Synonyms: brain; neurotransmitters; genetic expression
Modern neuroscience has made significant and sometimes dramatic gains into understanding how the brain and central nervous system are implicated in a range of problematic, criminal, and addictive behaviors. Indeed, the growth of modern neuroscientific findings can only be described as explosive, with new discoveries being published almost daily. These findings are often produced by highly advanced technologies—technologies that, at a broad level, allow scientists to view the brain in action and under experimental conditions. This same technology also now allows scientists to directly measure brain cell activity and the strength and integrity of connections between brain cells. Never before in human history have scientists had these tools at their disposal.
Criminologists, once primarily wedded to sociological explanations of crime, are beginning to incorporate many of these findings into their understanding of the development of criminal conduct. They are also using these findings to understand the process of chemical addiction, how environmental experiences affect brain development, and how best to intervene with anti-social youths and adult criminals.
Apart from understanding brain structures and their functions, neuroscience has made tremendous progress in understanding how the chemicals in the brain, known as neurotransmitters, affect a range of personality traits and behaviors. This knowledge has led directly to the development of entire classes of psychopharmacological drugs. These drugs, known as selective-serotonin-reuptake-inhibitors (SSRI’s), have been used to effectively treat a range of mood and psychological disorders. New stimulants, too, have been created that help individuals better regulate their own conduct, including those diagnosed with attention-deficit-disorder. The point is, what neuroscience reveals to us about behavior, including criminal and imprudent behavior, may lead to better, more effective interventions.
Evolutionary and Developmental Background
The human brain is the most complex computational device known to exist. It controls all necessary life functions, takes over 20 years to fully mature, and allows human beings to exercise tremendous control over their behavior and their environment. The human brain is a marvel of complexity and has been subject to evolutionary selection pressures for at least 250,000 years. Anthropological and genetic evidence indicates that the human brain has undergone at least two key evolutionary changes. The first is a mutation in the ASPD gene. The ASPD gene codes for the abnormal spindle protein homolog protein. Around 6,000 years ago a new allele of the ASPD gene emerged, resulting in progressively larger brains across humans of European, Asian, and Middle Eastern decent. Brain size is positively correlated with intelligence and is thought to allow for increases in the number of neurons and the number of connections between neurons. Growth in brain size, however, was not uniform. The area of the brain responsible for all higher-order thought processes, known as the cerebral cortex, expanded to the point where today it accounts for 77 percent of total brain volume. Mutation in the ASPD gene corresponds with the advent of writing, agriculture, and science; although, it is unlikely to have directly caused them.
The second mutation also had a profound effect on human development and activity. Approximately 100,000 years ago a mutation in the FOXP2 gene occurred. Scientists believe that the mutation eventually allowed for the development of language. The mutation spread rapidly across ancestral populations, in large part because language provided early hominids a tremendous advantage in terms of survival. The area in the brain responsible for the production of language was discovered by a French physician, Paul Broca, and is known as Broca’s area. The area of the brain that allows individuals to understand language was named after German psychiatrist Carl Wernicke, and is known as Wernicke’s area. The development of language has allowed for tremendous advancements among human cultures. Interestingly, research has found that language deficiencies are positively correlated with antisocial behaviors.
Over 60 percent of our genes code for the structure and function of the brain. In terms of nuclear material, the brain is the most expensive organ to create. The average adult brain weighs between 1300 and 1400 grams, or about 3 pounds. Because brain development is under strong genetic control, several regions of the brain appear to be strongly heritable, including regions that allow individuals to control their emotions, to plan for the future, and to engage in self-control. The heritability of brain structure and functioning necessarily means that the traits and characteristics of biological parents, both physical and behavioral, are likely to be shared and expressed by their offspring. Numerous behavioral genetic studies show that most traits, including self-control, intelligence, and specific personality features are modestly to strongly heritable. Part of the reason for the correlation between parent and offspring behavior and traits is likely to be due to similar brain architecture and functioning. Criminologists have known since the 1800’s that criminal behavior and other social pathologies concentrate within some families and can show high degrees of inter-generational continuity.
The brain begins to develop almost immediately after conception. Research shows that during in-utero development the brain is forming over 250,000 connections per minute. While much of brain development is genetically controlled, evidence indicates that environmental factors can also be influential. Because the developing embryo is connected to the mother by the umbilical cord and placenta, maternal behavior can influence in-utero brain development of the fetus. Maternal nutritional intake is the clearest example. The fetus depends on receiving sufficient levels of nutrients from the mother. Insufficient nutritional intake can have devastating neurological consequences for the fetus. Moreover, maternal stress, and the accompanying release of stress hormones, has been shown to affect brain development of the fetus. While the brain of the developing fetus is protected by layers of fatty lipids, known as the blood-brain barrier, molecules from a variety of substances can pass from one side of the barrier into the developing central nervous system of the fetus. Maternal drug use and abuse, for example, has been found to potentially compromise the brain and central nervous system of the developing fetus. Cocaine molecules, as an example, easily pass across the blood-brain barrier. Moreover, lead (Pb), which can be stored in the bone of the mother, becomes bioavailable during pregnancy, passes through the blood-brain barrier, and may compromise healthy nervous system development. Prenatal lead exposure has been linked to a range of conduct problems, including delinquency in adolescence and criminal conduct in adulthood.
The rate of synaptogenesis, the creation of new nerve cell connections, corresponds directly to the increases in motor and cognitive abilities witnessed across childhood. Indeed, the brains of two year olds are about two-times as active as adult brains. Yet by late childhood the brain begins pruning unused and unnecessary synaptic connections. Reductions in the number of unnecessary connections eventually results in an increase in brain efficiency. The brain, in other words, does not have to use as much energy to do the same tasks in adulthood as it did in childhood. In part, this is due to the strengthening of connections through repetition; however, this is also due to the sculpting away of unused connections. While the sculpting begins in late childhood, it is accelerated during puberty when the brain undergoes a substantial period of reorganization. The onset of puberty also signals the myelination of the front part of the adolescent brain. Myelin is a fatty substance that will eventually cover axons. By covering axons in myelin the electrical impulse across the axon is accelerated by a factor of 10 over unmyelinated axons. The end result of these developmental changes is an energy efficient, highly integrated adult brain.
It is worth pointing out that although the brain undergoes substantial reorganization and pruning in the earlier stages of life, this does not mean that once one reaches adulthood the brain remains unchanged. On the contrary, the brain continues to change over time. Scientists do not yet know, however, how these changes are connected to criminal behavior.
Structures and Functions
The first part of the brain to develop is lower brain, which includes the brain stem and reticular activating system (RAS). The RAS contains a dense bundle of nerves that help direct sensory information to the correct parts of the brain. Psychologists have also linked variation in the RAS to certain personality characteristics, with some of these characteristics, such as neuroticism and extraversion, relating to offending. The lower brain controls all autonomic activities, such as breathing, that support basic life functions.
Sitting on top of the lower part of the brain is the midbrain, also known as the limbic system or the reptilian brain. The limbic system is commonly thought of as the alarm system of the brain in that it prepares the body for fight or flight when faced with a threatening situation. The limbic system controls overall neuroendocrine functioning, which regulates the release of hormones throughout the body. Containing a series of unique but highly inter-related structures, the limbic system is deeply implicated in crime and violence.
One of the more important structures, at least as it applies to understanding violence, is the amygdala. The amygdala is an almond-shaped structure responsible for human emotions and emotional memory. The term “emotion” comes from the early Greek word for motion. The Greeks understood the powerful influence of emotions in human motivation and behavior. Emotions are chemical states generated by the amygdala which can serve as powerful motivators. Several emotions have been implicated in crime and violence, such as fear, anger, disgust, and contempt. The amygdala is also implicated in social learning. Information, situations, and events that carry with them strong emotions are more likely to be stored by the brain and recalled in the future.
Other structures, such as the hippocampus, hypothalamus, thalamus, and cingulate gyrus, compose the limbic system and are also implicated in problem behavior. For example, the hypothalamus, with connections to the pituitary gland, helps to control endocrine functioning. The endocrine system regulates the flow of hormones and androgens, sex-specific hormones, in the bloodstream. Human sensations of bonding, for example, are produced by the release of vasopressin and oxytocin, while aggression has been linked to increased levels of testosterone in males and to estrogen in females. Moreover, the hippocampus is associated with the anticipation of consequences and the ability of individuals to learn from prior experience—known as conditioning. One of the hallmarks of adult criminals is their inability to learn from prior experience. Finally, the cingulate gyrus has been repeatedly linked to human aggression. The anterior cingulate gyrus influences attention, concentration, and focus. Reduced ACC volume and activity correlate with conduct disorder, obsessive thoughts, and compulsive behaviors.
Numerous studies have also implicated the hypothalamus-pituitary-adrenal (HPA) system in a range of emotional, psychiatric, and behavioral problems. The HPA axis is a complex network that regulates nervous system responses to stress. When the brain is under stress, the hypothalamus will release corticotropin-releasing hormone (CRH) and vasopressin. These molecules will be transported to the pituitary gland where they will cause the release of adrenocortictropic hormone (ACTH). ACTH will then stimulate the release of cortisol, a stress hormone, from the adrenal gland. These chemicals will then act on the hypothalamus and pituitary gland to eventually suppress CRH and ACTH production.
Extended exposure to stress can harm brain functioning and, in turn, can bring about a variety of non-adaptive stress responses. This may be why dysfunction in the HPA axis has been correlated with everything from aggression, to mood disorders, to drug- addiction and abuse. Indeed, prolonged exposure to intense stress appears capable of altering HPA axis functioning. This possibility has encouraged scholars and criminologists to examine how stress filled environments influence neurological activity. For example, some children of depressed, neglectful, or physically abusive mothers show dysregulation of the HPA axis. Blunted HPA responses to stress may occur because of repeated exposure to abuse and neglect. In turn, the affected child may be neurologically compromised in his response to threatening and difficult environmental circumstances.
The cerebral cortex sits atop the limbic structures and is divided into two hemispheres by the corpus collosum, which connects the two hemispheres through a dense bundle of fibers. The cortex has been called the key to civilization because all higher-order, uniquely human capacities are distributed across the cortex. There are approximately 10 to 20 billion neurons in the cerebral cortex generating between 60 to 240 trillion connections. The cortex is the last part of the brain to develop, taking approximately 20 years to fully mature, and is more susceptible to insult and injury in males than females. The limbic system, moreover, is intimately connected to the cerebral cortex.
The cortex is divided into the prefrontal cortex (PFC), the dorsolateral prefrontal cortex (DLPC) and the orbitalfrontal cortex (OFC). Collectively these areas are responsible for what are known as “executive functions.” Executive functions include goal driven behavior, planning and execution of a plan, impulse control, delayed gratification, task switching, and emotional regulation. Deficits in executive functioning are of primary concern to criminologists. First, a large body of research evidence shows that criminal offenders are highly impulsive, frequently hedonistic, and have trouble delaying immediate gratification. A dominant theory in criminology refers to this syndrome as “low self-control”(Gottfreson & Hirschi, 1990). Medical experts are able to assess brain function through a number of imaging techniques. A growing body of imaging affected by mental or physical health disability. However, research evidence, including functional magnetic reasonance imaging (fMRI), positron emission tomography (PET), and single photon emission tomography (SPECT), reveals a pattern of under-activation in the in the left, but sometimes the right, PFC. The lack of activity is thought to correspond to deficits in the executive control functions of the brain. Self-control is known to be housed in the PFC, specifically the OFC. It is considered to be a relatively stable characteristic over long periods of the life-course. Activity levels in the OFC, however, can sometimes be adjusted with the use of prescribed stimulants.
Second, the PFC is the last part of the brain to fully develop, with myelination not complete until the start of the third decade of life. Because of this, adolescents appear to be more influenced by limbic impulses and emotional stimuli. Experimental studies have found that adolescents tend to over-estimate the potential emotional and social awards of a behavior, while they tend to under-estimate the actual risks associated with the behavior. Moreover, they appear to be more influenced by peer pressure than are adults. One of the developmental changes that takes place during adolescence is a shift from a reliance on initial limbic impulses to a reliance on the PFC as the primary brain mechanism used to solve problems. Interestingly, this evidence was used by the Supreme Court of the United States in Roper v. Simmons, 543 U.S. 551 (2005). The court held that the execution of individuals who were under the age of eighteen years at the time of their capital crimes is prohibited by the Eighth and Fourteenth Amendments..
Contemporary neuroscience theorizes that aggressive and violent impulses originate in the deep limbic structures of the brain, primarily in the amygdala, where they stimulate an autonomic nervous system response that prepares the body for violence. Part of this response is an increase in the amount of hormones, such as testosterone, entering the blood stream, as well as an increase in muscle oxygenation, a loss of peripheral vision, and an increase in heart rate. Depending on the context, however, this impulse can be shunted by a sufficiently developed and inter-connected PFC. The PFC causes a reduction in the initial limbic impulse and delays the behavioral actions that can accompany the impulse. This is why, for example, most people do not engage in violence even when they have been sufficiently provoked. Violent behavior can occur, however, when deficits in the PFC, either through a lack of active connectivity or insufficient connections to the limbic system, fail to filter the violent impulse.
Neurotransmission is the process by which electro-chemical messages are sent throughout the brain. Neurons are cells that facilitate this communication. There are approximately 100 billion neurons in the adult brain, and each neuron connects to between 1 and 10,000 other neurons. Neuronal activity is highly complex and is involved in every thought, emotion, and behavior engaged in by the human organism. Neuronal transmission occurs when a pre-synaptic neuron generates and sends an electrical charge down its axon, which is a tail-like extension emanating out of the cell. The electrical charge moves down the axon by passing through a series of positively and negatively charged sodium and potassium gates where it will eventually encounter the synaptic cleft. Electrical energy is then converted to chemical energy when the vesicle housing chemicals, known as neurotransmitters, releases molecules into synaptic space. Synapses, or the area between the axon of one cell and the dendrite of the another, is 1/600th the width of a human hair. Neurotransmitters pass through synaptic space and bind to specialized receptors on the post-synaptic neuron. Once the neurotransmitters have bonded to the receptors, an electrical charge is sent back down the post-synaptic neuron and the transmission cycle is completed. The neurochemicals used in the process are reabsorbed by the neuron in a process called reuptake, or they are enzymatically degraded for future use.
Neuronal receptors are highly specialized and vary in number and sensitivity. The number of receptors located on a dendrite, and their efficiency and sensitivity, is genetically controlled and varies across individuals. Receptor sites are also specific to each neurotransmitter. For instance, dopamine receptors accept only dopamine molecules while serotonin receptors accept only serotonin molecules.
Neurotransmission frequently involves groups of neurons working collaboratively. These “neural networks” are responsible for a number of complex functions carried out within the brain. With repetitive stimulation, these networks become more efficient and send messages more quickly. In contrast, neurons that are not used or that are stimulated irregularly are pruned. This has led to the development of Hebb’s rule: neurons that fire together, wire together.
Neurotransmitters can be classified as excitatory, causing an increase in cell firing, or inhibitory, which prevents cells from firing. Four neurotransmitters have been identified that are related to antisocial behavior. Dopamine is an excitatory neurotransmitter, which is involved with the reward and punishment systems in the brain, as well as cognitive functioning. Norepinephrine is another excitatory neurotransmitter. This chemical is tied to emotion and memory, and is involved in fight-or-flight responses. Gamma aminobutyric acid (GABA) and serotonin are inhibiting neurotransmitters. GABA is responsible for filtering stimulation from the environment to allow interpretation of experiences. Serotonin has been linked to mood and self-restraint. Each of these neurotransmitters has been found to be related to antisocial behavior.
Moderate levels of dopamine are associated with optimal cognitive functioning. It is known to enhance problem solving and to increase concentration. Dopamine is released in the brain when individuals participate in pleasurable activities, creating feelings of euphoria. This increases the likelihood that the behavior will be repeated. As dopamine is involved in the reward systems in the brain, it has been hypothesized that some individuals may experience reward deficiency syndrome (RDS). In these circumstances, individuals experience less neurological arousal from behaviors that would be experienced otherwise. For individuals suffering from RDS, behaviors such as eating or sex would not bring an optimal amount of pleasure. This condition is thought to lead individuals to be more impulsive, obsessive, or vulnerable to addiction. Commonly abused substances such as nicotine, caffeine, and a number of illicit drugs increase dopamine levels, and many substances such as cocaine, bind directly to dopamine receptors. As a result, dopamine is believed to play an important role in substance abuse and addiction.
Significantly high levels of dopamine have been associated with aggressive and criminal behaviors as well as reduced intelligence. A number of studies have shown that antipsychotic drugs known to reduce dopamine production have also reduced aggressive behaviors (Brizer, 1988; Mollinger, Lockner, Sauls, & Eisenberg, 1962; Yudofsky, Silver, & Schneider, 1987). However, some studies have suggested that the effect of dopamine on aggressive behavior may be insignificant (Raine, 1993).
Norepinephrine can be found in the cerebellum and hindbrain regions, as well as between the brain stem and cortex. It is distributed throughout the limbic system. During periods of heightened arousal norepinephrine is released, aiding individuals in responding to fright or anger. Provided that norepinephrine can be found in regions of the brain responsible for emotion and memory, these events are likely to remain in an individual’s memory for a long period of time. Some drugs can block the reuptake of this neurotransmitter, resulting in an excess of this chemical in the brain. Generally, studies indicate that excessive amounts of norepinephrine are related to aggressive behavior (Brizer, 1988; Fishbein, 2001; Volavka, 1999; Yudofsky et al., 1987); however, there is also evidence that lower levels of this neurotransmitter may be linked to antisocial behavior (Raine, 1993). The available research indicates that the relationship between norepinephrine and maladaptive behaviors may be curvilinear; meaning both high and low levels of this neurotransmitter can increase problem behavior.
The most prevalent neurotransmitter is GABA (Wan, Berton, Madamba, Francesconi, & Siggins, 1996). Primarily, GABA is used by the brain to regulate the functioning of the nervous system and cognitive functioning. This neurotransmitter works to balance excitation in the brain by inducing relaxation and sleep. Reduced levels of GABA can lead to increases in irritability, anxiety, and violent behavior. In contrast, excessive levels of this neurotransmitter depress cognitive functioning. Alcohol and the date rape drug gamma hydroxybutyrate (GHB) have effects on the brain similar to excessive amounts of GABA. The senses of the body are inhibited, and it becomes difficult to have a complete awareness of the immediate environment.
Serotonin is used by neurons found in the brain stem, the limbic system, and the frontal cortex. In addition to the influence genetics may have in the development of the serotonergic system, there are environmental factors such as seasonal changes that also seem to influence serotonin levels. This neurotransmitter has been found to play a large role in the regulation of behavior and mood. Low levels of this neurotransmitter have consistently been linked to criminal behavior among juveniles and adults (Brizer, 1988; Fishbein, 2001; Raine, 1993). Recent research has indicated that serotonin is important in inhibiting impulsive behavior, and that impulsivity that is tied to strong emotion is more likely to result in violent outcomes (Krakowski, 2003, Volavka, 1999). Lower levels of serotonin have also been associated with violent forms of suicide. It has also been found to correspond with higher levels of substance abuse and a number of clinical disorders. There is even evidence that suggests that low serotonin, alcoholism, and violence may share an association.
There is clear evidence that an excess or deficiency in neurotransmitters can result in modifications in behavior. While the amount of these chemicals in the brain is influenced by the body’s production of the chemical, it is also influenced by the regulation of the substance through its removal. One method of removing neurotransmitters that remain in the synaptic space after neurotransmission is through the process of enzymatic degradation. Monoamine oxidase-A (MAO-A) is an enzyme found within the brain which is responsible for metabolizing serotonin, norepinephrine, and dopamine. Having too little or too much MAO-A will interfere with neurotransmission. If an individual is deficient in MAO-A, an abundance of neurotransmitters may be left in the synaptic cleft after neurotransmission. In contrast, an overproduction of MAO-A may reduce the amount of a neurotransmitter in the transmission process. In either case, this interference can result in abnormal behavior.
Genes code for the level of MAO-A in individuals, and alleles for this enzyme are categorized as either high or low activity. Although research on the relationship between MAO-A and behavior is still in early stages, high and low levels of MAO-A have been associated with aggression and impulsivity among children and adults. There is also some evidence that suggests that varying levels of MAO-A may interact with environmental conditions producing maladaptive behavioral outcomes (Kim-Cohen, Caspi, Taylor, Williams, Newcombe, Craig, et al., 2006). For example, those that possess the low activity allele and experience maltreatment during childhood may be more likely to engage in antisocial behaviors during adolescence or adulthood.
While the available research indicates that various aspects of brain structure and function are implicated in the explanation of antisocial behavior, these relationships require further specification. For example, some studies have found that high concentrations of a neurotransmitter are related to maladaptive behaviors, but other studies demonstrate that lower concentrations of the same neurotransmitter also lead to maladaptive behaviors. The pattern of findings suggest that excesses and deficiencies result in behavior differences, but some scholars have also suggested that the relationship may be specific to types of offenses, such as property offenses, and to the nature of the offending, such as predatory or impulsive offending (Raine, 1993). Either way, it is fair to say that while great advances have been made that much research is still necessary.
Neurological findings, in general, and those associated with human aggression and violence specifically, challenge many long held views about the nature of free will and choice in human action. Some scholars assume that accepting biological factors as contributing to human behavior eliminates the possibility of free will. They point, for example, to the neurological evidence showing brain deficiencies in the frontal cortex as evidence that behavior cannot be freely chosen. Moreover, other scholars have argued that since certain afflictions, such as alcoholism and drug-addiction, are highly heritable, that individual responsibility for conduct associated with these afflictions should be reduced. This is no small matter. The criminal justice system currently operates under the assumption that individual’s make decisions based on free will. Neurological findings, however, have been used to question the extent to which free will exists and thus have been used to argue that the criminal justice system should more fully embrace a medical model—where the cause of criminal behavior is diagnosed and individual treatment, not punishment, provided.
Recent evidence adds to and expands this debate. Several studies have recently found that brain structure and function in the early life-course predicts brain structure and functioning in adulthood. The implications of these findings are not fully understood but they contrast with the images of rapid growth and change in brain functioning so prevalent in the research literature. It may be the case that the brain changes only within a limited range and that the changes detected in childhood through adulthood are bounded or restricted. If true, early brain activity may be a marker for problem behaviors later in life. While this information could be used to target youth for intervention, it could also be used to label youth as potential criminals. Hence, the ethics surrounding the use of neurological information in the identification and treatment of individual behavioral problems has yet to be sorted out. Do we, as a society, intervene in the lives of children and their families based on early neuro-scientific findings, or do we wait until problem behavior has emerged and someone has possibly been harmed? At this stage, the science is far ahead of our ethical understanding about the limits of the science.
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