Declustering aims to divide earthquake catalogs into independent events (mainshocks), and dependent (clustered) events, and is an integral component of many seismicity studies, including seismic hazard assessment. We assess the effect of declustering on the frequency–magnitude distribution of mainshocks. In particular, we examine the dependence of the ‐value of declustered catalogs on the choice of declustering approach and algorithm‐specific parameters. Using the catalog of earthquakes in California since 1980, we show that the ‐value decreases by up to 30% due to declustering with respect to the undeclustered catalog. The extent of the reduction is highly dependent on the declustering method and parameters applied. We then reproduce a similar effect by declustering synthetic earthquake catalogs with known ‐value, which have been generated using an epidemic‐type aftershock sequence model. Our analysis suggests that the observed decrease in ‐value must, at least partially, arise from the application of the declustering algorithm on the catalog, rather than from differences in the nature of mainshocks versus fore‐ or aftershocks. We conclude that declustering should be considered as a potential source of bias in seismicity and hazard studies.