Schoenthaler, S.J., Amos, S.P., Eysenck, H.J., Peritz, E. & Yudkin, J. (1991) Person Individ Diffs 12: 351-62
615 schoolchildren were examined on multiple measures of intelligence, randomly assigned to one of four treatment groups for 12 weeks, and post-tested on the same measures of intelligence.
One cohort received placebos while the other three were given different strength vitamin-mineral supplements. The trial was completed under ‘blind’ conditions, i.e. the subjects, the testers, and the scientists conducting the data analyses did not know any subject's group assignment. This created a triple-blind placebo-controlled, classical design capable of determining whether supplements could produce significant gains in standardized validated indices of intelligence/performance. The study was carried out in Stanislaus County, California, using 4 different schools.
Results showed that for non-verbal Wechler Tests there were highly significant improvements in I.Q., whereas for the verbal tests there were none; a conclusion expected on the basis that ‘fluid’ intelligence, as measured by the non-verbal test, might be improved by supplementation, whereas ‘crystallized’ ability tests (verbal tests) would be unlikely to be so improved. This difference was predicted on the basis of previous studies also finding similar results. Other tests (Raven's Matrices, the Comprehensive Test of Basic Skills, the Matrix Analogies Test, and measures of Reaction Time and Inspection Time) gave additional confirmatory evidence on the contribution which supplementation of the diet by vitamins and minerals can make to the improvement of I.Q.
A smaller replication of this study was carried out in Great Britain, under the supervision of Dr D. Tamir of Jerusalem. Results were generally supportive of those reported here and will be published in due course. The Iraqi War caused the call-up of Dr Tamir, who was thus unable to complete the account of the British study.