New research on how police officers decide which types of drivers to stop and search
for drugs and weapons suggests that they choose their search strategies to maximise
successful searches and not necessarily out of racial bias. The study by Professors
Nicola Persico and Petra Todd, published in the November 2006 issue of the
Economic Journal, looks at evidence from Wichita, Kansas, and finds that rates of
detection of contraband – ‘hit rates’ – are remarkably similar for drivers of all
races/ethnicities.
The researchers also find that hit rates are virtually identical for drivers aged over 50
and for 18-24 year olds, as well as for men and women. But, they point out, equalising
the hit rates in this way does not guarantee ‘maximum deterrence’. If the police
wanted to maximise deterrence, they would have to go after groups that are more
easily deterred, if there are any such groups, and thus might have to accept unequal hit
rates.
Over the last ten years, numerous lawsuits have been brought against US law
enforcement agencies alleging racial profiling. The pattern that prompts these lawsuits
is familiar: minorities are disproportionately the target of interdiction by law
enforcement. In ordering an investigation into possible racial profiling, President
Clinton condemned the practice as ‘the opposite of good police work where actions
are based on hard facts, not stereotypes’.
But precisely what police work should be considered lawful because it is based on
‘hard facts’ as opposed to unlawful because it is based on ‘stereotypes’? Recent
economic research has make progress in this direction. The test case is that of
Wichita, a town in Kansas that exhibits the type of interdiction pattern that gives rise
to concerns of racial profiling: black people are searched for contraband three times as
often as their representation in the Wichita population, and three times as often as
white people.
The disparity, of course, is not only between black and white people. Compared with
older people (those over 50 years old), for example, young people (those aged 18-24)
are 9 times as likely to be searched. What to make of these disparities? Are they
evidence of racial bias on the part of the police or do they reflect goal-oriented
policing?
To assess which of these is the correct motive, Professors Persico and Todd develop a
behavioural analysis of policing that is designed to simulate the decisions made by
police officers and drivers in Wichita.
In their analysis, officers choose which types of drivers to search. Drivers decide
whether to carry illegal contraband (such as drugs or weapons), taking into account
the probability that they will be detected through a search. As the probability of
detection increases, drivers are less likely to carry illegal contraband.
The key insight obtained from this analysis is that unbiased police should sample
drivers to the point where detection rates are equalised across different observable
categories of drivers. If the rates were not equal, then an officer could do better by
reallocating efforts to a category of driver with a higher detection rate.
On the other hand, racially biased police will tend to ‘over-search’ certain types of
drivers, for example, minority drivers, to the point where the carrying rate (and thus
the detection rate) is lower for that type. One can therefore infer racial bias by testing
whether detection rates (‘hit rates’) are equal across different categories of drivers.
When this ‘hit rate test’ is employed, the data show a remarkable equalisation in the
detection rates (about 22%) for drivers of all races/ethnicities. This finding is
consistent with the notion that police in Wichita choose their search strategies to
maximise successful searches and not necessarily out of racial bias. Moreover, the hit
rate is virtually identical for drivers aged over 50 (20.6%) and those aged 18-24
(21.7%), as well as for a variety other characteristics (such as gender of driver).
This finding is consistent with earlier work by these researchers with John Knowles
on Maryland data and with a review of racial profiling studies for a number of other
cities and states. The overall picture that emerges from employing the hit rates
analysis provides support for the conjecture that the difference in search rates among
categories of drivers arises mainly from a goal-oriented policing motive as opposed to
racial bias.
ENDS
Notes for editors: ‘Generalizing the Hit Rates Test for Racial Bias in Law
Enforcement, with an Application to Vehicle Searches in Wichita’ by Nicola Persico
and Petra Todd is published in the November 2006 issue of the Economic Journal.
The authors are at the University of Pennsylvania.
For further information: contact Nicola Persico on +1-215-898-7711 (email:
nicola.persico@nyu.edu); or Romesh Vaitilingam on 07768-661095 (email:
romesh@compuserve.com).