The ethnographic characteristics of the Chiriguanos allowed us to make up an experimental design to show the relationship between migration and mental pathology. The Chiriguanos are a South American ethnographic population characterized by a traditional migratory tendency. The Chiriguanos group is a resultant of the Tupi-Guaraní migrations. From the 15th century up to the 19th aboriginal guaraníes moved from the current area of Paraguay to the East zone of Bolivia; they conquered and mixed with the group that lived there and after that they resisted the European forces. The Chiriguanos history can be divided in different phases: 1) The establishment in the new zone; 2) The "Chiriguana war" with the dominant group; 3) The grouping with Franciscan missions and its community organization. This study of Transcultural Psychiatry can be considered as an ex post facto experiment in the field of Psychiatric Epidemiology. It allows us to analyse population phenomena related to changes in the prevalence of mental pathology. The facts presented in this report have been established thanks to the use of sampling techniques adequated to each population being studied (original and migratory groups): demographic structure, total fertility Grow's "evolutional intensity index", and mental prevalence rates in both groups. Differences in the biennial prevalence rates of mental morbidity were found. In the original Chiriguana community there is an evident "group endogamy"; on the contrary, the migratory groups integrated by individuals of different aboriginal culture, are really melting-pots, that originate a new genetic groupal structure. We have worked with human situations created by a natural social and cultural reality. We have worked with two ethnographic homogeneous populations, one of them stable and the other a migratory one. We intended to demonstrate: 1) migration changes the rates of mental pathology; 2) those changes are followed by changes in the genetic structure of the individuals.