NOW PUBLISHED: https://doi.org/10.1177%2F2515245920917961. The idea that in behavioural research everything correlates with everything else was a niche area of the scientific literature for over half a century. With the increasing availability of large datasets in psychology, and the heightened interest in falsifiability and null results, the ‘crud’ factor has however become more relevant than ever before. It is often referenced by researchers to discount minute – but statistically significant – effects that are deemed too small to be considered meaningful. This review tracks the history of the crud factor and examines how its use in the scientific literature has developed to this day. It highlights a common and deep-seated lack of understanding about what the crud factor is, whether it can be proven to exist or estimated, and how it should be interpreted. This makes the crud factor a convenient tool for psychologists to disregard unwanted results, even though the presence of a crud factor should be a large inconvenience for the discipline. To inspire a concerted effort to take the crud factor more seriously, this review clarifies the definitions of important concepts, highlights current pitfalls and poses questions that need to be addressed to ultimately improve our understanding of the crud factor. Such work will be necessary to develop the crud factor into a useful concept encouraging improved psychological research and theory corroboration practices.