The dearth of longitudinal studies involving the close opservation of expressions over time in the child, however, makes it impossible to infer with any certanty what mechanisms could conceivably be involved (p. 160).
出典:Charlesworth W. R., Kreutzer, M. A, (1973). Facial expression of infants and children. In P. Ekman (Ed.), Darwininan and facial expression (p.p. 91-168). New York: Academic Press