We present the results of two acceptability judgment and three self-paced reading experiments exploring the source of degradation effects on the acceptability of genitive subjects caused by having them not adjacent to the verb. From a syntactic perspective, Miyagawa (2011) attributes this adjacency constraint to his assumption that the genitive subject is situated in Spec-vP, lower than adverbials adjoined to vP or higher. This theory thus predicts that adverbials lower than vP can intervene between the genitive subject and the verb without degrading acceptability. To test this issue and examine the time course of the intervention effects, our experiments varied the number and the types of interveners. Whereas the results of our acceptability judgment experiments straightforwardly confirmed the purported intervention effects, the self-paced reading experiments provided results that cannot be explained in structural terms alone. First, the reading times of the verb and/or the head noun were greater when the genitive subject was adjacent to the verb, an effect that has not been reported in the previous literature. Secondly, the summed reading times of the preverbal regions showed opposite patterns: i.e., the reading times were greater when the genitive subject was not adjacent. Finally, regarding the types of interveners, we found that temporal adverbs and locative PPs, but not manner adverbs, were read significantly slower when they followed the genitive subject. We argue that a hybrid account incorporating a syntactic hypothesis with expectation-based incremental processing theories is called for to account for the whole range of data.