Coordinated terrorist attacks by the Islamic State Friday using a combination of suicide bombs and automatic weapons left at least 128 people dead across Paris.

As the city mourns, ordinary citizens around the world express sympathy for and solidarity with the French capital this weekend. World leaders have condemned the attacks, with French President Francois Hollande calling the assault “an act of war.” European governments have also beefed up security and called for heightened vigilance from the general public, reports Agence France-Presse.

How could something like this happen? It’s a question that security and intelligence officials around the world consider in order to prevent future incidents. Psychologists, social scientists and other researchers have also investigated this subject by studying the minds of terrorists.

CDC’s Dirty Dozen: 12 Diseases Watched as Terrorist Threats

Why Do Terrorists Attack Civilian Areas?

Yesterday’s attacks in the French capital targeted restaurants, a soccer game and a concert hall, exactly the kinds of places the average young Parisian or tourist would want to be on a Friday night.

In “Ciottone’s Disaster Medicine (Second Edition)“, which devotes a chapter to the psychology of terrorism, author Robert Ciottone explains:

Terrorists seek to destabilize individuals and societal organizations by undermining the cognitive, affective, and valuative perspectives they have of the physical, interpersonal, and sociocultural aspects of the world. In other words, terrorists seek to reshape the frame of reference by which people know about, have emotional reactions to, and attach relative importance to the world of objects and places, of people and of laws, and of rules, customs, and expectations.

Because of the disparity between the resources available to governments versus terrorist organizations, civilian targets offer terrorists the possibility of a greater return on investment, so to speak. These areas are chosen not because of military or political significance but for other strategic reasons because terrorists strike where populations feel most secure.

When Terrorism Targets Sports: Photos

How Do Terrorists Choose Their Targets?

Instead of seeking to maximize economic or physical impact, terrorists make decisions based on “emotional and visceral factors,” according to a study published in June in the journal Risk Analysis.

These targets are specifically considered not only to instill fear in a particular demographic but also demonstrate the capability of the organization to bolster support amongst its followers.

In “The logic of terrorism: Terrorist behavior as a product of strategic choice,” author Martha Crenshaw explained that “pectacular humiliation of the government demonstrates strength and will and maintains the morale and enthusiasm of adherents and sympathizers.”

Terrorists of course can never be assumed to be fully rational actors, which explains major flaws in the decision-making, but also makes it more difficult for security analysts to predict their next move, the immediate impact of which is extremists’ chief concern.

The authors of the Risk Analysis study note that “passions and visceral factors influence an agent to behave extremely myopically and to seek immediate rewards, disregarding any detrimental effects.”

VIDEO: Explainer: Terrorist Watch Lists

Who Are Terrorists Typically?

Aside from being collectively bound to an extreme ideology, terrorists generally have one other demographic connection: They’re often young men, which may be explained by behavioral biology, according to the recently released book “Evolutionary Psychology and Terrorism.”

In early adulthood, young men are more prone to risk-taking behaviors, previous studies have found. They also have higher levels of testosterone. Testosterone “makes them more susceptible to influence by other males, who are usually older,” Jason Roach of the University of Huddersfield said in a statement in September upon the book’s release. “That is why you will find that it is younger males who do the suicide bombing, but the direction comes from older men.”

Suicidal terrorists in particular “seek care and guidance from stronger personality figures,” according to the Risk Analysis study, and crave the attention they get for their extremely violent actions.

The psychological roots of terrorist actions can be difficult to pin down. Extremist thinking is “rigid, primitive, and unsophisticated” and their emotional state best described as “impassioned filled with disappointment, frustration, fear, disgust, anger, and hatred,” finds an article published in 2007 in the journal Aggression and Violent Behavior.

The author cited a variety of factors for these cognitive and psychological states, ranging from a poor education system marked by memorization, a personal life scarred by failure (to hold a job, to marry, to adjust to modernity, etc.), or dissatisfaction with a combination of “social, economic, political cultural, or religious conditions.”

No study of the terrorist’s mindset could ever completely explain their behavior. But the closer psychologists and policymakers get to understanding extremist psychological and cognitive patterns, the better equipped security and officials will be to deal with terrorists.