Inbreeding refers to situations in which individuals who are closely related to one another (such as siblings or cousins) procreate and produce offspring. Inbreeding can occur among humans as well as animals. Inbreeding is uncommon in most developed countries, but contributes a statistically relevant portion of the total number of births in certain developing countries.
The practice of marriages between close relatives (typically cousins) is known as consanguineous marriage. Although rare in the Americas and modern Europe, consanguineous marriage is notably common in North Africa and the Middle East, where it is a traditional and respected aspect of many Arab and Muslim cultures. In many countries, consanguineous marriages and inbreeding are considered illegal. Sexual relations between consanguineous couples is typically referred to as incest in these regions.
Although precise per-country data is rare, it is widely accepted that countries in the Middle East, North Africa, and parts of Western Asia have the highest rates of inbreeding in the world. Countries with traditionally high rates of consanguineous marriage and inbreeding include Pakistan, Saudi Arabia, Lebanon, Egypt, and Israel.
A certain degree of inbreeding can be difficult to avoid in isolated or cloistered societies, such as the populations of small islands or ethnic/cultural groups such as immigrants or Amish, where the range of possible marriage partners is limited. These conditions can artificially limit potential mates for members of the population, driving up rates of inbreeding.
For example, members of the Amish community may have higher rates of inbreeding because they tend to marry other Amish people, which shrinks the size of the population. Members of the Mennonite population may also have higher rates of inbreeding for the same reason. Finally, there are some genetic disorders that are more common among Hasidic and Haredi Jewish groups because they tend to marry members of the same population as well.
Inbreeding among humans has been proven to increase the risk of certain types of genetic diseases and disorders among certain pockets of the population. Due to the rates of inbreeding in these countries, genetic disorders such as hydrocephalus, neural tube defects, and congenital heart defects are more common. In Palestine and Lebanon, there are also very high rates of children born with cleft lips and palates.
However, limited inbreeding in the animal population, particularly when combined with similar-but-healthier methods such as crossbreeding, can be used to increase the prevalence of a favorable trait in that species’ population.
In general, mating between individuals who are closer than second cousins is considered inbreeding. As such, relationships between siblings, half-siblings, father and daughter, mother and son, uncles or aunts with their nieces and nephews, and grandparents and grandchildren are considered inbreeding. In many cultures, unions between first cousins are also considered inbreeding, and in some cultures the taboo extends all the way to seventh cousins.
Although relationships between people who are second cousins may also be viewed unfavorably by a given society, they are generally not close enough to be considered inbreeding in a biological or genetic sense.
Countries with the highest rates of people having children with close relatives include Burkina Faso, Pakistan, and Qatar.