mande wrote:I think the truth is, there's no "natural" rule here about why people do, or do not, have the desire to have sex with their siblings.
This is the truth. While it may be possible that we have a slight tendency to be less attracted to those that we've been raised around, there's certainly no biological block in place to prevent us from being attracted to our relatives. Relatives who meet but don't know that they're related aren't suddenly going to be sexually repulsed by each other.
The majority of peoples' aversion to incest is just social conditioning. People are taught that it is wrong early on and thus believe it to be wrong and become repulsed by it.
The reality is, it takes a while to screw up the gene pool.
It's a myth that for instance, if brothers and sisters mate, their kid will be deformed or retarded. It usually takes a couple generations of this to cause severe problems like that.
I admit that I'm going to be testing my memory here on my genetics courses, but what you said is only partially true. Incest does not, by itself, produce deformities. Whenever a person is born, they usually have a couple of small mutations in their genes. These are typically benign because they're recessive and get overridden by dominant genes.
When a parent has a child, that potentially bad recessive gene gets passed on to both of them. So, if those two kids have a child together, you're now pairing up that recessive gene with itself - bringing it out in the form of a birth defect, deformity, or what have you. In other words, inbreeding just provides a higher chance for recessive traits (many of which are undesirable defects) to match up than if you copulate with a stranger (who is far less likely to share that same genetic mutation).
The incest doesn't CAUSE the defects, it just makes it more likely that they'll dominant if they already exist in your genes. Incest where there are few detrimental recessive genes will produce as healthy a baby as any other pairing. But, that's a gamble given the muddy genetic pools that we all have.
