Incarcerated men with sexual sadism show distinct anatomical brain traits

A recent study published in the Journal of Psychiatric Research reveals that men who have committed sexually sadistic crimes possess enlarged brain tissue in areas responsible for processing visual information and understanding the minds of others. These anatomical differences offer a biological window into an extreme form of violence. The physical brain traits identified by the researchers might eventually help medical and legal professionals better understand and assess the motivations behind severe sexual offenses.

The societal impact of these offenses is massive. The United States Department of Justice reports over half a million victims of sexual assault every single year. The individuals who commit these crimes are driven by a wide variety of motives, making them a diverse and complicated group to study.

Sexual sadism is a psychiatric condition where a person experiences sexual arousal and gratification by inflicting physical or emotional pain on another person. When this desire is carried out without consent, it often leads to devastating violence and lasting trauma for victims. The criminal justice system heavily penalizes these acts, often resulting in longer prison sentences.

The psychological traits of people who commit such crimes have been documented extensively. Very little is known about the underlying biology of the disorder. Forensic settings such as prisons are highly controlled environments, making it difficult to bring in advanced medical equipment to scan the brains of incarcerated people.

Individuals might also be reluctant to volunteer for community studies out of fear of exposing their illegal behaviors. Prior research relied on physiological tests, like measuring arousal when subjects viewed images of violence. Functional imaging studies, which measure blood flow in the brain to track activity, showed unusual responses in brain regions linked to emotion and empathy.

Those earlier studies measured the brain in an active state. They could not determine if the actual physical structure of the brain was different in people with the condition. The physical makeup of the brain is often referred to as gray matter and white matter.

Gray matter consists primarily of the main bodies of brain cells, which process information and generate signals. White matter acts as the communication network connecting these processing centers. Structural abnormalities in gray matter often relate to differences in how people perceive the world around them.

Carla L. Harenski, a researcher at The Mind Research Network in New Mexico, led a team to investigate these physical structures. Harenski and her colleagues wanted to map the neurobiology of severe sexual offenses to see if distinct physical traits accompanied the psychological diagnosis. Their goal was to examine whether these specific behaviors corresponded to inherent structural differences in the brain.

To conduct the study, the research team used a mobile magnetic resonance imaging scanner. This large piece of equipment uses strong magnetic fields to take detailed pictures of the inside of the body. They transported the scanner directly to state prisons and a secure treatment facility in the United States.

The researchers recruited 232 incarcerated men to participate in the scans. Exactly half of these men had been convicted of one or more sexual offenses. The other half had no history of sexual offense convictions, though most had been convicted of other violent crimes like assault or robbery.

The participants had to meet strict criteria to be included in the project. They were required to have at least a fourth-grade reading level and a standard intelligence score of 70 or higher. The researchers also excluded anyone with a history of psychotic disorders or major central nervous system diseases.

The team then needed to determine which of the sexual offenders actually met the criteria for sexual sadism. They used a specialized clinical scale that relies on detailed institutional records and crime scene reports. Two independent raters scored each participant based on behavioral indicators, such as the use of psychological torture, excessive physical violence, or ritualistic behaviors during the crime.

Nineteen of the participants met the formal threshold for a diagnosis of sexual sadism. This number represented about sixteen percent of the sexual offender group. The researchers then compared the brain scans of these nineteen men against the men who committed sexual offenses without sadistic traits, as well as the men with no sexual offenses.

Harenski and her team used two distinct analytical techniques to evaluate the brain images. The first technique is a multivariate approach, which looks at spatial patterns to identify entire networks of brain regions where gray matter volume changes together across different people. This method helps researchers see how different areas of the brain might be structurally linked.

The second technique is a univariate approach. This method divides the brain into tiny, three-dimensional cubes called voxels and examines them one by one. It allows researchers to pinpoint specific, isolated regions where gray matter volume differs between groups.

Both analytical methods pointed to the exact same anatomical differences. Men who scored high for sexual sadism possessed enlarged gray matter volume in the posterior temporal cortex, a region located near the back and sides of the brain. This physical enlargement was not present in the other two groups of incarcerated men.

Specifically, the researchers observed increased tissue volume in the temporoparietal junction. This region is located at the intersection of the brain’s temporal and parietal lobes. It plays a major role in social cognition, empathy, and the ability to distinguish between oneself and others.

The temporoparietal junction is heavily involved in mentalizing, which is the process of appraising the thoughts and feelings of other people. In a previous functional imaging study, Harenski found that this exact area became highly active when men with sexual sadism viewed images of people in pain. The new structural scans suggest that this heightened activity might be rooted in a physical overabundance of gray matter.

The brain images also revealed increased gray matter volume in the fusiform gyrus among the men with sexual sadism. The fusiform gyrus is located at the bottom of the brain. It is primarily responsible for high-order visual processing, including the recognition of faces and bodily expressions.

Prior studies on other mental health conditions have linked the fusiform gyrus to the way people process fearful body expressions. While its exact role in sexual violence is not entirely clear, a heightened visual focus on facial expressions and body language aligns with the nature of sadistic behavior. The researchers noted that these visual processing areas are likely engaged during the commission of the crimes.

The team also found higher tissue volume in the putamen, a round structure situated deep within the center of the brain. The putamen helps regulate movement and influence various types of learning. It is also involved in modulating the sensory and motor aspects of pain perception.

The researchers controlled for several external factors to ensure their results were strictly related to sadism. They matched the groups based on age, intelligence, and overall brain volume. They also accounted for psychopathic traits, which often overlap with sadistic tendencies but represent a distinct psychological profile.

Psychopathy is characterized by callousness and a lack of remorse, which are common among incarcerated populations. The team confirmed that the gray matter differences in the temporal regions were tied uniquely to sexual sadism rather than general psychopathy. The researchers also confirmed that past traumatic brain injuries or substance abuse disorders were not statistically significant factors in explaining the brain structures they observed.

The researchers did not find volume differences in areas of the brain strictly associated with basic emotional arousal or reward. Areas like the amygdala, which processes fear, did not show structural abnormalities. The physical differences were confined entirely to regions handling social cognition, pain perception, and visual processing.

The researchers acknowledged several limitations in their study. The men who participated were older, and many had been incarcerated for a long time. It is possible that different anatomical traits might be visible in younger individuals or those who are closer in time to their most recent offense.

It is also possible that some of the participants committed additional crimes that were never discovered. A person in the non-sexual offense group might have a hidden history of sexual violence. Undisclosed crimes could affect the accuracy of the group classifications.

The results also may not apply to other forms of sadistic behavior. The findings are specific to nonconsensual, violent sexual sadism. It is entirely unknown if similar brain structures exist in people who practice consensual sadomasochism or those who exhibit non-sexual everyday sadism.

Future research will need to explore exactly how these structural differences develop. Scientists hope to investigate the age at which these gray matter changes first appear in the brain. They also aim to study how these physical differences directly influence the specific cognitive processes that drive sexually aggressive behavior.

The study, “Increased gray matter within temporal cortical networks in sexual sadism,” was authored by Carla L. Harenski, Fadwa Cazala, J. Michael Maurer, Keith A. Harenski, Nathaniel E. Anderson, Jean Decety, Vince D. Calhoun, Kent A. Kiehl.