Using Medical Science to Reform the Heat of Passion Defense

A defendant's unique brain structure and circumstances should be examined before arguing for the application of the "heat of passion" defense to homicide.

Photo by Matt Hobbs (Flickr)

During the Spring 2015 semester, the Campbell Law Observer held the first annual CLO Law and Medicine Writing Contest. This entry was selected as the runner-up by a panel of Campbell Law School faculty members.

BY: Max Ashworth, Campbell Law School

Max Ashworth

Max Ashworth

“A [criminal] defendant charged with murder may invoke the heat of passion defense and attempt to show that he was significantly provoked and, as a direct result, became emotionally charged, and killed his victim while in this uncontrolled state.”1  From its inception, the heat of passion defense has defined the “uncontrolled state” as an uncontrolled emotional state.  Thus, the heat of passion defense requires the fact-finder to determine a defendant’s culpability by inferring emotions.

Until recently, the debate concerning the heat of passion defense has focused on its merits and not on the events taking place inside a defendant’s brain at the moment he enters an “uncontrolled state” of aggression.  Due to neuroscience’s recent revelations, now is a prudent time to explore the physiology and psychology of a defendant who becomes so overwhelmed that he takes the life of another.2  Specifically, neuroscience and behavioral science allow us to learn more about the physical, tangible operating system of a defendant, his brain. 

The implications of neuroscience and behavioral science mandate amending the heat of passion defense.  Specifically, courts should use the Model Penal Code’s plain language and abandon—or, at least, amend—the heat of passion’s reasonable person requirement.

Heat of Passion Homicide

The Model Penal Code states that a criminal homicide is murder, manslaughter, or negligent homicide.  A murder occurs when the defendant commits a homicide “purposely or knowingly; or [when the murder] is committed recklessly under circumstances manifesting extreme indifference to the value of human life.”3  A defendant convicted of murder has committed a first-degree felony, commonly expressed as first-degree murder.  A defendant has reduced culpability, and is only guilty of a second-degree felony, when he perpetrates manslaughter.4  Manslaughter occurs when a defendant commits:

“A homicide…under the influence of extreme mental or emotional disturbance for which there is reasonable explanation or excuse. The reasonableness of such explanation or excuse shall be determined from the viewpoint of a person in the actor’s situation under the circumstances he believes them to be.”

In practical application, courts have supplanted a reasonable person requirement in lieu of the “reasonable explanation or excuse” language.5  As a result of abandoning the MPC’s plain meaning, defendant-specific factors, such as mental dysfunctions, are ignored and the legal fiction that is the reasonable person pervades the heat of passion defense.

When a defendant successfully invokes the heat of passion defense, he is culpable for the underlying homicide, but his punishment is reduced to a second-degree felony. In general, there are five elements a defendant must prove in order to show the homicide was committed in the heat of passion: (1) the defendant was provoked; (2) a reasonable person subject to the same provocation would have lost control of himself; (3) as a direct result of the provocation, the defendant “experienced uncontrollable rage or other extreme emotional disturbance;” (4) the defendant had not “cooled off;” and (5) a reasonable person would not have cooled off between the time of the provocation and the homicide. 6

Retributive philosophy and penal proportionality gave rise to the heat of passion defense. 7  Penal proportionality mandates that a defendant should be punished in regards to the culpability of his crime.  When the heat of passion defense is invoked, the defendant’s “emotional response to the provocation may be viewed to be so extreme that it diminishes his ability to think rationally and…prevents him from acting with malice aforethought and cold-blooded premeditation.” 8  In essence, the emotional response that the defendant undergoes while he kills must “be substantial enough that it prevents rational thought and reasoned behavior.”9

Combined with the reasonable person requirement, entrusting a jury with such a subjective consideration—inferring emotional states—can produce injustice on two levels: primarily, application of the heat of passion defense could “partially excuse[ ] reactive killings that are relatively more dangerous, cruel, and ferocious than those that are premeditated;” or the defendant could be susceptible to underlying cognitive dysfunctions that cause uncontrollable rage more frequently than a reasonable person. 10  As a consequence, the heat of passion defense can be exploited to downplay a defendant’s actual culpability, and individuals who are subjectively, but not objectively, provoked are not able to take advantage of the heat of passion defense.  Both of these potential pitfalls could be avoided, however, if courts utilized neuroimaging technology to aid the jury’s determination of a defendant’s emotional state.

Neuroimaging Technology in the Courtroom

The advance of medical technology now allows courts to peek inside the human brain.  Neuroimaging maps brain activity by “detecting and measuring specific signals of some property related to the brain with” highly advanced detection devices.  The detection devices capture three types of brain activity: electrical activity, radioactive events, and/or “magnetic dipoles.”11  Once the brain activity is recorded, an algorithm will project the activity onto a computer-generated brain model.12 Regardless of the type of signal captured, neuroscientists can discern distinctive patterns based on the signals a brain exhibits.13  These patterns can then be compared with the patterns of other brains.14 Upon making this comparison, anomalies in brain activity can be identified.  Specifically, individuals with a cognitive dysfunction will exhibit brain activity patterns different than individuals who are cognitively normal. 

The two most frequently used detection devices are position emission tomography (“PET”) and functional magnetic resonance imaging (“fMRI”).  Gathering data on “glucose metabolism, oxygen metabolism, and cerebral blood flow,” PET devices create a computer model and allow neuroscientists to see which portions of the brain are most active when a brain is subjected to external stimuli.  fMRI devices work in a similar fashion but gather data on hydrogen atoms in the brain.15  Observing the hydrogen atoms in blood, fMRI devices allow neuroscientists to map which portions of the brain are the most active when a brain is subjected to external stimuli.  Based on an analogical model, neuroscientists determine how a subject’s brain would react when the subject was confronted with external stimuli, such as a provoking event preceding a homicide.

Using PET and fMRI devices, neuroscientists can ascertain mental and emotional anomalies by examining the frontal lobe.  A cognitively dysfunctional individual exhibits an abnormal degree of brain activity in the frontal lobe.16 “Frontal lobes are anatomical masses of the brain located at the anterior of the skull behind the forehead.” 17  These anatomical masses “regulate socially appropriate behavior and suppress impulses.”18 Socially appropriate behavior and impulse suppression help control emotional responses to provoking events.  Thus, frontal lobe analyses offers neuroscientists “a biological glimpse into one’s behavior” in response to provoking events.

Pseudopsychopathy is the most prominent frontal lobe defect in murderers and results in impulsive behavior and irrational responses to social interactions.19 Disregarding wrongfulness, an individual suffering from pseudopsychopathy will respond disproportionately to environmental stimuli.  More importantly, a psuedopsycopathic individual is: unable to regulate emotions effectively; more likely to react violently when faced with a disadvantageous social interaction; and unable to control his emotional responses. 

A psuedopsycopathic experiences an emotional state different than a cognitively normal person when faced with the same provoking event.  Since the heat of passion defense relies on a reasonable person requirement, it fails to address scenarios when frontal lobe dysfunctions render a defendant more susceptible to violent emotional states. More specifically, whereas a cognitively normal person may not kill in a certain scenario, a psuedopsycopathic may resort to homicide in the same scenario. The reasonable person requirement is not applicable to the psuedopsycopathic because, quite simply, he is incapable of acting like a reasonable person.20

Disregarding frontal lobe anomalies results in injustice when a defendant is incapable of conforming to the MPC’s reasonable person requirement. The heat of passion defense does contain other elements that focus on the defendant’s subjective emotional state, but the crux of this mitigation theory is the reasonable person requirement.21 At the very least, the reasonable person requirement should be adjusted to account for a highly impulsive defendant or an emotionless defendant.  In this way, a defendant is not punished or rewarded for brain defects affecting his behavior.  While neuroimaging can shed light on a defendant’s emotional state, another field of study, behavioral science, can provide insight into a defendant’s emotional state and mental state.22

Behavioral Science in the Courtroom

Social-information processing (“SIP”) is a prominent behavioral science model that  “explains aggressive behavior as the behavioral result of multiple series of mental operations that are potentially active in response to social stimuli.”23 SIP contends that all individuals assess social stimuli through the same process.  Perceptions of social information are evaluated “in light of past experiences that are stored in and accessed from memory.”24 However, individuals prone to reactive homicides have difficulty encoding and interpreting social stimuli.  This difficulty is due to a culmination of negative life experiences that can trigger disproportionate responses to social interactions.  “For example, an individual who was victimized in his youth may develop a tendency to react with hostility to situations that involve ambiguously negative treatment of a child.”25

Individuals who kill in response to a provocation may “attribute harmful intent, hostility, and moral wrongfulness to stimulus actors in ambiguous provocation situations.”26 Studies have also shown that reactive killers “interpret benign cues as hostile.”27 These unjustified interpretations result from an inaccurate determination that provoking actors are intentionally causing harm to the reactive killer.  Such unwarranted conclusions are a result of brain development amidst an extremely hostile environment. 

Psychologists have developed a methodology to determine whether an individual has an SIP dysfunction.28 Specifically, an SIP dysfunction exists when two criteria are satisfied: attributing hostility to an actor who accidentally caused harm and attributing ill motives to an actor who has a benign motive.  These two criteria can be identified through psychological testing in which the test subjects are presented with hypothetical scenarios.  Individuals who exhibit these symptoms will show a pattern of “aggressive behavior that is typified by chronic fighting, being easily angered, and other characteristics that are associated with reactive violence.”29 SIP dysfunctions most often afflict those who suffer from “chronic social rejection” and those who spend a lot of time in communities with high rates of violent crime.  Akin to individuals suffering from a frontal lobe dysfunction, the reasonable person test may produce injustice when it is applied to an individual with an SIP dysfunction—i.e., a reasonable person may not react with violence but one suffering from an SIP dysfunction may.

On the other end of the spectrum, an SIP dysfunction can also manifest itself in a subdued manner.30 For instance, an “individual who is faced with an unambiguously horrific provocation but does not interpret it as such…may be far less likely to respond” with violence.31 This type of dysfunction is also troubling for the heat of passion defense.  Like the individual who has an abnormally inactive frontal lobe, those with a subdued SIP dysfunction can exploit the heat of passion defense and escape increased culpability.  Whereas a reasonable person may become violently provoked upon witnessing an act of child abuse, an individual with a subdued SIP dysfunction may commit a homicide even though he was not personally enraged. 

“Those who have firsthand knowledge of the defendant” can help the court determine whether the defendant “has a history of difficulties with respect to mindful, valid understandings of social events.”32 Specifically, psychologist can provide “testimonial evidence…regarding whether there exist[s] a persistent pattern or bias with respect to how the defendant interprets…ambiguous social events.”  Courts should utilize these experts to determine whether a defendant’s emotional and mental states suffer from an SIP dysfunction causing a propensity for unnecessary, violent reactions. If the court concludes the defendant is afflicted with an SIP dysfunction, then the reasonable person standard—and, as a corollary, the heat of passion defense—will be prone to injustice. However, a defendant hoping to utilize these theories must first overcome the issue of causality.


Neuroimaging evidence along with behavioral science evidence is subject to causality problems in the context of the heat of passion defense.33 Specifically, individuals suffering from emotional and mental dysfunctions may “still ha[ve] the ability to restrain [their] actions, because ‘the mind almost always has veto power over what the brain decides.’”

One theory suggests that the mind—or consciousness—acts as a middleman between the brain and the body; once the brain instructs the body to act, the mind is capable of restraining that action.34 In this scenario, frontal lobe and SIP dysfunctions could be managed and the dysfunctional brain would be capable of reasonable person decision-making.  Although neuroscientists disagree on the causal relationship between mental afflictions and criminal behavior, the scientific community unanimously agrees that emotional and mental dysfunctions are correlated to “certain manifest behaviors.”35

A prominent neuroscientist, Michael Gazzaniga, believes “that most people with brain damage or defects have some ability to restrain their actions.” (emphasis added).  Gazzaniga supports his contention by pointing to the case of Phineas Gage, a man who suffered a frontal lobe injury that drastically altered his personality.  Although Gage lost his ability to plan ahead and respond appropriately in social interactions, Gage did not result to criminal behavior.  Gazzaniga believes that Gage proves the mind has a veto power over the brain—i.e., your brain decides to punch someone before your fist moves, and your mind has the power to stop the flying fist.  Noting his expertise in neuroscience and not the law, Gazzangia sees science as a means to inform society, and issues with criminal responsibility should be “resolved by society and the law.” 

Unlike Gage, an un-named Virginia man suffered from a frontal lobe dysfunction when a tumor starting growing in his brain.  The otherwise normal school teacher suddenly became addicted to child pornography and was convicted of sexual offenses after he tried to sexually assault his stepdaughter.  Before he was sentenced, the man went to the hospital and told doctors that he could not control himself and was afraid he was going “to rape his landlady.”  Neurologists examined the man and discovered the large tumor growing on his frontal lobe.  The doctors noted such a distinct change in personality once the tumor was removed that they suggested the man was fit to return to society.  The sentencing court heeded the medical opinion and suspended the man’s sentence.”  The neurologists in this case were able to establish a direct causal link between the frontal lobe dysfunction and the impulsive criminal behavior.  While not all emotional and mental dysfunctions result in criminal behavior, there are cases, such as this one, in which a causal link can be established.

Although the issue of causality is currently in limbo, neuroscience and behavioral science are courtroom necessities.  When mentally afflicted individuals, such as Gage, commit criminal acts, science is necessary to disprove the causal link between the dysfunction and the criminal behavior.  On the other hand, when a mental dysfunction does cause criminal behavior, medical science is necessary to prove causality.

Including these sciences in courtrooms will result in an enhanced degree of equity—culpable defendants will not escape proper punishment and dysfunctional defendants, such as the man from Virginia, will receive the appropriate rehabilitative care.  At the very least, courts should apply the heat of passion defense in accordance with the MPC’s plain language.  Upon doing so, the heat of passion defense will account for physiological and neurological abnormalities—an adjustment all defendants should be afforded.

Max Ashworth is a 3L at Campbell Law School who will graduate in May 2015. He can be reached by email at