Evidence affects hypothesis judgments more if accumulated gradually than if presented instantaneously.

In a hypothesis comparison task involving quantifiable evidence, we investigated whether judgments of relative probability were affected by gradual evidence accumulation and by making a series of revised ratings, rather than a single final one. Each trial of our task required participants to rate the probability that a focal hypothesis, rather than its alternative, was correct. We manipulated (1) the strength of evidence supporting the focal hypothesis, (2) the strength of evidence supporting its alternative, and (3) whether that evidence was presented in three sequential portions (gradually accumulated evidence condition) or, instead, was all presented instantaneously (control condition). In a second experiment, we also manipulated (4) the number of successive ratings made within a trial with gradually accumulated evidence. Regardless of how many ratings were made per trial, gradual evidence accumulation increased the effects of evidence strength on ratings of relative probability.