This isn't to say that individuals who commit political violence don't talk to anyone before they attack. Recent research into 119 lone-actor terrorists in the United States and Europe, who were either convicted of such a crime or died during it, finds that they often expressed their extremist beliefs, grievances, and sometimes their violent intentions to others -- mostly friends and family or online communities. The good news should be that family, friends, and colleagues might be able to help prevent those close to them from engaging in political violence if, as a society, we were to adopt strategies that built trust of law enforcement in the public, particularly affected communities, rather than fear and suspicion. (But given the record these last years, don't hold your breath.)
On the other hand, the methods that the police and national security state seem to be exploring to deal with the issue -- like trying to determine what kinds of individuals will join terrorist groups or profiling lone wolves -- won't work. The reasons individuals join terrorist groups are notoriously complex, and the same holds true for politically violent people who act alone. After reviewing those 119 lone-wolf cases, for example, the researchers concluded, "There was no uniform profile of lone-actor terrorists." Even if a "profile" were to emerge, they added, it would be essentially worthless: "[T]he use of such a profile would be unwarranted because many more people who do not engage in lone-actor terrorism would share these characteristics, while others might not but would still engage in lone-actor terrorism."
As a group, such solitary terrorists differ from society at large in one crucial way: almost one out of three had been diagnosed with a mental illness or personality disorder before engaging in political violence. Another study concentrating on 98 U.S. perpetrators found that approximately 40% had recognizable mental health problems. The comparable figure for the general population: 1.5%.
Given such high rates of psychological disturbance, there's a chance individual attacks could be prevented if at-risk people got the mental health care they needed before they took a violent turn.
Fact vs. Fiction
Fortunately, what makes lone wolves so difficult to detect beforehand renders them more impotent when they strike.
Because such individuals don't have a larger network of financing and training, and may be disturbed as well, they are likely to have a far less sophisticated skill set when it comes to arming themselves or planning attacks. Terrorism researcher Ramon Spaaij of Australia's Victoria University created a database of 88 identified lone wolves who perpetrated attacks between 1968 and 2010 in 15 countries. What he found should dispel some of the fear now being associated with lone-wolf terrorism and so the increasingly elaborate and overzealous government planning around it.
Spaaij identified 198 total attacks by those 88 solo actors -- just 1.8% of the 11,235 recorded terrorist incidents worldwide. Since lone wolves generally don't have the know-how to construct bombs (as the Unabomber did), they usually rely on firearms and attack soft, populated targets, which law enforcement responds to quickly. Therefore, Spaaij found that the average lethality rate was .062 deaths per attack while group-based terrorists averaged 1.6 people per attack.