The dual systems model, also known as the maturational imbalance model, is a theory arising from developmental cognitive neuroscience which posits that increased risk-taking during adolescence is a result of a combination of heightened reward sensitivity and immature impulse control. In other words, the appreciation for the benefits arising from the success of an endeavor is heightened, but the appreciation of the risks of failure lags behind.
The dual systems model hypothesizes that early maturation of the socioemotional system (including brain regions like the striatum) increases adolescents' attraction for exciting, pleasurable, and novel activities during a time when cognitive control systems (including brain regions like the prefrontal cortex) are not fully developed and thus cannot regulate these appetitive, and potentially hazardous, impulses. The temporal gap in the development of the socioemotional and cognitive control systems creates a period of heightened vulnerability to risk-taking during mid-adolescence. In the dual systems model, "reward sensitivity" and "cognitive control" refer to neurobiological constructs that are measured in studies of brain structure and function. Other models similar to the dual systems model are the maturational imbalance model, the driven dual systems model, and the triadic model.
The dual systems model is not free from controversy, however. It is highly contested and debated within developmental psychology and neuroscientific fields whether or not when the prefrontal cortex is said to be fully or efficiently developed. Most longitudinal evidence suggests that myelination of gray matter in the frontal lobe is a very long process and may be continuing until well into middle age or greater, and major facets of the brain are recorded to reach mature levels in one's mid-teens, including the parts that are responsible for response inhibition and impulse control, suggesting that many later age markers may ultimately be arbitrary.
Historical perspective The dual systems model arose out of evidence from developmental cognitive neuroscience providing insight into how patterns of brain development could explain aspects of adolescent decision-making. In 2008, Laurence Steinberg's laboratory at Temple University and BJ Casey's laboratory at Cornell separately proposed similar dual systems theories of adolescent risky decision-making. Casey et al. termed their model the maturational imbalance model.
The majority of evidence for the dual systems model comes from fMRI. However, in 2020 the model gained support from a study looking at brain tissue structural measures. Volumetric analysis and mechanical property measures from magnetic resonance elastography showed that individual differences in tissue microstructural development correlated with adolescent risk taking, such that individuals whose risk taking centers were more structurally developed relative to their cognitive control centers, were at greater likelihood to take risks.
Models
Maturational imbalance model Both the dual systems model and the maturational imbalance model conceive of a slower developing cognitive control system that matures through late adolescence. The dual systems model proposes an inverted-U shape development of the socioemotional system, such that reward responsivity increases in early adolescence and declines thereafter. The maturational imbalance model portrays a socioemotional system that reaches its peak around mid-adolescence and then plateaus into adulthood. Further, the dual systems model proposes that the development of the cognitive control and socioemotional systems is independent whereas the maturational imbalance proposes that the maturation of the cognitive control system leads to dampening of socioemotional responsivity.
Driven dual systems model Recently, another variation of the dual systems model was proposed called the "driven dual systems model". This model proposes an inverted-U shaped trajectory of socioemotional system responsivity, similar to the dual systems model, but hypothesizes a cognitive control trajectory that plateaus in mid-adolescence. This cognitive control trajectory differs from that proposed by the dual systems model and maturational imbalance model which continues to increase into the early 20s. Similar to the driven dual systems model, a model has been proposed including a hyperactive socioemotional system that undermines the regulatory ability of the cognitive control system. These later models hypothesize that cognitive control development is completed by mid-adolescence and attribute increased risk-taking during adolescence to the hyperarousal of the socioemotional system. The dual systems model and maturational imbalance model propose that cognitive control development continues into early adulthood and that increased risk-taking in adolescence is attributable to a developmental imbalance where the socioemotional system is at its peak of development but the cognitive control system developmental trajectory lags behind.
Triadic model The "triadic model", which includes a third brain system responsible for emotion processing and primarily implicating the amygdala. The triadic model proposes that this emotion system increases impulsivity during adolescence by increasing the perceived cost of delaying decision-making. This model posits that impulsivity and risk seeking in adolescence is due to a combination of hyperactive reward systems causing adolescents to approach appetitive stimuli, emotion processing systems causing adolescents to enhance perceived costs of delaying behaviors and reduce avoidance of potentially negative stimuli, and an underdeveloped cognitive control system that is unable to regulate reward-seeking behaviors.
Adolescent risk-taking Risk-taking in certain, but not all, domains peaks during adolescence. Most notably, mortality and morbidity rates increase significantly from childhood to adolescence despite the fact that physical and mental capabilities increase during this period. The primary cause for this increase in mortality/morbidity among adolescents is preventable injury. According to the Center for Disease Control, in 2014 about 40% of all adolescent deaths (ages 15–19 years) were caused by unintentional accidents. From 1999 to 2006, almost one-half of all adolescent deaths (ages 12–19 years) were due to accidental injury. Of these unintentional injuries, about 2/3 are due to motor vehicle accidents, followed by unintentional poisoning, unintentional drowning, other land transportation accidents, and unintentional discharge of firearms.
The dual systems model proposes that mid-adolescence is the time of highest biological propensity for risk-taking, but that older adolescents may exhibit higher levels of real-world risk-taking (e.g., binge drinking is most common during the early 20s) not due to greater propensity for risk-taking but due to greater opportunity. For example, individuals in their early 20s compared to mid-adolescence have less adult supervision, greater financial resources, and greater legal privileges. The dual systems model looks to experimental paradigms in developmental neuroscience for evidence of this greater biological propensity for risk-taking.
There is also a consistent relation between age and crime with adolescents and young adults being more likely to engage in violent and non-violent crime. These findings are linked to increases in sensation-seeking, which is the tendency to seek out novel, exciting, and rewarding stimuli, during adolescence, and continued development of impulse control, which is the ability to regulate one's behavior. The dual systems model points to brain development as a mechanism for this association.
Reward seeking Across many species including humans, rodents, and nonhuman primates, adolescents demonstrate peaks in reward-seeking behaviors. For example, adolescent rats are more sensitive than adult rats to rewarding stimuli and show enhanced behavioral responses to novelty and peers. Adolescent humans show peaks in self-reported sensation-seeking, increased neural activation to monetary and social rewards, greater temporal discounting of delayed rewards, and heightened preferences for primary rewards (e.g., sweet substances).
Sensation-seeking is a type of reward seeking involving the tendency to seek out novel, exciting, and rewarding stimuli. Sensation-seeking has been found to increase in preadolescence, peak in mid-adolescence, and decline in early adulthood.
Impulsivity Impulsivity has been found to exhibit a different developmental trajectory than reward or sensation seeking. Impulsivity gradually declines with age in a linear fashion. Around mid-adolescence when impulsivity and sensation-seeking are at their peak is the theoretical peak age for risk-taking according to the dual systems model.
Social influence Adolescent risk-taking is more likely to occur in the presence of peers compared to adults. Animal studies have found that adolescent mice, but not adult mice, consume more alcohol in the presence of peers than when alone. In humans, the presence of peers has been found to result in increased activation in the striatum and orbitofrontal cortex risk-taking, and activation in these regions predicted subsequent risk-taking among adolescents but not adults. Age differences in activation of the striatum and frontal cortex have been interpreted to suggest heightened risk-taking in the presence of peers is due to the influence of peers on reward processing rather than the influence of peers on cognitive control.
Socioemotional system The term socioemotional brain network or system (also known as the ventral affective system) refers to the striatum as well as the medial and orbital prefrontal cortices.
Dopamine Evidence from rodent studies indicates the dopaminergic system, the pathway connecting the ventral tegmental area to the nucleus accumbens and olfactory tubercle, plays a critical role in the brain's reward circuitry and the dopamine-rich striatum has been implicated as a key contributor to reward sensitivity in the brain.
During puberty, the dopaminergic system undergoes significant reorganization. Increased dopamine projections from mesolimbic areas (e.g., the striatum) to the prefrontal cortex have been observed during mid- and late-adolescence. These projections are pruned/decline in early adulthood. Adolescent-specific peaks in dopamine receptors in the striatum have been observed in humans and rodents. Additionally, dopamine concentrations projecting to the prefrontal cortex increase into adolescence as do the dopamine projections from the prefrontal cortex to the striatum (namely the nucleus accumbens).
Hyper- versus hypo-sensitivity to reward The striatum has been linked to reward processing, learning, and motivation.
Hyperactivity Neuroimaging studies using functional magnetic resonance imaging (fMRI) have shown that the ventral striatum is more active among adolescents compared to children and adults when receiving monetary rewards, primary rewards, and social rewards. Peaks in striatal activity as associated with increased self-reported risk-taking.
Hypoactivity Some studies have found that striatum activity is blunted compared to children and adults when anticipating rewards, which has been linked to greater risk-taking behaviors. The theory linking this hypoactivation to greater risk-taking is that adolescents experience less gratifying experience from anticipating rewards and they are therefore motivated to seek out more reward-inducing experiences to achieve the same level of reward sensation as other age groups.
Current consensus