U.S. drone strikes carried out in Pakistan appear to have little impact on insurgent violence in neighboring Afghanistan, according to a new meta-study published by the

But the study also finds that strikes carried out by unmanned aerial vehicles cause fewer civilian casualties than other kinds of combat and that those deaths don’t appear to be linked to further violence against U.S. forces and allies.

“We know little about how effective [drones] are as tools of punishment and deterrence,” according to James Igoe Walsh, the author of the study titled The Effectiveness of Drone Strikes in Counterinsurgency and Counterterrorism Campaigns.