This paper demonstrates a novel approach in experimental syntax, leveraging psychometric methods to resolve a decades-old puzzle.  Specifically, gaps in subject position are more acceptable than gaps in object position in non-islands, while the reverse is true in islands (the Island Boundary-Gap Effect).  Attempts at explaining this asymmetry generally attribute it to a violation of the same constraint that renders gaps unacceptable after the overt complementizer `that' (the That-Trace Effect).  However, the two effects involve distinct syntactic structures, and there is no a priori reason to believe they are the same beyond the elegance of a unified account.  One limitation has been the difficulty of testing for equivalence in the Null Hypothesis Significance Testing framework: when two constructs behave similarly, it generally constitutes an uninterpretable null result. Experiments 1 and 2 use standard experimental methods to circumvent this problem, but ultimately provide evidence that is at best just consistent with equivalence.  Experiment 3 demonstrates a novel approach which shows that individual differences in the That-Trace Effect correlate with individual differences in the Island Boundary-Gap Effect, after removing correlated variance from carefully-chosen controls.  This psychometric approach provides positive evidence that the two effects do indeed derive from the same underlying phenomenon.