=+a. Notice the direct effects of Self-Esteem and Locus of Control on Social Studies Achievement. Focus on the effect of GPA on Self-Esteem and Locus of Control. Is 8thgrade GPA a common cause of these variables and Social Studies Achievement? Now remove the 8th-grade GPA variable from the model. What happens to direct effects of Self-Esteem and Locus of Control on Social Studies Achievement? Explain the difference in effects from the original model.