In 2015, worldwide deaths from terrorism fell 10% from the previous year, despite an increase in the impact of terrorism around the globe, according to the Global Terrorism Index (GTI) released by the Institute for Economics and Peace (IEP).
The 29,376 terrorism-related deaths recorded in 2015 were a 10% decrease over 2014, marking the end of a four-year upward trend.
But terrorism-related violence spread, with 23 countries having their highest number of deaths from terrorism last year, over the previous high of 17 registered in 2014.
Among Organization for Economic Cooperation and Development countries, deaths from terrorism increased 650%, with 21 of 34 member-states recording at least one terrorist attack. The majority of terrorism deaths among OECD countries took place in Turkey and France.
Iraq, Afghanistan, Nigeria, Pakistan, and Syria accounted for 72% of all terrorism deaths in 2015. The Islamic State group (ISIS), Boko Haram, the Taliban, and Al Qaeda — operating within those five countries — were responsible for 74% of all terrorism-related deaths.
The spread of terrorism's influence was driven by the expanded activities of ISIS and Boko Haram, which has pledged allegiance to ISIS.
ISIS, with attacks in 252 cities resulting in 6,141 deaths, passed Boko Haram as the most lethal terrorist group last year.
ISIS increased its activity from 13 countries in 2014 to 28 last year, many of which were in Europe. Denmark, France, Germany, Sweden, and Turkey all saw the most deaths from terrorism in a year since 2000, and more than half the 577 deaths were related to ISIS, which orchestrated deadly attacks in Paris, Brussels, and Ankara.
Boko Haram, a terrorist group that originated in Nigeria, spread into Niger, Cameroon, and Chad, driving the number of people killed in those countries up by 157%.
Both groups seemed to welcome the recent election of Donald Trump as US president.
Abu Omar Khorasani, a top ISIS leader in Afghanistan, called the president-elect "a complete maniac," saying Trump's "utter hate towards Muslims will make our job much easier because we can recruit thousands."
Boko Haram leader Abubakar Shekau exhorted followers to "not be overwhelmed by people like Donald Trump and the global coalition fighting our brethren in Iraq, Syria, Afghanistan and everywhere."
Boko Haram's seven-year conflict against the Nigerian state has taken more than 20,000 lives.
"We remain steadfast on our faith and we will not stop," Shekau said in the hour-long message. "To us, the war has just begun."
"While on the one hand the reduction in deaths is positive, the continued intensification of terrorism in some countries and its spread to new ones is a cause for serious concern and underscores the fluid nature of modern terrorist activity," Steve Killelea, executive chairman of IEP, said in a release.
The GTI report found issues like youth unemployment, accessibility of weapons, and lack of trust in electoral processes as some of the "most statistically significant factors correlating with terrorism" in OECD countries, according to a release.
In developing countries, the history of conflict, corruption levels, and inequality correlated most significantly to terrorism.
Overall, the cost of terrorism rose to $89.6 billion in 2015, with Iraq suffering the highest economic impact, equal to 17% of its GDP.
SEE ALSO: Despite 15 years of occupation, Afghanistan’s opium production has only risen
Join the conversation about this story »
NOW WATCH: 'He's the founder of ISIS': Watch Trump and Obama trade insults throughout the years