The Body Type Theory, also known as the Somatotype Theory, is one of the earliest biological explanations of criminal behavior. It was proposed by William Sheldon in the 1940s, who believed that a person’s body shape was linked to their personality and likelihood of committing crime.

Although the theory is now considered outdated, it played an important role in shaping early criminology and the study of biological influences on behavior.

What Is Somatotype Theory?

William Sheldon suggested that people can be divided into three main body types, and each body type is connected to specific personality traits. According to Sheldon, these traits can influence whether someone is more likely to engage in criminal behavior.

1. Endomorphs

Body Type: Soft, round, heavy, with more body fat.

Personality Traits (Viscerotonia):

  • Relaxed and calm
  • Love comfort and food
  • Sociable and friendly
  • Tolerant, easy to get along with
  • Communicate emotions easily

Criminal Tendency: Sheldon believed endomorphs were least likely to commit crimes because they preferred comfort and social connection over risky behavior.

 

2. Mesomorphs

Body Type: Muscular, strong, athletic, and physically powerful.

Personality Traits (Somatonia):

  • Assertive and confident
  • Adventurous and energetic
  • Enjoy power, control, and dominance
  • Like taking risks
  • Sometimes aggressive and ruthless

Criminal Tendency: Sheldon believed mesomorphs were most likely to commit crimes, especially violent or aggressive crimes, due to their strength, confidence, and risk-taking nature.


3. Ectomorphs

Body Type: Thin, fragile, delicate, and lightly muscled.

Personality Traits (Cerebrotonia):

  • Quiet, introverted
  • Prefer privacy and solitude
  • Highly sensitive
  • Easily stressed or anxious
  • Less social

Criminal Tendency: Ectomorphs were believed to be least physically capable and emotionally too cautious to commit crimes.

 

Is Body Type Enough to Explain Crime?

Today, scientists agree that body type alone cannot determine criminal behavior. However, Sheldon’s theory opened the door to studying biological and genetic factors, which modern research explores in much more depth.

This leads to the next major question: How Do Genetics Influence Criminal Behavior?

Modern criminology looks beyond body type and focuses on genes, environment, and their interaction. Several areas of research help explain how genetics may contribute to antisocial or aggressive behavior.

1. Behavior genetics examines how both genes and the environment shape human behavior. Research shows that about 50% of antisocial behavior is influenced by genetic factors, while 15–20% comes from shared environmental conditions such as family life. Importantly, genetics do not act alone—supportive and caring parenting can reduce the genetic risk for antisocial behavior, while abusive or harsh environments can increase it.

2. Studies involving more than 77,000 twin families reveal that genetics play a major role in the development of antisocial behavior. At the same time, environmental factors such as parenting style and peer influence have a stronger effect on children who are genetically at risk.

3. Adoption studies provide clear evidence that children with antisocial biological parents are more likely to show antisocial behavior themselves. However, this genetic risk becomes even stronger when these children grow up in stressful, abusive, or unstable environments.

4. Modern molecular genetics has identified specific genes related to aggression. One of the most studied genes is the MAOA gene, often called the “warrior gene.” Individuals with the low-activity version (MAOA-L) may show higher aggression, but only when this genetic factor combines with childhood abuse or trauma. This means genes require environmental triggers to influence behavior. Other genes have also been found to affect the prefrontal cortex—the part of the brain responsible for impulse control and decision-making. When this area is damaged or functions poorly, it can lead to impulsive or violent behavior.

5. Earlier research suggested that men with an extra Y chromosome (XYY) had a higher tendency to engage in violent behavior. Later studies corrected this misunderstanding by showing that although XYY men appear more frequently in prison populations, they are mostly involved in non-violent crimes. The confusion arose because XYY men often face social disadvantages: they tend to be very tall, may have slightly lower IQ scores, and often experience social difficulties. These challenges—not genetics alone—may increase the risk of criminal involvement. Other chromosomal conditions, such as XXYY, also show links with antisocial tendencies, but again, environmental influences play a major and decisive role.

 

References

  1. Sheldon, W. H. (1940). The Varieties of Human Physique: An Introduction to Constitutional Psychology. Harper.
  2. Rhee, S. H., & Waldman, I. D. (2002). Genetic and environmental influences on antisocial behavior: A meta-analysis of twin and adoption studies. Psychological Bulletin, 128(3), 490–529.
  3. Stadler, C., Kroeger, A., & Clement, H. (2010). Molecular genetics and aggression. Neuroscience and Biobehavioral Reviews, 34(1), 4–6.
  4. van der Pol, L., & Boomsma, D. I. (2018). Twin studies in antisocial behavior. Current Opinion in Psychology, 27, 76–81.
  5. Witkin, H. A., Mednick, S. A., & Schulsinger, F. (1976). Criminality in XYY and XXY men. Science, 193(4253), 547–555.