Identifying helpful information from large-scale online reviews has become a core issue in studies on harnessing wisdom-of-crowds. We investigated whether online reviews expressing dissenting opinions (i.e., outlier reviews) can provide helpful information. Using statistical and simulation methods with a large-scale dataset, we found that, compared with other online reviews, outlier reviews were deemed more helpful because they provided more sufficient, neutral, and concise information. To interpret these results, we considered that in collective behaviours, a prevalent social psychological process—conformity (i.e., changing one’s behaviour in response to pressure from others)—pressured reviewers expressing dissenting opinions. This motivated them to provide more convincing evidence (i.e., sufficient, neutral, and concise information). This study offers a simple yet effective approach for eliciting helpful information from many online reviews and deepens the understanding of the mechanism underlying collective online behaviour. Specifically, conformity was considered to cause biases in the collective behaviour of humans; however, this study revealed that conformity can elicit valuable outcomes in collective behaviour.