Since Black Lives Matter protests gained national prominence following the 2014 police killing of Michael Brown in Ferguson, Mo., the movement has spread to hundreds of cities and towns across the U.S. Now a new study shows police homicides have significantly decreased in most cities where such protests occurred.

Black Lives Matter (BLM) began when Oakland, Calif.–based activist Alicia Garza posted a message of protest on Facebook after George Zimmerman, a neighborhood-watch volunteer who followed and fatally shot 17-year-old Trayvon Martin in Sanford, Fla., was acquitted of murder in 2013. Patrisse Cullors, another Oakland community organizer, began sharing Garza’s message on social media, along with the hashtag #blacklivesmatter. The slogan soon spread, building into a largely leaderless movement against structural racism and police violence. Last year, spurred by a Minneapolis police officer’s killing of George Floyd, millions of people demonstrated in hundreds of BLM protests throughout the U.S.

“Black Lives Matter represents a trend that goes beyond the decentralization that existed within the Civil Rights Movement,” says Aldon Morris, a sociologist at Northwestern University, who was not involved in the new study. “The question becomes, ‘Are Black Lives Matter protests having any real effect in terms of generating change?’ The data show very clearly that where you had Black Lives Matter protests, killing of people by the police decreased. It’s inescapable from this study that protest matters—that it can generate change.”

The study, posted in February as an online preprint item on the Social Science Research Network, is the first of its kind to measure a possible correlation between BLM and police homicide numbers. It found that municipalities where BLM protests have been held experienced as much as a 20 percent decrease in killings by police, resulting in an estimated 300 fewer deaths nationwide in 2014–2019. The occurrence of local protests increased the likelihood of police departments adopting body-worn cameras and community-policing initiatives, the study also found. Many cities with larger and more frequent BLM protests experienced greater declines in police homicides.