First cousins have a shared ancestor. Second cousins have a great-grandparent in common. Third cousins have a great-great-grandparent in common (the grandparent of a grandparent). Fourth cousins have a great-great-great-grandparent in common (the grandparent of a great-grandparent). And so on. Because they share a double-great-grandparent, first cousins are twice as likely to be born within one year of each other as they are to be born five years apart. Also, because they share a double-great-grandparent, first cousins are more closely related than second cousins who don't share any other family members. There are 7,291,566 first cousins living in the United States today. That's about 1 in 8 people! Here are some other facts about first cousins: Almost half of all first cousins are married to other cousins. The others are paired up with someone else in the drawing of names from the same family tree. It is very unusual for two unrelated people to have four first cousins. When this happens, the chances are high that there is some type of mistake in reporting of parents or grandparents. Sometimes these mistakes are found after doing a DNA test and learning that you are not really related at all. We all know at least one first cousin. They are those relatives who share one of our grand-parents.