Numerous two-mode models of cognition and behavior exist in psychology (Barrett, Tugade, & Engle, 2004
; MacDonald, 2008
; Strack & Deutsch, 2004
). The core idea is that the nervous system is organized such that people simultaneously process experiences in two ways, one more basic (evolutionarily primitive) than the other (Epstein, 1994
). The two processing modes appear to use different aspects of available information, they seem to learn in different ways, and they create potentially competing paths to action. The more primitive mode operates largely outside consciousness. The other is the familiar symbolic processor of the rational mind.
Characterizations of the more primitive mode typically use such terms as impulsive, reflexive, reactive, implicit, heuristic, and associative. This mode is responsive to situational cues of the moment, schematic associations, and strong emotions. Its strengths are its quickness and its low demand on processing resources. It spontaneously creates action when its schemas or production systems are sufficiently activated. It thus can act even with little available information and high time urgency.
Characterizations of the other mode typically use such terms as reflective, explicit, strategic, deliberative, and logical. Its strength is its ability to take into account circumstances that go beyond the immediate present. This mode requires substantial processing resources and thus loses efficiency when cognitive capacity is limited.
The premise of two processing modes also connects to increasingly influential literatures on implicit versus explicit attitudes, self-concepts, and so on. Theories on such topics hold that people build up networks of associations that are not easily accessed verbally, and also form (separately) verbally mediated logical structures. The associations and the logical structures both influence behavior. Of most interest are cases in which the influences conflict with each other. The connection here is that the associative network represents the reactive system, and the logical structures represents the deliberative system.
There are also influential developmental versions of the two-mode viewpoint (e.g., Rothbart, Ellis, & Posner, 2004
). These are perhaps clearest in indicating that each mode can promote either action or inaction (). That is, in the reactive mode, strong reward focus can yield impulsive action; strong punishment focus can yield immobility or withdrawal. Immobility might not seem impulsive (i.e., no action), but the automatic nature of the response suggests an impulsive quality (i.e., impulsive as automatic). Either of these reactive tendencies can be overridden, once the capacity for what is called effortful control develops. Effortful control (which closely resembles what Depue & Lenzenweger, 2005
, called nonaffective constraint) can counter the more basic reactions, overcoming either an impulse to act or an impulse to become immobile.
Fig. 1 Three temperamental influences on behavior. Panel A shows a reactive temperament for approaching rewards and a reactive temperament for avoiding punishment compete for influence; in the absence of effortful control (whether due to early stage of development, (more ...)
The properties that differentiate the reflective from the reflexive mode resemble what are generally termed executive processes. An example is working-memory capacity (WMC), the ability to maintain information in working memory and shield it from interference or distraction from competing stimuli, including strong emotions. Another example is the ability to remove information from working memory when it is not relevant to the task at hand. Both WMC and the ability to disengage from irrelevant information help the deliberative system operate.
Variables such as these are subject to both individual differences and situational influences, yielding variations in the ability to exert effortful control. For example, when WMC is high, people act according to their explicit attitudes (i.e., WMC helps them use the deliberative, planful system); when WMC is low, people act according to implicit attitudes (i.e., in this circumstance the reactive system guides behavior; Hofmann, Gschwender, Friese, Wiers, & Schmidt, 2008
Effortful Control: Serotonergic Function
There is a substantial basis for arguing that the relative influence of reactive and effortful systems is affected by the serotonergic system. The processes by which this system operates are complex and not fully understood (Hensler, 2006
), but clearly more is involved than simply the level of serotonin. Other factors include sensitivity and density of several kinds of serotonin receptors, efficiency of reuptake of serotonin from the synaptic cleft, dietary intake of certain amino acids, and recent history of the cell’s firing. Each of these factors can influence the key issue, the overall “functioning” of the system.
There are several ways to study serotonergic function, which vary in how direct they are. One way is to challenge the system; systems with lower functioning show greater perturbation. Another is to experimentally increase or decrease tryptophan (a precursor to serotonin) or administer drugs that affect serotonergic function. There also are genetic markers of serotonergic function. In general, results converge impressively across methods.
Experimentally increasing serotonergic function reduces responsiveness to negative emotional stimuli, decreases aggression, and increases cooperativeness and social potency. Experimentally lowering serotonergic function increases behavioral impulsivity. Experimentally lowering serotonergic function also increases aggression, though the effect appears to depend at least partly on preexisting tendencies: Persons low in aggressiveness sometimes are unaffected and sometimes even decrease aggression when serotonin function is lowered.
Naturally existing low serotonergic function has also been linked to behavioral impulsivity, particularly impulsive responses to anger (Manuck, Kaplan, & Lotrich, 2006
). In genetic and drug-challenge studies, low serotonergic function related to self-reported hostility, sensation seeking, and impulsiveness. Large-scale studies link low serotonergic function to low conscientiousness, low agreeableness, and elevations in the anger-hostility and depression facets of neuroticism.
These various associations seem readily interpretable via the two-mode models described earlier. Associations of low serotonergic function with hostile and depressive tendencies reflect intrusion of strong emotions, which characterizes reflexive-mode processing. Associations with impulsive aggression (and other behavioral impulsiveness) may reflect limited effortful control over spontaneous action. Associations of higher serotonergic function with higher agreeableness and conscientiousness suggest that higher serotonergic function promotes a general broadening of perspective: Conscientiousness implies taking future contingencies into account; agreeableness implies taking the needs of others into account.
Low serotonergic function appears to tune the person to be more responsive to cues of the moment, particularly emotional cues, fitting the reflexive system in two-mode models. People with low serotonergic function tend to be highly reactive. They grab what they want when they want it rather than delaying gratification. They react to associative cues of the moment rather than thinking matters through. They react impulsively to salient or intense emotions (Spoont, 1992
Disorders of Impulse
Fitting the idea that low serotonergic function yields impulsive, reactive behavior, evidence links low serotonergic function to childhood conduct disorders and attention-deficit hyperactivity disorder (ADHD), and to emotionally explosive disorders of adulthood, including violent aggression, borderline personality disorder, and violent suicide. Indeed, impulsive externalizing problems are probably the most widely known manifestations of low serotonergic functioning in disorders (cf. Raine, 2008