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Aggressive behavior is exceptionally costly and has inspired decades of research. Over the years, scientists have largely agreed upon the following definition of aggression: "Any intentional attempt to harm another person who does not want to be harmed."
Let's break this down. First, aggression must be intentional. This means that accidentally hurting others does not count as aggression. Why this distinction? Because accidents happen, and aggression researchers are interested in hostile acts that are motivated by the intent to harm. This focus on intent is reflected in our judicial system, in which unintentional harm is met with far more lenient sentencing.
Second, an aggressive act only has to be attempted, even if the attempt is unsuccessful. For example, swinging your fist at someone is still an aggressive act, even if you don't land the blow. What determines the success of an aggressive act is largely a product of circumstance. Although it's important to render aggressive attempts unsuccessful, we're better off studying how to make people not even try to hurt others in the first place.
Another important corollary of the word "attempt" here is that aggression is a behavior. Indeed, aggression is not an invisible psychological process, such as an emotion (for example, angry feelings) or cognition (for example, hostile biases). Aggression is an act that requires some behavior on behalf of the perpetrator.
Third, the intent has to be to harm the person. This is the core aspect of aggression, in which harming the target is a motive underlying the act. Other motives can be present, but harm-doing must be among them. Harm can take many forms, from physical injury to a deep psychic wound to a loss of social status or employment. What kind of harm is intended is important for determining the sub-type of aggression that took place, but as long as harm is the goal, then it's aggression.
Fourth, aggression must be directed at another person. Individuals can also harm themselves or inanimate objects, but neither of these counts as aggression. Non-suicidal-self-injury (NSSI) and suicidal behavior do involve harming a person, but the target being the self has meant that they are not studied under the same conceptual umbrella as aggression (though they are correlated). Further, harming inanimate objects does not count as aggression because the harm done is not reflected in a mind and is less costly than injuring a precious human life.
Fifth, the victim must not want to be harmed. Consensual acts of harm-doing abound, which are readily showcased in shows such as Jackass. Aggression does not count here, as it is the desired outcome.
However, people seem to weigh this consideration much less when they decide whether a potentially-aggressive act is deemed immoral or not. Indeed, it would still be a violent crime to kill or maim someone, even if you had a signed affidavit that the victim consented to the act. Investigators seeking to establish this final definitional criterion of aggression must also ensure that it meets modern definitions of consent.
These five dimensions of the definition of aggression are important and have been adopted by the field because they provide applicable rules-of-thumb that define what is and isn't aggression. However, even with these hard and fast rules, there are still many cases in which this current definition does not resolve whether an act is a true aggression or not.
Lingering Questions
What about harm done to non-human animals? It remains undetermined whether non-human animals would fall under the aggression umbrella, or if harming them is something distinct from aggressing against humans. My position is that this distinction is arbitrary, and the definition of aggression should be expanded to include non-human animals. Though should this rule apply to all lifeforms? If someone applies an anti-bacterial agent to their kitchen counter, murdering untold numbers of micro-organisms, should this count as aggression? Where should we draw this line?
What about harm done to human-like inanimate objects? With the advent of robotics and artificial intelligence, many inanimate objects become more and more human-like. Many households contain an Alexa or Siri or Google Home that people can interact with and have feelings towards. As robots and virtual entities become more and more like us in both their appearance and capabilities, will harming them ever count as aggressive behavior?
What about self-defense? According to this definition of aggression, protecting yourself from an attacker by harming them counts as an aggressive act. This is problematic as we often think of acts of self-defense as fundamentally different from the maliciously-motivated acts of harm-doing that we construe when we think of prototypical acts of aggression.
Yet if we exclude acts of self-defense from the conceptual aggression umbrella, we run into yet another problem. Many people who have committed violent acts have believed, to some extent, that they were protecting themselves. Military actions are often based on claims that they supposedly pre-empted and prevented an imminent attack. As you can see, excluding acts of self-defense from aggression would severely complicate the determination of whether they are aggressive or not.
What about defending or avenging others? Many harmful acts are motivated by a desire to protect others from harm or to avenge the harm that has already been done to someone else. Many people would say that such acts fall outside of the maliciously-motivated behaviors that typically qualify as aggression. Such prosocially-motivated behavior surely is different, right? Maybe not. Antisocial and prosocial motives are not mutually exclusive, and they can often co-occur.
Think of the parent who punishes their child in the hopes that it will correct problematic behavior. They are currently harming their child, but the overall intent is prosocial. Terrorists often commit heinous acts of violence that they claim are driven from a desire to protect their ingroup from oppressive forces. Serial killers have even justified their homicidal acts as a form of protection for others. If we exclude aggressive acts that coincide with the perpetrator's prosocial motivations, we may have to discard a substantial portion of what we currently deem as "aggressive behavior."
What if the victim doesn't detect the harm? If harm happens to a person, and the person doesn't realize it, was it still aggression? The conventional definition of aggression would say that this consideration is tangential. However, if an act of harm is so ineffectual that it doesn't even register with the victim, does it still count as aggression?
What about victims with whom the perpetrator shares considerable self-other overlap? The self-other distinction is crucial for determining what counts as aggression and what doesn't. However, the distinction between the self and others becomes blurry between people in close relationships (parents and children, spouses, etc.) and people from more interdependent cultures. For many of our relationships, the self-other distinction does not hold true. So at what point does harming a close other become a form of self-harm? And does this disqualify the behavior as "aggressive"?
What about coerced acts of harm-doing? Imagine that someone has a gun to your head, puts another gun in your hand, and orders you to shoot someone, or they will shoot you. If you complied, even just to save your own life, this would be considered an aggressive act.
Think about child soldiers who were stolen from their families and were subsequently indoctrinated to violently harm others. Are their actions in combat truly aggressive? Our current definition of aggression does not correct for situations in which free will has been constrained, and choices are made that do not reflect our own true intentions.
On one side, these lingering issues are bewildering and frustrate the desire for a unified, parsimonious definition of aggression. On the other hand, such conceptual challenges are bound to happen for such a complex behavior as aggression. These areas of ambiguity should inspire more research into whether these boundaries that we draw between aggression and other concepts are useful and valid. With sharper tools (that is, clearer definitions of what counts as aggression and what doesn't), we can hope to more precisely and effectively excise the tumor of aggression from our society.
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