The reason is a lack of training, the researchers said.
The higher death toll contradicts claims by President Barack Obama and other U.S. officials that unmanned aerial vehicles controlled by on-board computers or by remote pilots on the ground or in other vehicles were more precise than their manned counterparts.
The study used classified military information to examine airstrikes in Afghanistan from mid-2010 to mid-2011 and the civilian casualties they caused.
Combat drone strikes were "an order of magnitude more likely to result in civilian casualties per engagement" than strikes with human pilots on board, an unclassified summary of the study said.
An order of magnitude refers to powers of 10, so an order of magnitude more likely means 10 times as likely.
Study co-author Larry Lewis was more specific when he spoke with British newspaper The Guardian, saying drone strikes were not just 10 times as likely to kill Afghan civilians as jet fighters -- they actually killed 10 times as many.
He wouldn't say how many Afghan civilians were killed by drones versus manned aircraft, citing classified information.
But "the fact that I had been looking at air operations in Afghanistan for a number of years led me to suspect that what I found was in fact the case," said Lewis, a senior research analyst at the Center for Naval Analyses, a federally funded research group in Alexandria, Va., with close ties to the U.S. military.
Lewis is also a field representative to the Joint Chiefs' Directorate for Joint Force Development, focusing on joint forces' operational effectiveness.
Co-author Sarah Holewinski, executive director of the non-governmental organization Center for Civilians in Conflict in Washington, said the reason for drones' higher civilian death toll is because drone operators receive less training to avoid civilian casualties than fighter pilots.
"These findings show us that it's not about the technology -- it's about how the technology is used," Holewinski told The Guardian.
"Drones aren't magically better at avoiding civilians than fighter jets," she said. "When pilots flying jets were given clear directives and training on civilian protection, they were able to lower civilian casualty rates."
But the military demand for drone strikes in Afghanistan's war zone creates pressure to reduce training, Holewinski and Lewis wrote in an article about the study appearing in a National Defense University journal.
"Adding or improving training on civilian casualty prevention is a resource decision in direct tension with the increasing demand for more UAS [unmanned aerial systems] and more operations, since additional training on civilian protection means time must be taken from somewhere else including the mission itself," Lewis and Holewinski wrote in Prism, a journal published for the NDU Center for Complex Operations. The center, authorized by Congress, focuses on training to fight counterinsurgency and irregular warfare.
Obama acknowledged drone civilian casualties in a May 23 speech at NDU.
He called them "a risk that exists in all wars."
And while he said he would curtail the use of drones in Afghanistan, he also defended the program as more precise than manned missile strikes.
"Conventional air power or missiles are far less precise than drones, and likely to cause more civilian casualties and local outrage," he said.
CIA Director John Brennan said April 30, 2012, when he was homeland security adviser, U.S. drones "targeted strikes against specific al-Qaida terrorists."
"With the unprecedented ability of remotely piloted aircraft to precisely target a military objective while minimizing collateral damage, one could argue that never before has there been a weapon that allows us to distinguish more effectively between an al-Qaida terrorist and innocent civilians," he told an audience at the Woodrow Wilson International Center for Scholars in Washington.
Brennan is now responsible for the CIA's drone program in Pakistan. That program killed a top member of the Pakistani Taliban along the Afghan border less than a week after Obama's May 23 NDU address.
Senate Intelligence Committee Chairwoman Dianne Feinstein, D-Calif., said Feb. 7 during Brennan's confirmation hearing to be CIA director combat drones killed only "single digits" of civilians a year.