Moral reasoning typically relates unexpectedly weakly with both Machiavellianism and authoritarianism. Although researchers often explain this by pointing to apparent shortcomings in both the construct and the measure of moral reasoning, such explanations are questionable given the many instances of support for hypotheses involving moral reasoning using the same construct and measure. As these latter cannot only sometimes be flawed, we explored the possible influence of moral reasoning index choice on observed results by using multiple indices available in the Defining Issues Test (DIT). In a sample of 201 employed persons surveyed in 1998, with results reported for the first time, advanced moral reasoners tended to be neither Machiavellian nor authoritarian. However, the specific moral reasoning index employed was critical to detecting these hypothesized inverse relationships. Specifically, we proposed (and determined) that currently unavailable D scores would be the relevant index for examining inverse relationships with Machiavellianism and that P scores would be most appropriate in the context of inverse relationships with authoritarianism, particularly among persons inclined to utilize their characteristic moral reasoning (assessed with U scores). We extrapolate the conceptual logic underpinning such relationships, and suggest that appropriate index choice flows from this logic and is essential to hypothesis testing across a broad array of constructs. Future research could adopt this logic to examine relationships involving constructs with implications similar to those found in Machiavellianism and authoritarianism.