A current issue about causal attribution is whether people take simple contrast-factor attributions or complex joint attributions in contrast situations. For example, a stone does not dissolve in water, and a piece of salt dissolves in water. That the piece of salt dissolves in water is due to which? (A.The influence of the piece of salt; B. The influence of the water; C. The joint influence of the piece of salt and the water). For such questions, we propose a mechanism-based sufficiency account. It argues that causal attributions are guided by mechanism-based explanatory sufficiency, and people prefer a causal attribution based on mechanism knowledge if it is sufficient for an effect than other available attributions. This account predicts the sufficient joint attribution (the C option), whereas the conventional covariation approach predicts the contrast-factor attribution (the A option). Two experiments investigated whether contrast situations affect causal attributions for compound causation with explicit mechanism information and simple causation without explicit mechanism information, respectively. Both experiments showed that contrast situations didn't affect causal attributions. In both the presence and absence of contrast situations, the majority of participants preferred sufficient joint attributions to simple contrast-factor attributions regardless of whether explicit mechanism information was present. These findings favor the mechanism-based sufficiency account rather than the covariation approach and the complexity account. In contrast situations, the predominance of joint attributions implies that explanatory complexity affects causal attributions by the modulation of explanatory sufficiency, and people prefer mechanism-based joint attributions that provide sufficient explanations for effects. The present findings are beyond the existing approaches to causal attributions. Joint Attributions in Causal Contrast Situations 3