He studies involvement and engagement with terrorism, with a focus on disengagement and deradicalisation from terrorist movements.
He has been described by the European Eye on Radicalization research group as the "world’s most distinguished expert in the psychology of terrorism".
[8] While at the University of Massachusetts, Lowell he directed their Center for Terrorism & Security Studies;[9] while at Pennsylvania State University, he directed for their International Center for the Study of Terrorism.
In 2010, following his consultancy work with the FBI's Behavioral Analysis Unit (BAU), he was appointed to the Research Advisory Board of the FBI's National Center for the Analysis of Violent Crime (NCAVC) until its official disbandment in 2012.
[1] In 2019, Horgan and a group of researchers at Georgia State University were awarded $250,000 by the U.S. Department of Homeland Security to research the growth and spread of the incel subculture, a group that Horgan described as "one of the purest hotbeds of Internet radicalisation I’ve ever seen".