A well-characterised sequence length poly- morphism in the serotonin transporter promoter region (5-HTTLPR) influences individual behavioural traits and cognitive abilities in humans and rhesus macaques. Maca- ques have been classified into four continuous grades on the basis of their behavioural attributes, ranging from highly hierarchical and nepotistic species to the most egalitarian and tolerant ones. A comparative study of several species that spanned these grades revealed only rhesus macaques to be polymorphic at the 5-HTTLPR and concluded that the polymorphism was responsible for their despotic and aggressive behaviour (Wendland et al., Behav Genet 36:163–172, 2006). We studied wild populations of three other species and found that the egalitarian and tolerant bonnet and Arunachal macaques are also polymorphic while liontailed macaques, although belonging to the same group, are monomorphic. We thus reject a role for this particular polymorphism in interspecific behavioural vari- ability and show that polymorphic species enjoy greater ecological success possibly due to their higher infraspecific variability in individual behavioural traits.