Third, it is important to remember that causal conclusions can only be drawn about the manipulated independent variable.
But can causal conclusions be drawn about the conjunction of the manipulated and non-manipulated variables? Would Schnall and colleagues be justified in concluding that disgust and body consciousness together affect the harshness of moral judgments?
Schnall and her colleagues also demonstrated an interaction because the effect of whether the room was clean or messy on participants’ moral judgments depended on whether the participants were low or high in private body consciousness. If they were high in private body consciousness, then those in the messy room made harsher judgments. If they were low in private body consciousness, then whether the room was clean or messy did not matter.
Re. my comment above, is this interaction effect evidence of causation? If some third variable were effecting both body consciousness and severity of moral judgments, that wouldn't explain why those high in body consciousness only made harsher moral judgments in a messy room (assuming that's what the experiments showed).