Brain Association Studies in 2024: A Systematic Review of Sample Sizes and Preregistrations.

Within the field of research aiming to link psychological variables to individual differences in brain structure and function, there is a widespread consensus that (1) samples sizes at least in the hundreds are necessary to detect the presumably modest effects of typically rho < 0.20 with sufficient statistical power and that (2) a large number of equally defensible options for data preprocessing and analysis exist that have a marked impact on the final results (i.e., an issue typically addressed via a detailed preregistration). However, among 515 relevant studies published in 2024 and identified in the current systematic review the median sample size was only 101 participants (80 in studies that presumably involved data collection rather than reanalysis of preexisting data) and only 5% featured a preregistration. Whereas 96% of the studies reported at least one significant relevant association, only 2.3% reported a significant preregistered association. This pattern suggests that many of the published associations are likely false positives and that significant changes in research practices are required. The current systematic review provides a blueprint for testing whether recent research in other fields faces similar problems.