Over 80% of marriages within the Spanish branch of the Habsburg dynasty were consanguineous, which caused a series of disorders, including mental retardation. Now imagine what happens when a whole nation keeps marrying consanguineously for generations. The nation may widely suffer from similar sort of disorders Habsburgs suffered from.
Fortunately, modern science and technology knows how to prevent inborn diseases—assuming, of course, that the population is aware of the risks. Unfortunately, some populations ingore the risks associated with inbreeding.
Note that the consanguinity rate in the Gaza Strip exceeds that of Spanish Habsburgs. Bad news.