Student performance in basic math and reading skills in the United States trails behind other developed countries, providing the rationale for more research to determine how performance might be improved. Following evidence to conclude that multilingualism enhances cognitive, neuro-linguistic and meta-linguistic development, it is proposed that improved student performance might be achieved if the U.S. educational system mainstreamed foreign language teaching as early as elementary school. The research hypothesis of this quantitative causal-comparative study was therefore that multilingual students should achieve significantly higher scores in standardized tests of basic skills relative to monolingual students. The target population was 3,559 students at 69 schools throughout the European region of the American overseas school system (AOSS). Simple random samples of 500 monolingual and 500 multilingual students in grades 5 through 8 with similar socio-economic status were drawn from the target population. The instrument was the TerraNova-Multiple Assessments, second Edition. Threats to internal validity including sampling bias, subject characteristics, history effect, instrument decay, and mortality were limited. MANCOVA was used to test for the main effect (multilingual or monolingual) on the two correlated dependent variables (TerraNova math and reading subtest scores) controlling for the ages of the students (indicated by their grades). The assumptions of MANCOVA with respect to the homogeneity of the covariance matrices and equality of variance across the groups were satisfied by weighting the scores with the reciprocal of the variance. There was a significant difference between the combined TerraNova scores of the monolingual and multilingual students (Hotelling's Trace = 0.028, F (2, 996) = 14.118, p less than 0.001). The mean math and reading scores of multilinguals were respectively 4.3 and 2.3 higher than the scores of monolinguals. Although the effect size eta [superscript 2] = 0.03 was small, the outcome has practical significance. The results of this and other studies should influence educational policy on language teaching. Future qualitative research interviewing monolinguals and multilinguals and classroom teachers to obtain a multi-perspectival view of why multilinguals perform better is recommended. Urgent action is demanded from educational administrators and curriculum specialists to make foreign language teaching an integral part of the curriculum. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]