If birth control is used, is there any moral reason for siblings from abstaining from sex?  When I asked this on a religious forum a few muslims were disgusted with me.

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Chuckles!

Are you saying it is socially abhorrent for a person with a known genetic defect to have children?  Same result, different process.

 

It's hard to keep your prejudices from tripping over one another.

One thing it is is imbued with ego... "ooh I MUST procreate, I am sooo precious"

As an only child, I always find it hard to judge sibling dynamics. While I do find sibling intercourse to be repugnant for reasons I can't really define, I still just don't really understand the issue at hard. I'm not saying that if I had a hypothetical brother or sister that I would sleep with him or her; I guess I just feel disinterested by the scenario in general because that sort of familial relationship is utterly foreign to me.

But about the increased genetic abnormality resulting from incest: there was a thread on here about incest a few months ago where I think I recall several people listing reasons why this is a popular misconception with little actual science to back it up. Basically, I think the point was that there is nothing intrinsically wrong with the pairing of sibling gametes; any issues arise purely from the increased likelihood that siblings will share similar negative gene mutations. I think the point was that this chance for negative gene mutations was far less likely than commonly perceived; therefore, the idea of incest resulting in mutant children is highly overblown. I'm late for school right now but I'll try to find it later.

Adriana, resident scientist of the thread, is this accurate or am I jumbling up my memories into clouds of misinformation?
Don't forget certain specific regions of North America too - I know of a couple 8^)
CBS's search feature sucks, but I recall the Amish story on 60 Minutes. While the argument may be called anecdotal, the consistent results speak for themselves amongst isolated populations. And in the case of the Amish, we are really only talking about roughly 20+ generations for the problem to be noticed within a non-scientific community. So how many generations back did the problem really start? Additionally, these aren't even simply siblings but more distant relatives.
I found the thread that I was thinking of where this was discussed before:

Incestuous Progeny (24 April 2010)

So if the real danger posed by siblings mating is the increased chance of common genetic disorders, shouldn't the matings of groups with little genetic diversity (like the Amish and others who were mentioned) also be considered dangerous to a certain degree and likewise repulsive? I'm being rhetorical, but I just wonder why we only find incestuous unions to be disgusting and not the interbreeding of a tiny, isolated population. It just seems like a weird contradiction that nearly all cultures abhor sibling breeding while simultaneously retaining a sense of xenophobia towards breeding with outsiders. (I'm speaking in massively vague generalities.)
The risk of concentrating bad recessive genes is the same as that of contracting usefull recessive genes. In conclusion YES the risk of genetic abnormalities are increased, BUT these genetic abnormalities stand at least equal chances of being usefull vs damaging. Damaging recessive genes are progressively weeded out from the genetic pool when they negatively affect reproductive success. However usefull genetic abnormalities would not be weeded out of the genetic pool because they would increase reproductive succes. Which means YES the risk of negative genetic abnormalities is highly overblown due to religious taboos.
Rationally, the main opposition to siblings (or other close relations) having sex is the increased likelihood of genetic abnormalities in their potential offspring. Even this varies from culture to culture, from little cultural taboo even for siblings to even second cousins or more distant kin being verboten in others.

There does seem to be an innate repulsion to the idea, however. Undoubtedly part of this is due to good evolutionary reasons, perhaps partially due to the undeniable connection between sex and reproduction. Even when birth control or sterility is a factor, it's undeniable that the underlying purpose of sexual activity is for reproduction. The fact that it can also be fun and recreational is a pleasant side effect.
Familiarity, as much as it can be a vehicle for sexual attraction, can be a sexual repellent in a larger dose. Step-siblings or step relatives would be an example of this abhorrence with no genetic basis.

Also, I have seen reports of studies (can't say much of their veracity) that say that innate senses play a role in deciding sexual attraction. I know that there were some women (well in my past) that I found attractive in appearence, but found unattractive due to their scent (not that they smelled bad) and the relationship did not last. Was this a way for me to avoid a poor genitic match? I don't know if that is the case, but it seems to make sense on the surface.

I enjoy my wife's scent and we made a beautiful baby! I'm just going to call it "case closed". ;-)

Familiarity, as much as it can be a vehicle for sexual attraction, can be a sexual repellent in a larger dose.

 

Familiarity could be the best clue available, even in a pure family. In any case, the whole purpose of sex in living things is to enhance genetic variety, so aversion to inbreeding or mating with the most familiar faces (at least in a typical tribal setting) seems reasonable.

purpose of sex in living things is to enhance genetic variety

 

That is simply untrue, there is not a single class in biology or anthropology that teaches such nonsense. The outcome of reproduction is a BALANCE between outbreeding and inbreeding, too much of either can be the death of a population.

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