A serial killer is typically defined as someone who murders three or more people over a period of time, with a cooling-off interval between each murder, and without a single, impulsive burst of violence that would characterize a spree killing. This operational definition is widely used by investigators like the FBI to distinguish serial killings from mass murder and spree murder.

Core criteria commonly cited

• At least three victims, separated by time gaps, not committed in a single event.

• A discernible pattern or modus operandi that evolves across killings, indicating planning and intent.

• Preference for vulnerability in victims and a psychological distance or control dynamic the killer seeks to maintain.

Common factors researchers discuss

• Psychological factors: many serial killers exhibit antisocial traits, neurodevelopmental risks, and difficulty regulating emotions, with a tendency to compartmentalize or rationalize their actions. Loss of empathy or moral disengagement can be part of the pattern.

• Behavioral patterns: some typologies emphasize motives such as power/control, sexual/erotic components, or financial gain, though not all killers fit a single category.

• Social and developmental context: histories of trauma, unstable family environments, and early exposure to violence are frequently noted in case studies, though they do not determine behavior in a simple cause-effect way.

Important caveats

• Definitions vary by jurisdiction and agency, so terminology can differ (e.g., serial killer vs. spree killer vs. mass murderer). The three-victim threshold with cooling-off periods is a conventional baseline, but not universal.

• Public discussion and media portrayals often oversimplify, overlooking the diversity of backgrounds, motivations, and psychological profiles among individual cases.

If you’d like, I can tailor this to a specific region’s definitions, or summarize key academic theories (e.g., trauma, personality pathology, social learning) with examples from notable cases.