Abstract Minimalist grammars have been used recently in a series of papers to explain well-known contrasts in human sentence processing in terms of subtle structural differences. These proposals combine a top-down parser with complexity metrics that relate parsing difficulty to memory usage. So far, though, there has been no large-scale exploration of the space of viable metrics. Building on this earlier work, we compare the ability of 1,600 metrics to derive several processing effects observed with relative clauses, many of which have been proven difficult to unify. We show that among those 1,600 candidates, a few metrics (and only a few) can provide a unified account of all these contrasts. This is a welcome result for two reasons: First, it provides a novel account of extensively studied psycholinguistic data. Second, it significantly limits the number of viable metrics that may be applied to other phenomena, thus reducing theoretical indeterminacy.