Police presumptions about criminal career trajectories have been little studied. The exploratory study reported here involved 42 police staff of varying rank and experience. Participants were asked to complete a questionnaire that asked them to predict the type of offence that an individual with a specified prior record would most probably commit next. Participating police personnel substantially overstated the homogeneity of criminal careers, that is, the nature of prior offences determined their prediction of their next offence more than available official data would deem reasonable. An incidental finding was that officers who rated the probability of further offending highest were also those who thought criminal careers most specialised. The implications for operational police decision‐making were discussed and held to be profound. Copyright © 2013 John Wiley & Sons, Ltd.