The choice of an appropriate sample size for a study is a notoriously neglected topic in behavioural research, even though it is of utmost importance and the rules of action are more than clear – or are they? They may be clear if a formal power analysis is concerned. However, with the educated guesswork usually applied in behavioural studies there are various trade-offs, and the degrees of freedom are extensive. An analysis of 119 original studies haphazardly chosen from five leading behavioural journals suggests that the selected sample size reflects an influence of constraints more often than a rational optimization process. As predicted, field work involves greater samples than studies conducted in captivity, and invertebrates are used in greater numbers than vertebrates when the approach is similar. However, it seems to be less important for determining the number of subjects if the study employs observational or experimental means. This is surprising because in contrast to mere observations, experiments allow to reduce random variation in the data, which is an essential precondition for economizing on sample size. By pointing to inconsistent patterns the intention of this article is to induce thought and discussion among behavioural researchers on this crucial issue, where apparently neither standard procedures are applied nor conventions have yet been established. This is an issue of concern for authors, referees and editors alike.