9.19. During the past decade, awareness about the situation of persons who are forced to leave their places of usual residence for a variety of reasons has been rising. Because there is no single definition of internally displaced persons, estimates of their number vary, as do the causes for their migration. However, it is generally accepted that those causes range from environmental degradation to natural disasters and internal conflicts that destroy human settlements and force people to flee from one area of the country to another. Indigenous people in particular are in many cases subject to displacement. Given the forced nature of their movement, internally displaced persons often find themselves in particularly vulnerable situations, especially women who may be subjected to rape and sexual assault in situations of armed conflict. Internal displacement is often a precursor to outflows of refugees and externally displaced persons. Returning refugees may also be internally displaced.