It's not socially accepted and hasn't been for hundreds of years. However it used to be quite alright to marry your cousin.
Cosmos is right though, if you had a small population of monkeys, who were all brothers and sisters, and left them in captivation, tehy would die out eventually.
You get half your genes from your parents, so that's 25% per parent, if you keep narrowing it down you do not get a purer person, you get too many of the same chromosomes, which is the cause of downs syndrome.
However most people with birth defect are not the product of incest.
If anyone wants immediate advice don\'t hestitate to contact to me on AIM (AOL instant messenger) - aim: vodkasmodka
Eccentric! Not egocentric!