Stacked mass assembly histories (MAHs) in three mass sub-samples, corresponding to galaxy clusters (light blue, thicker lines), high-mass groups (orange), and low-mass groups (turquoise, thinner lines). Dark shaded regions indicate the 1σ uncertainty around each mean MAH. Light shaded regions indicate the population standard deviation (just shown for the galaxy clusters sub-sample for clarity). Dashed (dotted) lines show the MAHs expected for each mass range according to the analytic (semi-analytic) model by Correa et al. (2015) (van den Bosch et al. 2014).