Main stages of studying of development of twins as a specific unique phenomenon

The theoretical and methodological basis of the study of formation of a personality of an adolescent twin in the system of additional education includes the study of 3 main stages of development of the science about twins as an extraordinary and psychological and pedagogical phenomenon since the middle of the 19th century up to the 21st century. The article provides the model of the mechanism of interaction of biological, psychological and social factors in shaping the orientation of a personality of an adolescent twin in a twins' situation.