This research hypothesized that preformal-operational children (before age 11) can improve their ability to write persuasive essays at an age earlier than Jean Piaget's developmental stage model would predict. A Vygotskian social interactionist approach, which claims that adult intervention can help children achieve what may have seemed beyond their abilities, was used. Fourth and sixth graders (n=153) wrote two persuasive essays, with 3 weeks of instruction intervening for the experimental group. Instruction consisted of daily 45-minute lessons for 3 weeks, involving brainstorming, supporting, conferencing, and editing on different topics. The essays were evaluated on the basis of "claims,""data," and "warrants." Students in the experimental group on the whole performed better than those in the comparison group. Girls performed better across all three aspects than did boys. Of the three measures, "warrants" seemed to be the most difficult. The experimental treatment was questionable for boys on this trait, as performance of fourth grade comparison group boys was higher than that of experimental group boys. The study concludes that formal operations are not a necessary precondition for writing persuasive essays; a Vygotskian approach to the teaching of writing is supported; girls acquire formal operations sooner than boys; and girls have superior verbal abilities that allow them to perform generally better than boys at writing tasks. (Contains 19 references.) (JDD)