Background: Police kill more than 300 black Americans-at least a quarter of them unarmed-each year in the USA. These events might have spillover effects on the mental health of people not directly affected.

Methods: In this population-based, quasi-experimental study, we combined novel data on police killings with individual-level data from the nationally representative 2013-15 US Behavioral Risk Factor Surveillance System (BRFSS) to estimate the causal impact of police killings of unarmed black Americans on self-reported mental health of other black American adults in the US general population. The primary exposure was the number of police killings of unarmed black Americans occurring in the 3 months prior to the BRFSS interview within the same state. The primary outcome was the number of days in the previous month in which the respondent's mental health was reported as "not good". We estimated difference-in-differences regression models-adjusting for state-month, month-year, and interview-day fixed effects, as well as age, sex, and educational attainment. We additionally assessed the timing of effects, the specificity of the effects to black Americans, and the robustness of our findings.

Findings: 38 993 (weighted sample share 49%) of 103 710 black American respondents were exposed to one or more police killings of unarmed black Americans in their state of residence in the 3 months prior to the survey. Each additional police killing of an unarmed black American was associated with 0·14 additional poor mental health days (95% CI 0·07-0·22; p=0·00047) among black American respondents. The largest effects on mental health occurred in the 1-2 months after exposure, with no significant effects estimated for respondents interviewed before police killings (falsification test). Mental health impacts were not observed among white respondents and resulted only from police killings of unarmed black Americans (not unarmed white Americans or armed black Americans).

Interpretation: Police killings of unarmed black Americans have adverse effects on mental health among black American adults in the general population. Programmes should be implemented to decrease the frequency of police killings and to mitigate adverse mental health effects within communities when such killings do occur.

Funding: Robert Wood Johnson Foundation and National Institutes of Health.