gofish – I grew up always hearing that if you married cousins your children will come out retarded. When I grew up I met several couples that are cousins and have large families with not a single retarded (mentally challenged) kids. On the contrary, chazal praise marrying within the family. Do a Google search for “Klausenberger Rebbe marrying cousins”. The question was posed to him adding the dangers that the gentile medical field warns against.
Here is a quote of part of his response:
“On the contrary says the Klausenberger Rebbe. There is a mitzva for a person to marry within his family as the Rabbeinu Bachaye says in Parshas Chayei Sara by Yitzchok and Rivka, and the Ibn Ezra says in parshas Shemos by Amram and Yocheved.”
Not that it is necessary, but here is a quote from a goyish website on the subject:
“In modern western society, marrying your cousin is not well accepted, particularly in the United States. Through a combination of old prejudices and present-day conventional wisdom about inherited birth defects, first cousin marriage is seen by many as a little too close for comfort, as well as a bad idea if you want children.