This work is the second fragment of the investigation of the nature of individual differences in physiological principles of infant temperament. The aim of this stage of the study was to estimate the contribution of genotypic and environmental factors to the formation of individual differences in childhood temperament of the second half-year of life. The experimental group consisted of 172 infants from mono- and dyzygotic twin pairs, aged 8–12 months. The estimations of the level of psychomotor development (by Bayley’s Scales) of all the children were normal. The temperament was estimated by Balleyguier’s Questionnaire, which has a number of substantial advantages over other questionnaires. The contribution of genetic and environmental factors was statistically analyzed by model fitting at the levels of individual temperamental features (questionnaire scales) and integral dimensions (the basic factors of the factor structure of estimations by the scales). It was shown that the nature of individual differences in the temperamental features of children of the second half-year of life markedly differs. The features such as activity, irritability, aggression, and negative emotional background in neutral everyday situations are under rigid genetic control. The sensitivity to punishment signal is to an equal extent determined genetically and by the factors of shared (family) environment, while the individual differences in sociability are predominantly determined by the family environment. A significant discordance between the observed phenomena and the data of recent literature is discussed.
