Dynamics of environmental influences on similarity in twins were studied with the help of a mathematical model represented by Markov's incidental processes with discrete states and continuous time. The model takes into account genetic factors and individual environmental influences. Histograms were used as characteristics for analysis. The minimum x2 method was used to estimate the required meanings. The above approach was used to analyze longitudinal data on mono- and heterozygotic twins. It has been discovered that the same environmental influence can produce different contributions to the final dispersions of a psychological characteristic. The values of these contributions depend on the genetic factor, represented by the distributions of the parameter under analysis at the moment of birth.