## Introduction

“Serial killer” is a term used in the United States to describe an offender who commits multiple murders across separate events. While you may see “three or more victims” in older sources, current U.S. law enforcement practice—especially within the FBI—typically uses “two or more victims in separate incidents,” with a cooling-off period between crimes. This guide explains what that means, how serial killing differs from other types of multiple homicide, what common patterns researchers discuss, and brief case summaries you’ll likely see in textbooks and media coverage.

This article is for readers who want a clear, responsible overview that avoids sensational detail and keeps the focus on terms, patterns, and documented examples.

## What You’ll Learn

- How U.S. agencies define “serial killer” and how that differs from older definitions
- The difference between serial, mass, and spree killings
- Key terms: modus operandi (MO), signature, and victimology
- Common traits discussed in research and what they do—and do not—mean
- Short profiles of Ted Bundy, Jeffrey Dahmer, and Aileen Wuornos
- Practical uses for students, educators, and informed readers
- A quick-reference table with core terms and sources

## Core Concepts

### Definition and U.S. Criteria

- Current FBI practice: Two or more murders committed as separate events, often by one person, with time passing between crimes. This “cooling-off period” can range from days to years.
- Older definition you may still see: Three or more victims over more than a month, with significant time between killings.
- Why the change matters: The two-victim standard helps investigators link cases earlier and encourages consistent reporting across jurisdictions.

Important note: “Serial killer” is a descriptive, investigative term. It is not a medical diagnosis or a charge by itself. Prosecutors still bring specific counts of murder based on state or federal law.

### Serial vs. Mass vs. Spree

- Serial murder: Multiple killings in separate incidents, with a cooling-off period.
- Mass murder: Four or more victims killed in a single incident, generally at one location (some jurisdictions may use a threshold of three).
- Spree killing: Multiple victims in different locations in a short period, without a clear cooling-off break.

Understanding the differences helps analysts group cases correctly and apply the right investigative strategies.

### Modus Operandi, Signature, and Victimology

- Modus Operandi (MO): Practical methods used to commit the crime and avoid detection. MO can change as an offender learns or circumstances shift.
- Signature: Behaviors that serve a psychological or emotional need, separate from what’s required to commit the crime. Signatures are more stable than MO and help link cases.
- Victimology: The study of victim characteristics (age, gender, lifestyle, occupation, routine). Patterns in victim choice can guide case linkage and risk assessment.

### Psychological and Behavioral Patterns

- No single profile: Serial killers are not all the same. They come from different backgrounds and show a range of behaviors.
- Traits often discussed in research: Antisocial behavior, lack of empathy, manipulativeness, fantasy life, and, in some cases, sexual deviance. Some have histories of childhood abuse, neglect, or head trauma, but many people with those histories never commit violent crimes.
- Stressors and triggers: Life events (job loss, relationship breakdown, legal trouble) sometimes precede an offense, but these are not causes on their own.
- Diagnoses: Some offenders meet criteria for disorders such as antisocial personality disorder or exhibit psychopathic traits; others do not. Diagnosis requires clinical evaluation and should not be inferred from limited information.

Takeaway: Behavioral concepts can help make sense of patterns, but they do not predict individual actions and should be used cautiously and ethically.

## Key Examples or Case Studies

- Ted Bundy (1974–1978, several U.S. states)
  - Number of victims: Confessed to 30; true total remains debated.
  - Typical pattern: Approached young women in public places, used charm or pretexts, then abducted and killed them.
  - Why studied: Case illustrates targeted victim selection, organized planning, and interstate offending.

- Jeffrey Dahmer (1978–1991, Wisconsin and Ohio)
  - Number of victims: 17 confirmed.
  - Typical pattern: Lured young men to his residence, where he killed them; he also committed additional crimes with the bodies, including cannibalism.
  - Why studied: Case highlights issues of victim vulnerability, missed intervention opportunities, and apartment-based crimes that complicated detection.

- Aileen Wuornos (1989–1990, Florida)
  - Number of victims: 7 confirmed.
  - Typical pattern: Shot men at close range; she stated the killings were in self-defense while she was engaged in sex work.
  - Why studied: One of the rare female serial offenders; case raises questions about victim-offender interaction, motive, and public narratives.

These cases recur in courses and documentaries because they document different offender types, victim pools, and investigative challenges. They should be discussed with respect for victims and without glamorizing offenders.

## Practical Applications

- For students and educators
  - Use consistent definitions (FBI/DOJ language) when writing papers or teaching.
  - Compare serial vs. mass vs. spree cases to avoid mixing categories.
  - When discussing psychology, distinguish between research findings and what can be said about a specific person without a full evaluation.

- For informed readers and journalists
  - Be careful with labels. Avoid calling an offender “serial” until there is evidence of separate incidents with a cooling-off period.
  - Avoid unnecessary detail that sensationalizes harm. Focus on verifiable facts, investigative steps, and prevention lessons.

- For public safety and community programs
  - Encourage reporting of suspicious activity to local law enforcement when immediate safety concerns exist.
  - Share crime prevention basics: meet strangers in public, let others know your plans, verify identities for online meetups, and use available campus or community safety resources.

- For policy and research discussions
  - Consider how data systems (like the FBI’s ViCAP) help link cases across jurisdictions.
  - Support victim services and cold case resources, which can improve both prevention and resolution.

## Summary Checklist

- Know the current U.S. practice: two or more murders in separate events with a cooling-off period.
- Distinguish serial, mass, and spree killings by timing and location.
- Separate MO (practical method) from signature (psychological need).
- Use victimology to understand patterns, not to blame victims.
- Recognize there is no single “profile” and no predictive checklist.
- Discuss well-known cases without glamorizing offenders or sharing graphic detail.
- Rely on reputable sources (FBI, DOJ, peer-reviewed research, court records).
- Remember that serial murder is rare; most homicides do not fit this pattern.

## Quick Reference

| Concept       | U.S. Source/Context        | Key Takeaway                                                 |
| ------------- | -------------------------- | ------------------------------------------------------------ |
| Serial murder | FBI (NCAVC)                | Two or more murders, separate events, with a cooling-off gap |
| Mass murder   | FBI/DOJ                    | Multiple victims in a single incident, usually one location  |
| Spree killing | FBI                        | Multiple victims in a short period, no clear cooling-off     |
| MO            | Investigative practice     | Practical method; can change with experience or circumstance |
| Signature     | Investigative practice     | Behavioral imprint serving an internal need; more stable     |
| Victimology   | Criminology/Investigations | Study of victim traits and routines to inform case linkage   |

Note: Definitions can vary slightly by jurisdiction and over time. When in doubt, cite the specific source (e.g., FBI publications or DOJ materials) you’re using.

Serial Killers: Definition, Traits, and Notorious U.S. Cases