This study tested an internalization model of emotional development proposing that emotional expression decreases during childhood in situations in which emotions serve only self-regulation. This model was tested by inducing joy and disappointment in solitary versus interpersonal conditions in 3 gender-matched, 20-member groups of 6-, 7-, and 8-year-olds. Results supported the model: Expression--but not self-reported feeling--decreased in solitary conditions as a function of increasing age, whereas both expression and feeling remained stable in the interpersonal condition. This effect also correlated positively with the ability to discriminate between expression and feeling on a conceptual level. Results are discussed in relation to the major developmental trend toward creating a mental level of self-regulation--first described by Vygotsky.