Van Hulle, C.A., Corley, R., Zahn-Waxler, C., Kagan, J., & Hewitt, J.K. (2000a). An exploration of the genetic and environmental etiology of heart rate in infancy and middle childhood. Twin Research, 3, 259-265.

Analysis of resting heart rate recorded at ages 14, 20, 24, 26 months and 7 years indicates that genetic factors contribute to the stability of individual differences over time. Shared and non-shared environmental factors tend to be occasion specific, with non-shared environment contributing substantially to the individual variation at each age.