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<p>Perhaps that may be because admission to graduate and professional degree programs uses standardized tests (and MD and JD program admissions tend to place very high importance on test scores), and standardized test performance is likely a good predictor of performance on other standardized tests. So the standardized test non-submitters (presumably those who scored lower on them) may be among those who do worse later on tests like the MCAT, LSAT, etc. and therefore have less chance of admission to professional degree programs.</p>