Conflicts of interest and the evolution of decision sharing

Abstract

Social animals regularly face consensus decisions whereby they choose, collectively, between mutually exclusive actions. Such decisions often involve conflicts of interest between group members with respect to preferred action. Conflicts could, in principle, be resolved, either by sharing decisions between members ('shared decisions') or by one 'dominant' member making decisions on behalf of the whole group ('unshared decisions'). Both, shared and unshared decisions, have been observed. However, it is unclear as to what favours the evolution of either decision type. Here, after a brief literature review, we present a novel method, involving a combination of self-organizing system and game theory modelling, of investigating the evolution of shared and unshared decisions. We apply the method to decisions on movement direction. We find that both, shared and unshared, decisions can evolve without individuals having a global overview of the group's behaviour or any knowledge about other members' preferences or intentions. Selection favours unshared over shared decisions when conflicts are high relative to grouping benefits, and vice versa. These results differ from those of group decision models relating to activity timings. We attribute this to fundamental differences between collective decisions about modalities that are disjunct (here, space) or continuous (here, time) with respect to costs/benefits.

Figure 1

ESSs depending on the grouping benefits to type 1 of being in a group of three ($B_3^{\prime }$, x-axis) or two ($B_2^{\prime }$, y-axis), respectively, relative to the potential consensus costs; (a) both types with same grouping benefits to consensus cost ratio (i.e. $B_3^{\prime }$ (type 1)=$B_3^{\prime }$ (type 2); $B_2^{\prime }$ (type 1)=$B_2^{\prime }$ (type 2)), t=0.5; (b) both types with same grouping benefits to consensus cost ratio, t=0.1/0.9; (c) type 1 with four times higher grouping benefits to consensus cost ratio (i.e. $B_3^{\prime }$ (type 1)=4·$B_3^{\prime }$ (type 2); $B_2^{\prime }$ (type 1)=4·$B_2^{\prime }$ (type 2)), t=0.1; (d) type 1 with four times higher grouping benefits to consensus cost ratio, t=0.5; (e) type 1 with four times higher grouping benefits to consensus cost ratio, t=0.9 (black areas, 1 ESS, all individuals play always ω_high, resulting in complete segregation; horizontally striped areas, no ESS, individuals oscillate between playing ω_medium and ω_high, resulting in varying leading and partial segregation; light grey areas, 1 (or 2) ESSs, the type with the highest potential consensus costs or which is more common plays always ω_high, the other type plays always ω_medium, resulting partially in leading by first type and partially in segregation; dark grey areas, 1 ESS, the type with the highest potential consensus costs or which is more common plays always ω_high, the other type plays always ω_low, resulting in leading of decisions by first type; vertically striped areas, 2 ESSs, one type plays always ω_high, the other type always ω_low, resulting in ‘leading by type 1’ or ‘leading by type 2’; spotted areas, 1 ESS, all types play always ω_medium, resulting in equally shared decision making; white areas, 3 ESSs, ‘leading by type 1’, ‘leading by type 2’ and ‘equally shared decision making’).

Figure 2

Schematic summary of model predictions. The area above the dotted line refers to groups that are above optimal group size; the area below the dotted line refers to groups of suboptimal size.