One of the many causes of conflict within the labour process is choice of an appropriate criterion for the selection of labour and the allocation of work tasks. While merit, as a proxy for productivity or efficiency, is appropriate according to economic rationality and the principles of business management, evidence abounds of the prevalence of seniority as the criterion most used. In an attempt to explain this apparent contradiction it is hypothesised that, in many cases, management is obliged to utilise seniority because the costs of insisting upon merit are too high. These costs, in turn, are claimed to result from three factors: the difficulties of measuring merit, workers' attachment to the seniority principle and external constraints on having merit predominate. Evidence from a variety of case studies is presented in support of the hypothesis.