In human populations, inbreeding refers to two biologically related people who mate and have children. Degree of relationship isn’t always fully defined, but a marriage between two cousins or second cousins is an example, and these marriages are still legal in many places today, and were considerably common in most parts of the world several hundred years ago. From a health perspective, inbreeding poses problems because it can cause greater likelihood of genetic defects or vulnerability to certain diseases. This is why many countries and cultural groups discourage its practice.
The actual health risks for inbred children depend on the degree of relationship of the parents and past history of family inbreeding. Where it is common for people with many relationship ties to each other to produce children, it’s also more likely that the available gene pool gets smaller with each generation. In areas that are cut off geographically from other locations or where, for other reasons, it’s very likely that inbreeding occurs, certain genetic defects begin to present more frequently, and overall blood relationship between mates often leads to offspring having similar characteristics like being smaller in size and having lower fertility levels.
If a whole population area or other group regularly practices inbreeding, one thing that occurs in an attempt to correct this is a process called culling. A high infant mortality rate, child death rate, or inability to reproduce can reduce the amount of negative traits that get passed on, only leaving those people who have the strongest genetic makeup. This could increase survival rates of future generations and select for the most desirable genes.
There are well-known examples of systemic inbreeding. For many centuries, the European monarchy typically only married within the nobility, and many marriages took place between people with one or more relational ties to each other. This practice, repeated over time, led to the expression of serious illnesses that often affected infant mortality. Long before the existence of a European monarchy, other cultural groups like the ancient Egyptians practiced marriage between much closer relatives. Egyptian kings frequently married their sisters.
Culturally, there are taboos that have arisen regarding inbreeding, and many of these have existed long before the genetics of the matter were fully understood. Strong incest taboos exist in many cultures, forbidding the routine practice of mating siblings, or parents to children. Some cultures extend this and feel that marriage between cousins is unacceptable, too. Conversely, in other cultures past and present, marriage is only acceptable if the two people have a defined level of relationship; e.g. nieces must marry uncles. Alternately, some groups seem to be so aware of the potential genetic risks of inbreeding that people are not allowed to marry anyone from their own geographic area and must find mates in other towns or tribes.
Related people desiring marriage could use genetic testing to determine some risks to offspring. Tests don’t screen for everything but might eliminate worries about passing on certain diseases, such as autosomal recessive inherited disorders, passed to children at a 25% rate when both parents have a gene for the condition. With related mates, the likelihood of both carrying a gene for these types of disorders is elevated.