12) Robinson, J.L., Emde, R.N., & Corley, R.P. (2001). Dispositional Cheerfulness: Early Genetic and Environmental Influences. In R. Emde & J. Hewitt (Eds.), Infancy to Early Childhood: Genetic and Environmental Influences on Developmental Change (pp 163-177). Oxford University Press: New York, New York.

In this chapter researchers explore the idea that a disposition towards expressing positive emotions, “dispositional cheerfulness”, is evident in the second year of life. Approximately 100 MZ and 100 DZ twins were observed in two low stress contexts (administration of the Bayley Scales of Infant Development and 20 minutes of free play) and two high stress contexts (response to prohibition and response to the distress of another). The Hedonic Tone Scales were used to measure the strength of positive and negative emotion during each of the observed situations. The results pointed to a disposition to cheerfulness observable in individual differences in children’s emotional responses. Researchers also found that all children’s positive expressions tended to increase with age, and they offer several hypotheses as to why this may be the case.