No , it's not a good idea for human first cousins to interbreed: http://www.parapundit.com/archives/003130.html http://www.isteve.com/cousin_marriage_conundrum.htm (...) Of course, there are also disadvantages to inbreeding. The best known is medical. Being inbred increases the chance of inheriting genetic syndromes caused by malign recessive genes. Bittles found that, after controlling for socio-economic factors, the babies of first cousins had about a 30% higher chance of dying before their first birthdays.(...) http://isteve.blogspot.com/2008/02/darwinian-sweet-spot-3rd-cousin.html Dogs and humans are both mammals, the same laws of genetics apply: http://terriermandotcom.blogspot.com/2006/05/inbred-thinking.html http://terriermandotcom.blogspot.com/2008/12/flawed-paws.html http://terriermandotcom.blogspot.com/2008/08/inbreeding-of-kennel-club-dogs-data.html http://terriermandotcom.blogspot.com/2008/08/closed-registries-breed-diseased.html http://terriermandotcom.blogspot.com/2008/12/following-is-from-great-scots-magazine.html http://terriermandotcom.blogspot.com/2008/12/pedigree-dogs-exposed-in-just-one-click.html