Activity levels as a dimension of temperament in toddlers: its relevance for the organismic specificity hypothesis.

The objective of the present study was to follow up on hypotheses derived from earlier research on activity level as a possible mediator between environment and development. Of particular theoretical interest was the organismic specificity hypothesis, which states that early experiences differentially influence development because the impact of the environment is mediated by individual differences in the organism. Subjects were 52 15-month-old toddlers who were observed in their homes 6 times over a 4-week period. Measures of the caretaker's attention-focusing behaviors were aggregated across 4 of the visits. Measures of the child's home environment and exploratory ability were also obtained. ANOVA analyses indicated a significant interaction between activity level and intensity of stimulation in predicting both exploratory competence and total exploration. The findings provided clear support for the organismic specificity hypothesis. Implications of the results for clinical work and future research on activity level and individual differences are discussed.