text: If a person can trace their family history back far enough they will usually come across one or more cousin marriages in their tree.  First, until almost 1870-1880 is was socially acceptable.  Second the further you go back in history the less likely someone was going to marry outside of their community because people just couldn't meet people from other communities very easily and third it was sometimes even encouraged especially when a family was in a certain social group.\n\nThe reason cousin marriages are discouraged is because if there is defective gene the chances of inbreeding that one gene is higher when you marry within a certain degree of family;  if the genes are clean then there is nothing biologically wrong but in today's society most marriages below that of third cousins are considered unacceptable.