In their paper “Nestmate recognition in social insects: over-coming physiological constraints with collective decisionmaking”, Johnson and co-authors present agent-based mod-els of collective decision-making to show that multiplediscriminators (guards) may together mitigate individualdecision errors and achieve near-perfect, colony-level dis-crimination of nestmates from non-nestmates (Johnson et al.2011). The authors also conduct a literature review of dis-crimination errors in social insects to evaluate, in contrast totheir model, the theory of optimal acceptance thresholds,which predicts that falsely rejecting nestmates and correctlyrejecting non-nestmates should be negatively correlated.They conclude that as false rejections are exceedingly rareacross species, there can be no correlation between the twoerrors. However, we believe that the conclusions drawnfrom both the model and the literature review are based onfalse assumptions, which we will detail later.The models put forward by the authors test the effect ofguard density and guard–intruder encounter rate on theacceptance levels of nestmates and non-nestmates. Overall,they demonstrate that the probability of rejection increaseswith increasing encounters. For example, in their first mod-el, the overall probability of acceptance falls to less than 1%after only three encounters at an 80% per-encounter rejec-tion probability, but this also occurs after 21 encounters at20% per-encounter rejection probability. Given that themodel’s results show that increasing the discriminator inter-actions increases the probability of rejection, the authorssuggest that recruitment of additional guards may be analternative to the optimal acceptance threshold theory,whereby guards exhibit an individual, context-dependentresponse to incoming intruders that is based on the frequen-cy of intruder contact and the cost of making errors (Reeve1989; Starks et al. 1998).In our opinion, their hypothesis and models are intriguingto consider and could in principle work. The idea thatcolonies up-regulate the number of guards at the entrancewhen frequency of robbing increases has already been bothobserved in a natural situation and tested experimentally(Downs and Ratnieks 2000; Couvillon et al. 2008, 2009).When nectar supplies dwindle (for example, as we movefrom summer into autumn), honeybees are more likely tointrude into neighbouring colonies to rob them of storedhoney, to which colonies respond over several weeks/months by stationing more guards at the nest entrance(Downs and Ratnieks 2000; Couvillon et al. 2009). Thisup-regulation of guard number is also present over shortertime scales: when robbing is experimentally induced, hon-eybee colonies respond in 15 min by increasing the numberof guards by 20% (Couvillon et al. 2008). These datainitially appear to support the authors’ model of collectivedecision-making.However, we believe that a few issues have been over-looked. One major problem is that, despite fitting their finalmodel to the biology of the honeybee (Apis mellifera), theauthors do not consider previous empirical work investigat-ing individual-level responses of guards to increased intru-sion. In addition to demonstrating the rapid up-regulation in