Did More Stop and Search by Police Cause Less Knife Injury in London? Evidence from 2008–2023

Abstract Objectives This study investigates the impact of police stop and search encounters (SSEs) on knife injuries and homicides in public places in London. While prior research has studied SSE impact on crime in general, we focus specifically on SSE relations to weapon-related injuries and deaths: whether conducting more SSEs over time has reduced such crimes. Methods The study analyzes 15 years of data (183 months) from the Metropolitan Police Service (MPS) in London, including 58,503 recorded knife injuries and 4.3 million police SSEs. Two quasi-experiments and Autoregressive (AR) models were employed to examine correlations between changes in SSE volumes and trends in knife injuries and homicides over time. Results AR models revealed statistically significant reductions in knife murders and injuries in response to increased SSEs. Specifically, if SSEs were conducted at the 2008–2011 rate of 45,000 per month, there would be an estimated 30 fewer knife murders per year. Additionally, changes in SSE frequency were associated with notable crime rate shifts. A 66% reduction in SSEs from May 2014 led to 44 more knife murders and 1276 more injuries than expected. Conversely, a 55% increase in SSEs in January 2018 resulted in 27 fewer knife injuries per month. Conclusions The results suggest that increased SSEs can significantly reduce knife-related injuries and homicides in public places. This reduction translates into preventable healthcare costs of approximately £216,000 per month. These findings highlight the potential effectiveness of formerly higher levels of SSEs in preventing knife crime, with one fewer injury occurring every day in London.