A model of nature-nurture interaction in school situations emanating from a longitudinal Swedish twin project is presented. This model implies that interactional effects measured by MZ-DZ within-pair comparisons over time are related to the type of behavior studied, as well as teacher and pupil influences at different levels. In a more permissive and stimulating school situation, hereditary factors are hypothesized to be more influential (decisive for behavioral variation) than in a more restrictive and nonstimulating situation. A study of such interactional effects will require longitudinal measurements of pupil behavior as well as teacher and parental influences. To acquire a variation in permissiveness/restrictiveness and stimulation/nonstimulation at the societal level, comparisons are made between twins attending grade 4-6 in compulsory schools in Sweden and twins of the same age attending Israelian Kibbutz schools.