This paper is on the influences of the classroom context on the course and
malleability of aggressive behavior from entrance into first grade through the transition into
middle school. Nineteen public elementary schools participated in developmental
epidemiologically based preventive trials in first and second grades, one of which was directed at
reducing aggressive, disruptive behavior. At the start of first grade, schools and teachers were
randomly assigned to intervention or control conditions. Children within each school were
assigned sequentially to classrooms from alphabetized lists, followed by checking to insure
balanced assignment based on kindergarten behavior. Despite these procedures, by the end of
first quarter, classrooms within schools differed markedly in levels of aggressive behavior.
Children were followed through sixth grade, where their aggressive behavior was rated by
middle school teachers. Strong interactive effects were found on the risk of being highly
aggressive in middle school between the level of aggressive behavior in the first grade
classrooms and each boy's own level of aggressive, disruptive behavior in first grade. The
more aggressive first grade boys who were in higher aggressive first grade classrooms were at
markedly increased risk, compared both to the median first grade boys, and compared to
aggressive males in lower aggressive first grade classrooms. Boys were already behaving more
aggressively than girls in first grade; and no similar classroom aggression effect was found
among girls, although girls' own aggressive behavior did place them at increased risk. The
preventive intervention effect, already reported elsewhere to reduce aggressive behavior among
the more aggressive males, appeared to do so by reducing high levels of classroom aggression.
First grade males' own poverty level was associated with higher risk of being more
aggressive, disruptive in first grade, and thereby increased their vulnerability to classroom level
of aggression. Both boys and girls in schools in poor communities were at increased risk of being
highly aggressive in middle school regardless of their levels of aggressive behavior in first grade.
These results are discussed in terms of life course/social field theory as applied to the role of
contextual influences on the development and etiology of severe aggressive behavior.