an indicator of the presence of a certain trait in both representatives of the group (most often in twins). If the sign manifested itself in both twins (for example, both have mental disorders), we speak of concordance in this couple according to this sign. High concordance in monozygotic twins and low in dizygotic twins indicates a significant role of heredity. The similarity of indicators in both types of twins indicates a small role of the genetic factor. Low values in both groups indicate the predominance of environmental influences.