First generation has the same chance of birth defects as the mother being over 30.
Past that, it depends on what double recessives are hiding in your genetic code.
There's also some promollgation that even cousin marriage lowers IQ, but it seems to be a "Just so" explanation, in that they don't want to say that the groups the extantly practice cousin marriage are perhaps less intelligent to begin with.
There's the Westermark effect. Though, even it's study basically said it only exists in the females. My personal thought is that this is because they can already extract any resource they want out of a childhood friend/sibling without needing to offer anything else, so they don't. Also, familiarity tends to breed contempt.