Would you be surprised to know that as you study your genealogy you will probably find first-cousin marriages in your family? You shouldn’t be, and if you are, you will get over it quickly if you spend much time analyzing your family tree. Marriages between cousins were very common in many cultures, including many western countries, such as the United States and Europe until the early to mid-1900s. And, today many cultures still practice cousin marriages.
A cousin marriage is technically a consanguineous marriage. Genealogy classifies families based on generation and following direct descendant or biological child relationships (consanguinity) and immediate in-law (affinal) relationships. When you share a direct descendant, you are related by blood to another person, and you can trace your lineage all the way back to a common ancestor through biological parent-child relationships. A consanguineous marriage is a union between two people who share an ancestor.
Prior to relationship laws being passed, cousin marriages were popular in the US and Europe, especially among the wealthy and upper class. Cousin marriages were encouraged to keep wealth and property within a family, to keep cultural values intact, to maintain geographic proximity, and for various other reasons. Queen Victoria and Prince Albert were first cousins, and Charles Darwin married his first Cousin Emma Wedgwood. Sometime in the 1880s 13 states and territories passed laws prohibiting first cousin marriage. By 1920 there were 26 states with laws prohibiting first cousin marriages. But, as previously mentioned, cousin marriages are still common in some cultures. In fact, according to a study cited by Wikipedia, worldwide 10% of marriages are between first or second cousins.
Today most in people in western countries consider first-cousin marriages taboo. The Roman Catholic Church and the Easter Orthodox Church have a long history of marital prohibitions limiting consanguineous marriages. At one point these churches prohibited a person from marrying up to and including a sixth cousin. The Marriage Act of 1986 changed the rules of who churches can prohibit a person from marrying, but most laws and customs still prohibit the marriage of a person to their parents, siblings, first cousins, aunts, uncles, nieces, and nephews.
As you study your genealogy, don’t be surprised if you find more first cousin marriages in your family. And don't be surprised if you find more than you knew about or expected. Remember, not all that long ago, in many western cultures and communities, including the United States, cousin marriages were considered ideal and actively encouraged.
 consanguinity: A noun derived from two Latin words "con" meaning common or shared and "sanguineus" meaning blood and refers to a relationship between two people who share a common ancestor or blood.
 affinal: An adjective meaning of a family relationship by marriage of a relative (through affinity), as opposed to consanguinity; an in-law.