Kissing cousins is an English idiom that generally refers to two or more things that are somehow alike, but in a vague or distant way. The idiom probably derives from the practice of cousin marriage, in which two distant relatives marry and start a family. In some cultures, popular belief has long held that the practice of marrying a relation could result in physically or mentally handicapped children, due to the genetic effects of inbreeding. In cultures where cousin marriage is accepted, it is generally nevertheless believed that marriages between first cousins should be avoided, since the partners could be too closely related to one another to produce healthy offspring. Marriages between second, third, and even more distantly related cousins, however, are often approved of, so one might refer to one's distant cousins as one's "kissing cousins" because, in many cultures, these distant relations may still be considered acceptable marriage partners.
The practice of marriage between cousins is believed to have a long history worldwide. Historically, and even today, powerful families have used cousin marriage to maintain the family's status and assets. Marriage between two members of the same family tends to ensure that the family's wealth stays within the family. In aristocratic families, cousin marriage has been practiced as a way to prevent perceived contamination of the family bloodline.
While the practice of cousin marriage is largely frowned upon, and even illegal in some parts of the United States, most other Western countries are known to allow the practice of marriage between cousins of any degree, even first cousins. It is believed that social taboos against the marriage of first cousins originated in the mid-19th century, when fears of birth defects due to inbreeding first began to appear in the public consciousness. With the advent of modern genetics, however, researchers are discovering that the risk of birth defects in children born to first-cousin parents is usually low, comparable to the risk of birth defects children born to unrelated parents older than 35 to 40 years of age.
Some researchers have gone so far as to suggest that marriages between kissing cousins may have genetic benefits for the species as a whole. An Icelandic study found that partners who are kissing cousins are often likely to have more children. These children tend to be healthier, and have more children themselves, than do the offspring of unrelated couples. Researchers believe that this occurs because a certain level of consanguinity helps to ensure the healthy development of the fetus. Couples who are kissing cousins may also marry earlier in life, and begin having children earlier, than couples who aren't related.
Some believe that, historically, most marriages have been between partners who were cousins. While the marriage of first cousins may remain frowned upon in some parts of the West, marriages between more distantly related kissing cousins, as they are called, are usually considered more acceptable.