Upper panels show the virial mass distributions of the samples of fossil (empty) and ΔM₁₂ < 0.5 mag (filled) groups in the MS, while lower panels show the assembly time distributions. Left panels: “Mass-matched” samples. Non-fossils that were selected to reproduce the same virial mass distribution of fossil groups in order to avoid a mass bias. Right panels: “Assembly-matched” samples. Fossils and non-fossils were selected to have both similar virial mass and assembly time distributions.