想这样的创伤经历意味着一切皆有可能的.
尤其是这个地球只有两人，相处几年后，
就算是亲兄妹也会合在一起的.不要低估了人类对爱和友谊的需求.
对有些人来说，孤独的感觉真的很可怕.
哦，一开始一切看起来都是一样的。
但几年后，要想熬过严冬，唯一的办法就是合用一张单人床，
两人尽可能多地分享各自身体的热量.
是的，很有可能兄弟姐妹之间的关系会慢慢变成“生活伴侣”.
谁知道呢？预测你将如何改变来适应一个有压力的环境，是有点困难的.
我觉得，只有两人的情况下，由性本能来驱动，应该更符合幸存者的互相“照顾”的行为.
比如两个10岁的小弟弟/妹妹，
几年后他们开始有高荷尔蒙/性欲，
他们每个人都是这个死气沉沉的世界里对方唯一的情感慰藉者，
他们还在学习社会习俗，走自己的路，
而酒类商店也可以经年积月地销售“好”酒.
避孕用品可能都不需要，因为几年后，它们就会过时的.
所以，在导致人类灭绝只剩兄妹两人的灾难发生5年之后，
当他们到15岁的时候，荷尔蒙就开始起作用了，
他们会在某个晚上喝很多酒……嗯，啊，哎呀，
“不可描述”就这样自然发生了.
也就是说，我想说，在经历了多年的孤独之后，
“不可描述”的事更像是一场意外，而不是故意的。
并不是对所有人。如果兄弟姐妹们最终开始发生性关系，
并且到了生育年龄，而且不小心，那么，是的，他们可能最终会有很多或多或少有基因缺陷的孩子，
那些小孩，多数会在分娩时就死掉，要不然也是活个几年就死掉.
不过，以我和妹妹的年龄，我们的性欲可能还在，但比以前低了。
我们有更加根深蒂固的社会美德和道德准则。
所以，是的，我们只是让人类永远“休息”。
没有食物，没有空气，没有水，你无法生存，
没有性生活死不了人的.

Debdoot Das
The incestuous offspring is not the main concern here. First of all, as you mentioned about the high chances of genetic deformities, let us suppose that the offspring produced as a result of in-breeding among the first generation(me and my opposite sex sibling) be without any genetic deformities(highly unlikely). The next(third) generation of offspring would have to be produced either by intercourse between organisms of the first generation and the second generation, or by intercourse between the second generation. These offspring would have extremely high chances of being deformed. The fourth generation would thus have ever higher chances.Thus, in short reproducing with a sibling would do little to save humanity. Every species needs to have a number of organisms greater than its Minimum viable population so that it can survive.
Anthropologist John Moore has declared this number to be about 150 - 180, for providing 60 - 80 generations of humans beings. A study by Rutgers University has stated that 70 human beings have populated Americas, ie, the MVP is 70. In other words, a population of 2(two siblings) can’t reproduce to save humanity. A Population bottleneck is reached quickly if two people start trying to repopulate the world.Also, the scenario mentioned above was an ideal one. In the real world, there are also other factors, like absence of advanced healthcare and doctors(since only two of the siblings are there, and it is unlikely that both of them are super-specialists in all fields of medicine as well as surgery), resource gatherers, etc.The only reason two siblings would engage in sexual intercourse would be for mutual pleasure, because there are not any other people left to socialize with. And they would have to take strict steps for protection, so that no offspring is born. This might be morally wrong, but then again, no one else is there to question morals!

不！我不要乱伦.
主要问题不是说乱伦就没有后代，而是，首先，正如您提到的，乱伦的遗传，很有可能是畸形的.
让我们先假设第一代近亲繁殖没有遗传畸形(这可能性很低的).
下一代(第三代)则必须通过第一代和第二代生物体之间的交配来繁殖的，或通过第二代之间的交配来繁殖.
这些后代就极有可能产生畸形婴儿了.
因此，第四代产生的畸形婴儿就变得比前面的更高了.
因此，简而言之，兄弟姐妹交配繁殖是不可能繁衍出人类的.
任何物种，想要繁殖下去，都必需要有一定数量的个体.
人类学家约翰·摩尔说，这一数量约为150至180个.
这样交配出来的后代能达到60—80代.
罗格斯大学的一项研究表明，所有美国人是由70个祖先繁衍下来的.
换句话说，2人(两个兄弟姐妹)的种群是无法繁殖人类的.
两个人来繁衍的话，人口瓶颈很快就会出现.
此外，上面提到的场景是一个理想的场景。
在现实世界中，还有其他因素，
比如缺乏先进的医疗保健和医生(因为只有兄弟姐妹中的两个人在那里，而且他们不太可能都是所有医学和外科领域的超级专家)、资源收集者等等
兄弟姐妹之间发生性关系的唯一原因是为了共同的快乐，
因为没有其他人可以和他们交往。
他们必须采取严格的保护措施，这样就不会有后代出生。
这可能在道德上是错误的，但话说回来，没有人会质疑道德的!

Randell Nonya
the likelihood of any child being born with a birth defect between you and your sister/brother would be at maximum 7% so if you and your sibling had 100 children only 7 of them would have any birth defect. and while the danger really increases each successive generation of inbreeding it still would never reach 100%. eventually there would be a margin of separation between genetic similarities

兄妹结婚，生出的孩子，最多只有7%的可能会有先天缺陷.
也就是说，你们结婚的生100个孩子，只有7个残疾.
虽然每一代近亲繁殖生出的孩子，残疾的可能性在增大
但是，可能性并不会达到100%.
基因相似性之间会有一定的界限的.
原创翻译：龙腾网 https://www.ltaaa.cn 转载请注明出处

Jim Birch
You can be pretty certain that incest has been practised quite often in past and it is probably the reason you are here. Humans have lived in small groups for most our history. Certainly, consanguineous marriage - ie, between cousins - has been completely normal in the past, and still is in a many places. If you grow up in a tribe of a hundred people you are usually all pretty closely related. In larger and more modern societies it provided a way of keeping property in the family.From a gene pool perspective, attempting to recreate a healthy population from two individuals is problematic. This is because the species has a limited set of genes to draw on, to cope with new environmental conditions, diseases, and so on. However, I doubt it is as impossible as some people are suggesting, especially if the initial brother and sister are in good health and have no serious defects. Obviously this means that human genetics take a different and specific path but there is evidence that this sort of genetic bottleneck has happened before. Certainly, the idea that the kids must be weird looking idiots is a myth. They will look like their parents and be about as smart as them. (While "hybrid vigour" is a genuine effect, hybridization also produce a lot of defective individuals.)From an ethical or religious point of view: whatever. Being the only two people on earth is going to produce some serious problems but being forced into a moral tradition to avoid the approbation of others is not one of them. You are it. You have a cultural history but its hold will weaken over time in a very different world. There appears to be a evolutionary trick that acts against close inbreeding. Animals - including humans - generally don't feel attracted to siblings, or more exactly, the kids they were raised with, even if not genetically related.
This effect is observed in various animals, for example, adolescent female chimpanzees will jump troupes, to join unfamiliar males. (This would be too dangerous for a young male chimp, they would literally tear the new guy apart.) It is also observed in the failure of human child marriages to produce offspring. This is probably the origin of the incest taboo: that it produces asexual, childless unxs, not that it produces defective children. If this applies, you simply may not feel like it, even if you are keen to continue the species for philosophical or other reasons. On the other hand, I would not be surprised to find that, after a while, the effect of limited alternate opportunities and radically changed social circumstances does something to whatever moral indoctrination, ideas or feelings you thought you had, and, as they say, the rest is history.

可以肯定的是，在古代，乱伦是经常发生的，
这可能也是你来这里的提问的原因.
在人类历史上的大部分时间里，人类都是群居的.
这样，近亲婚姻——也就是堂兄弟姐妹之间的婚姻——在过去是完全正常的，就是现在，在许多地方也是存
在这样的现象的.
如果你在一个有一百人的部落里长大，你们通常都是近亲。
在更大、更现代的社会中，近亲结婚也是一种保护家族财产的方法.
从基因库的角度来看，试图从两个个体中重建一个健康的群体是有问题的。
这是因为这个物种只有有限的一组基因可以利用，
只有一组基因来来应对新的环境条件、疾病等等。
然而，我怀疑这并不像有些人说的那样不可能，
特别是，如果最初的兄弟姐妹身体健康，没有严重的缺陷的话，
我觉得一对兄妹也可以繁衍人类.
显然，这意味着人类遗传学走了另一条不同的道.
有证据表明，这种遗传瓶颈以前也发生过.
觉得近亲交配生出的孩子一定是长相怪异笨傻，这种想法一定正确.
乱伦的生出的孩子，他们的长相会像他们的父母，和父母一样聪明.
(虽然“杂交活力”一种基因组合的效果，但杂交也会产生大量的缺陷个体的。)
从伦理或宗教的角度来看:乱伦没什么大不了的.
作为地球上唯一存在的两个人，面临的问题是很多的，
但是，道德问题并不是其中之一.
如果是你的话，那么，一开始，对于乱伦，你是有思想抵触的.
但是，随着时间的推移，整个地球只有两个人的话，
那么，你的抵触思想会慢慢减弱的.
抵触乱伦的思想，似乎是一种保护进化的思想，
这种思想可以防止近亲繁殖.
动物——包括人类——通常不会被兄弟姐妹所吸引，
或者更确切地说，不会被和他们一起长大的孩子所吸引，
即使他们没有血缘关系。
这种效应在各种动物身上也能观察得到的，
例如青春期的雌性动物黑猩猩不会要自己群体里的雄性猩猩，而是要群体外的陌生的雄性.
(这对年轻的雄性黑猩猩来说是很危险了，雌性猩猩所在的群体坑会把新来的家伙撕成碎片).
在早起远古时代的人类婚姻中，也能观察到这种现象
这可能是不可以乱伦的（生物）依据:乱伦会生出无性的后代，而不是生出有缺陷的孩子.
如果地球上只剩下你们兄妹二人，即使你出于哲学原因，想要延续人类这个物种，你也可能不喜欢乱伦.
不过，另一方面，我觉得，你们兄妹二人在那种彻底不同的环境中相处一段时间后，你们的道德教化、思想或情感会有改变的，正如他们所说的“世事都是历史，无关道德”.