Middle-school school psychologist wants to determine which program decreases unfriendly behaviors in adolescents. he implements an anti-bullying program, and the students’ teasing behaviors decrease. he then implements an afterschool “random acts of kindness” program and again the unfriendly behaviors decrease. however, when the psychologist stops observing the adolescents, their unfriendly behaviors return to their original levels. the ____________ effect may account for these results.