Consider this list:
Charles Darwin married his first cousin.
Albert Einstein's parents were first cousins. Then Albert married his own first cousin. Elsa Lowenthal, Einstein's second wife, was his first cousin on his mother's side. In fact, they were also "double cousins." Lowenthal also happened to be Einstein's second cousin on his father's side.
Franklin Delano Roosevelt and Eleanor Roosevelt were fifth cousins, once removed (a chart showing their relationship is available at http://www.gwu.edu/~erpapers/teachinger/q-and-a/q6.cfm).
John Adams married his third cousin, Abigail Smith.
John F. Fitzgerald, former mayor of Boston and grandfather of John F. Kennedy, married his second cousin, Mary Josephine Hannon.
Rudy Giuliani, former mayor of New York, married his second cousin once removed, Regina Peruggi
Samuel F. B. Morse, inventor of the Morse Code, took his first cousin once removed, Sarah Elizabeth Griswold, as his second wife.
Johann Sebastian Bach married his second cousin, Maria Barbara Bach.
Jerry Lee Lewis married his first cousin, who was 13 years old at the time.
Edgar Allan Poe married his cousin Virginia Clemm in Baltimore in 1835. She was 13 years old at the time.
At the age of 21, Queen Victoria married her first cousin, Prince Albert of Saxe-Coburg-Gotha. For the next 20 years, they lived in close harmony and had a family of nine children, many of whom eventually married into the European monarchy.
Princess Mary of Teck (later to become Queen Mary) married her second cousin, once removed, King George V.
The above is an abbreviated list. A much longer list of notable couples who were cousins may be found at http://en.wikipedia.org/wiki/List_of_coupled_cousins
Thousands of people marry their first or second cousins every year. One article I read claims that twenty percent of all married couples in the United States are cousins. That reference comes from an article at http://abcnews.go.com/2020/Stossel/story?id=2395516&page=1 that offers no source citations. I am not sure I believe it.
Once you go further apart than second cousins, the issue becomes more difficult. After all, can you name all your third cousins and fourth cousins and those even further away in the family tree? Very few people can do that. There is always a strong possibility that you unknowingly married a distant cousin. The only way to find out is to research both of your family trees. To be sure, all of us are related to each and every other person somewhere back in history. Therefore, you probably married a distant cousin. The difficulty lies in proving it.
In America, marrying your first cousin is legal in 25 states. Some lists claim that 26 states allow cousin marriage. The confusion apparently stems from the laws of North Carolina, where double cousin marriage is specifically prohibited, but marriages amongst "normal" cousins (those who are not double cousins) are allowed. (N.C. Gen. Stat. § 51-3). All first cousin marriages are permitted in the other 24 states.
Six states ban marriage between first cousins once removed, i.e., marrying the son or daughter of your first cousin. No states ban marriages between second cousins.
Some states have more "interesting" laws, such as Arizona: first cousins may marry only if both are sixty-five years of age or older.
A list of laws in each state may be found at http://www.cousincouples.com/?page=states. Click on the appropriate state in the map to view each state's laws.
No European country prohibits marriage between first cousins. It is also legal throughout Canada and Mexico to marry your first cousin. The U.S. is the only western country with cousin marriage restrictions.
Many people believe that the Bible is a higher authority than the laws of men. In Leviticus, Chapter 18 (KJV), God tells us that we are not to have sexual relations with a long list of relatives, but the word "cousin" is absent from the list. Leviticus does deny marriage to "any close relative," and scholars have long debated whether or not that includes cousins. However, the Bible contains many instances of cousin marriage. For instance, Zelophehad's daughters, Mahlah, Tirzah, Hoglah, Milcah and Noah, married their cousins on their father's side (Numbers 36:1-11). According to that book, the important result was that “their inheritance remained in the tribe of their father’s clan.”
The Catholic Dictionary finds that Christ's parents-- Joseph & Mary-- were first cousins. Not all Protestants agree. More cousin marriages listed in the Bible may be found at http://www.cousincouples.com/?page=religion
The most common argument against cousin marriages is the increased risk of inherited diseases and birth defects. Indeed, there is some increased risk although the numbers appear to be small.
Non-related couples have a 2-3% risk of having children with birth defects. Couples who are first cousins double that risk to a 4-6% risk. Second cousins have little, if any, increased chance of having children with birth defects, according to the book "Clinical Genetics Handbook." (Source citation: http://www.cousincouples.com/?page=facts)
Many genetic diseases are caused by recessive genes. To get the disease, you must obtain the bad gene from both parents. The greater the genetic similarity between your parents, the greater your chance of getting two copies of the bad gene.
Charles Darwin, who knew something about genetics, married his first cousin, and they raised exceptional children. All of Darwin's children were healthy, had no known birth defects, and most of them apparently were either geniuses or near geniuses.
An interesting side issue is called pedigree collapse. When two individuals share an ancestor, the number of distinct ancestors of their offspring will be smaller than it could otherwise be. For example, a single individual today has more than 30 generations going back to the High Middle Ages with roughly a billion ancestors, more than the total world population at the time.
NOTE: This is impossible, of course. Nobody has more ancestors than those on the face of the earth. Duplicate ancestors (the same person appearing in multiple places in the family tree) will always appear in every person's ancestry chart if we research and find every ancestor. However, for the moment, let's focus only on the math involved.
A single individual occupies multiple places in the family tree when the parents of an ancestor are cousins (sometimes unbeknownst to themselves). For example, a person with no cousin relationships amongst his or her parents or grandparents has eight great-grandparents. Anyone whose parents were cousins only has six great-grandparents instead of the normal eight. One set of great-grandparents will show up twice in the ancestors chart. Going back to earlier and earlier generations results in a smaller and smaller number of ancestors when compared to the ancestry charts of the offspring of "unrelated" couples. This reduction in the number of ancestors is called pedigree collapse. It collapses the binary tree (two parents, four grandparents, eight great-grandparents, and so on in a binary progression).
Aside from the genetics, the legal issues, and the genealogies involved, social problems also arise. For instance, having a marriage is one thing, but going through a divorce is something else. You can move on from an ex-spouse or ex-lover, but there's no such thing as an ex-cousin. How are your parents and your ex's parents supposed to handle a nasty divorce or a breakup? How can they support their kids without antagonizing their siblings and their siblings' kids? You've wrecked your whole family.
Avoiding one's undesirable mother-in-law is already difficult in a normal marriage. What do you do if she is already “in the family” before marriage?
Each of us has millions of distant cousins. My belief is that most Americans are tenth cousins (or closer) to every other American. Some geneticists believe that everybody on Earth is at least 50th cousin to everybody else.
Chances are that you and your spouse were already related before your marriage.