PROFILING POLICY FOR CRIME DETECTION: MINIMISING THE COSTS TO SOCIETY OF COMPLETED OFFENCES, PUNISHMENT AND POLICE WORK

What motivations should govern the choice of a profiling policy in law enforcement where decisions to search for evidence of crime – say drug sales or terrorism – may vary with observable characteristics of the people at risk of being searched? That is the question explored by Professor Charles Manski of Northwestern University, writing in the November 2006 issue of the Economic Journal .

Policies that make search rates vary with personal attributes such as race and ethnicity are variously defended as essential to effective law enforcement and denounced as unfair to classes of persons subjected to relatively high search rates.

Choice of a profiling policy should depend on (a) what society wants to accomplish and (b) policy-makers know about the effectiveness of alternative strategies. Professor Manski's analysis clarifies the interplay of these forces.

Consider what society wants to accomplish. One possibility, assumed in previous research by John Knowles, Nicola Persico and Petra Todd (KPT), is that the police aim to maximise the ‘hit rate' of search (the chance of detecting a crime) minus the cost of performing searches.

In contrast, Manski supposes that society wants to minimise a ‘social cost function' with three components: the social harm caused by completed offences; the cost of punishing offenders who are apprehended; and the cost of performing searches.

He shows that the difference between these objectives is consequential. KPT concluded that, in the absence of discriminatory intent, police should choose a search rule that equalises hit rates across racial and ethnic groups.

Manski finds that equalisation of hit rates is not generally optimal if the objective is to minimise the net social cost of crime. Indeed, there are circumstances in which it is optimal to search a less crime-prone group and not to search a more crime-prone group. This may occur if members of the former group are deterrable and those in the latter group are not.

Consider what policy-makers know about the effectiveness of alternative search strategies. The consequences of candidate search rules depend on the extent to which search deters crime. Deterrence is expressed through the ‘ offence function' , which describes how the offence rate of persons with given characteristics varies with the search rate applied to these persons.

Previous economic studies of profiling have assumed that the police have full knowledge of the offence function. This assumption seems unrealistic, so Manski studies how a social planner with only partial knowledge of the offence function might reasonably choose a search strategy.

To demonstrate general ideas, Manski supposes that the planner observes the offence rates of a study population whose search rule has previously been chosen. He knows that the offence rate weakly decreases as the search rate increases, but he does not know the magnitude of the deterrent effect of search.

In this setting, Manski first shows how the planner can eliminate some search rules that are definitely inferior. He then examines some ways to choose reasonably among the strategies that remain.

ENDS

Notes for editors : ‘Search Profiling with Partial Knowledge of Deterrence' by Charles Manski is published in the November 2006 issue of the Economic Journal .

Charles Manski is at Northwestern University.

For further information : contact Charles Manski on +1-847-491-8223 (email: cfmanski@northwestern.edu); or Romesh Vaitilingam on 07768-661095 (email: romesh@compuserve.com).