Relatedness, Conflict, and the Evolution of Eusociality
The threshold model is that assumed in Figure 4 of Nowak et al.  (m = 3, b0 = 0.5, d0 = 0.1, d = 0.01, α = 0.1, η = 0.01), with no benefits of working below colony size 3 (two workers). The step model is identical except one worker benefits the queen half as much as two workers do. The data used to make this figure can be found in S1 Dataset.