As part of the Council of Trent in the sixteenth century, the Catholic Church established rules that controlled who could marry within the church and who could not. Aside from being members of the Church in good standing, candidates for marriage could not be consanguineous within the fourth degree.
What exactly is consanguinity? If you are consanguineous with someone, you are blood-related. If you share consanguinity with someone in the first degree, you have the same parents-you are siblings. If you share consanguinity with someone in the second degree, you have the same grandparents-you are first cousins. To marry in the Catholic Church, couples could not be consanguineous within the fourth degree: they couldn't share the same great-great-grandparents. In other words, if they were third cousins or shared any closer blood relation than that, they could not marry in the Catholic Church, unless they got a special certificate of permission from the bishop known as a marriage dispensation.
Aside from the rules set forth by the Catholic Church, just how common was cousin marriage in Europe? Well, it depended on where you lived and what your social class was. Upper-class nobility were much more likely than the lower classes to marry their relatives, because they had to marry someone of their same rank and social standing, and sometimes a cousin or second cousin was the only one who fit the description. Those who lived in small isolated communities were more likely to marry relatives as well, simply because there weren't many marriage partners to choose from. In fact, marrying one's cousin because there was no one else of one's same social standing or no one else in one's small town to marry was such a common situation that there was even a special term coined for it in Latin and used in obispal marriage dispensations.
One of the first scientific studies that were done on cousin marriage was conducted by George Darwin, son of Charles Darwin. George Darwin wanted to find out how many marriages of the current population of Britain were first cousin marriages (Charles Darwin himself married his first cousin, Emma Wedgewood). George Darwin's study showed that about 3.5 percent of marriages in Britain were first-cousin marriages. Not surprisingly, he found that there were far more first-cousin marriages among the upper class than the lower class.
Contrary to popular opinion, cousin marriages do not cause genetic defects-unless a series of cousin marriages are perpetuated over many generations. When this occurs, there is not as much genetic diversity in one's gene pool, and so anomalies become more pronounced and undesirable recessive traits can come to the front. One of the most infamous examples of this is the Hapsburg dynasty in Europe. For more than five hundred years, the Hapsburgs were among the most powerful European families, controlling the Holy Roman Empire and, at times, Spain and its New World holdings. To maintain their status among Europe's powerful and elite, the Hapsburgs married other royalty-often their own cousins or distant cousins. As a result of centuries of cousin marriages, they became known for their protruding "Hapsburg jaw" and many were infertile or had children who died very young as a result of genetic disorders. One of the most extreme examples of Hapsburg inbreeding was Prince Carlos of Spain (1545-1568) who had only four great-grandparents instead of the usual eight. Not surprisingly, he was unable to produce an heir to the throne.
If you want to calculate the degree of consanguinity you share with someone or to see how your ancestors are related to each other, you can use use the OneGreatFamily Relationship Calculator located on your Family Dashboard.