Abstract
In comparison with children whose parents are not related, the offspring of parents who are first--degree relatives are 17-40% more likely to suffer and die from diseases, especially hereditary ones. The offspring born from the relationships of parents who are related to each other are called inbred offspring, while the process of creation (conception) and possible upbringing of such offspring is called inbreeding. Inbred offspring tend to demonstrate a reduced biological fitness, referred to in the literature as inbred depression. The article presents the natural conditions of humans that protect the human race against the occurrence of the above-mentioned phenomena. In addition, it contains a scientific discussion of the behaviors that reinforce these phenomena, occurring in some societies of the world.