You can marry your cousin in most states outside of the Eastern portion of the US.
When Brits first came here, the wealthy would marry their cousins to keep the money in their own family. Securing a future for their offspring. This of course created a class system that made everyone jealous, so laws were passed for the "good of the nation" that required a further distance between those betrothed.
It's one of the earliest cases of wealth redistribution in the United States. The increased percentage of having a child with a birth defect is similar to that of an 18-24 year old woman having a child, which is when a great portion of the population begins to reproduce anyway. The exception is if there is already a genetic mutation present in your family, and it's more likely to show up if the married couple are related on the father's side of the family.