Today, it would be extremely odd for someone to marry their cousin – first, second or otherwise. However, this was not always the case. In fact, it used to be perfectly normal to marry your first or second cousin.
The founding population of homo sapiens was only about 700 to 10,000 people. A certain amount of inbreeding in such a population was inevitable. As time went on, cousin marriage was used to protect cultural values over generations, ensure spousal compatibility, and preserve family wealth. Other reasons included strengthening family ties and avoiding health problems in a lesser-known spouse.
Cousin marriage was legal in the United States before the Civil War. After the war though, views changed and people began to see it as more advantageous to marry someone you were not related to. After the 19th century, cousin marriage in the US, Great Britain and other western countries began to decline. It is still prominent in other regions, such as the Middle East. Overall, it’s estimated that 80% of marriages in human history have been between second cousins or closer.