# Table 1 Key parameters of the model for an agent i.

Parameter(s) Description Role in model Subject to Natural Selection
ui, di, si Rules for i’s donation based on self comparison of the potential recipient j’s reputation, while applying prejudice if j is out-group Governs i’s donation behaviour Yes
α i Prejudice level of i Defines the level by which i reduces the universal reputation of out-group agents. Used to define commonality with others to create an in-group $${G}_{{t}_{i},{\alpha }_{i}}$$ Yes
t i Fixed trait held by i Controls the sub-population to which i belongs No
$${G}_{{t}_{i},{\alpha }_{i}}$$ The in-group to which i belongs, defined by agents with both the same fixed trait and prejudice level as i Defines the in-group boundary for i, determining whether or not i applies a prejudicial view of other agents Yes, as a consequence of αi evolving
S i The probability that i plays the donation game with a randomly chosen member of the in-group, rather than from an out-group Governs how well-mixed the interactions are between agents in different groups Optional – can evolve (Fig. 2) or exogenously fixed (all other Figs.)
P i The probability that i reproduces by selecting a social-comparison heuristic from in-group, rather than from the whole population Governs how the in-group focused agents are, from learning by copying the social comparison heuristics of others Optional – can evolve (Fig. 2) or exogenously fixed (all other Figs.)
$${r}_{i}^{U}$$ Universal reputation for i Status of the donation behaviour of i, based on generalised standing, assuming prejudice is not legitimate when considering the recipient’s reputation No
$${r}_{i}^{G}$$ In-group reputation for i Status of the donation behaviour of i, from the in-group perspective, which assumes that prejudice is applicable to out-group reputation No
tp i total payoff for i Accumulates costs and benefits from making and receiving donations No
1. Note that these parameters populate an agent’s social comparison heuristic, denoted Hi = (si, ui, di, αi, Pi, Si). Natural selection occurs with respect to Pi and Si in Fig. 2, but Pi and Si are exogenously fixed in all other Figures, as a means to consider scenarios where these variables are set by the external context.