Verified by 
Key points
- Most animals are reactively aggressive, but humans are often proactively aggressive.
- Reactive aggression has some obvious signs, but proactive aggression doesn't.
- Humans evolved to be curious about the motives of dangerous people.
- Manifestos offer a window into the mind of a dangerous person.
Each time a mass shooting is covered by the news, the first question people ask is: why?
They don’t want to know why for legal culpability. There’s no legal loophole for a school shooter by which they can be found innocent. People also don’t want to know why for reasons of moral culpability. Nobody would find a reasonable excuse for someone who marched through a school and shot innocent children.
Yet, people still want to know why the killer did what they did. They want to know the mindset, the thoughts, and the beliefs that the killer possessed. This is clear from news stories as well.
The first big question? The killer’s motive.
If there’s a manifesto, many people are typically intrigued enough to read it, despite its moral irrelevance or the disgust they may feel while reading it. It’s not just the public—the FBI also wants access to a manifesto if one is available. It’s considered an important piece of evidence, even when there’s no question about guilt.
Proactive Aggression in Humans
Humans often engage in something called proactive aggression. This differs from the typical reactive aggression we see in other animals because it involves planning and doesn’t immediately occur in response to a transgression.
Humans are fairly unique in their propensity for proactive aggression. While reactive aggression has been around for hundreds of millions of years, proactive aggression has only been around for a few hundred thousand years. Prior to the advent of language, humans also relied largely on reactive aggression to settle disputes and respond to transgressions. Because reactive aggression is evolutionarily old and tied closely to size and strength, there are reliable signals that someone is more likely to be reactively aggressive.
In humans, reactively aggressive males have a short temper and lash out in response to fear or anger. They behave dominantly in social situations. You might even be able to picture what this type of male looks like in your mind. Try to picture his face. What do you see?
As it turns out, people are in some agreement about what reactive aggression looks like. Males with higher facial width-to-height ratios (fWHR) are perceived by others to be more aggressive, and perhaps for good reason. Males with higher fWHR self-report being more aggressive and dominant in social situations. And their self-reports appear to be accurate. Studies show that males with higher fWHR do, in fact, compete more aggressively.
We’ve evolved to signal formidability to others and to understand those signals of formidability. This helps our species navigate violent interpersonal conflict in the most efficient way.
However, all of this was thrown out the window when proactive aggression entered the scene. With the advent of language and the expansion of the prefrontal cortex around 300,000 years ago, humans developed the ability to plan their aggression. While a more formidable foe wins in a fair and direct fight, getting the jump on someone is a massive advantage in conflict. The sudden appearance of proactive aggression in human conflict meant that a less formidable foe could prepare and catch their target off-guard. Instead of a propensity for aggression being worn on the bone structure of the face, it was now hidden away in the mind, festering until the time was right to strike.
Morbid Curiosity and Motivations of Dangerous People
Proactive aggression set the stage for a new selection pressure to emerge. In order to predict when others would be aggressive, humans now needed to know something about the motivations that drive the aggressive behavior in other people. We needed information about when and why someone might become aggressive. Much of this is locked away in the mind of the dangerous person, with a few clues to their true nature sometimes slipping through in their day-to-day actions.
Since proactively aggressive people aim for discrepancy, it’s difficult to learn the signals of who might be aggressive until after they’ve already acted on their aggression. We can really only learn about proactive aggression retroactively. This may be why true crime stories are so popular. When we hear about someone who plotted a murder, we become curious about their motivations and their reasoning because it can help us collect clues for our rolodex of wariness. When a killer writes a manifesto that clearly lays out their motivation and reasoning, our mind sees this as a gold mine of critical information. The clues are coming straight from the source.
In my research, I've found that curiosity about the minds of dangerous people is the most common form of morbid curiosity. This is true across countries, ages, and genders. In fact, I have even used a paragraph from a killer’s manifesto as an experimental stimulus. In the study, I asked people if they’d rather read an excerpt from a Nobel Prize winner’s speech or an excerpt from a serial killer’s manifesto. Unsurprisingly, the manifesto was chosen by many participants, particularly by those higher in trait levels of morbid curiosity.
This powerful hold over our curiosity is a key reason why we frequently want to see a killer’s manifesto, despite how we feel about the killer. In fact, the more awful the scenario, the more interested we are likely to be—not because we think we will find exculpatory evidence, but because evolution has crafted our desire to understand the darker side of the human species that is hidden in the shadows of the mind.
