Quote from Sayoko;742800:
Because incest has a high chance of causing recessive debilitating traits to show up. Hemophilia, progeria, etc.
http://en.wikipedia.org/wiki/Progeria
http://www.youtube.com/watch?v=hAAx6l9nCv4
I'm a Geneticist. You don't need to tell me about the effects.
In my professional opinion, it's not nearly as bad as it's painted to be. It would take multiple generations of Incest to have any real danger. Check out these articles:
http://www.thetech.org/genetics/ask.php?id=243
http://www.thetech.org/genetics/ask.php?id=105
With that said...
I don't think having children and having sexual and/or romantic relations are mutually exclusive.
Marriages among first cousins are prohibited in 24 out of 50 states, and only allowed in special circumstances in another 7. There's only a 2-3% higher chance of anything bad happening from any other couple. These are purely socially based laws, not on any kind of scientific foundation.
As for the more close pairings like bother/sister, father/daughter, mother/son, etc... they are indeed considerably higher. Though, I don't think marriage nor their relationships should be regulated nor prohibited by law.
Their having children however, I do think should be prohibited by law. And Federally regulated. If they wish to have children then either adoption or designer babies then become an option. Or, they can be tested first to see if there is a low risk, and if so then it could be allowed. But for high risk pairings that have considerably similar genes, they shouldn't be allowed to have children together, outside of adoption or a
designer baby.
This goes into a much deeper discussion for me however, about how I think all child birth should be regulated and parents should have to pass parenting tests to be allowed to have children.
Eventually we're going to have to decide whether we want a low birth rate or a high death rate. We can't have both.
That is, when we do come out with a way to prolong human life considerably (say 1000 years or more of healthy life), then it will be a very large problem that we will very seriously have to deal with.
Though, I digress. This probably deserves its own thread at this point.