Section Summary

The studies described in this section appear to be relatively consistent in suggesting that experimentally increasing serotonergic function reduces responsiveness to negative emotional stimuli, decreases aggression, and increases cooperativeness; there also were increases in what might be thought of as social potency (see also Spoont, 1992; Young & Leyton, 2002). Reducing serotonergic function impaired performances on tasks with emotional elements; it also increased expression of aggression and deterioration of cooperativeness, though these effects often occurred only among persons who were relatively high in aggressive tendencies by disposition.

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Serotonergic Function, Impulse-Related Clinical Disorders, and Violence

Evidence linking serotonergic function to lack of executive control, impulsiveness, and aggression in nonclinical samples suggests that low serotonergic function should also relate to disorders that are characterized by these qualities. There are in fact intriguing parallels between phenomena described in the preceding section and certain phenomena that are more clinically relevant. This section considers some of those parallels, though it is not an attempt to review externalizing disorders in general (for broader treatment see, e.g., Krueger et al., 2002). Specifically, evidence is briefly considered here regarding serotonergic involvement in attention-deficit/hyperactivity disorder (ADHD) and externalizing behaviors such as conduct disorder, disruptive disorders, and antisocial personality disorder.⁶

ADHD

There is a substantial basis for believing that ADHD is actually a disorder of inability to control behavioral impulses (Barkley, 1997; Nigg, 2001, 2003; Quay, 1988). Symptoms include distractibility, difficulty following instructions, disorganization, and talking out of turn. Evidence of actual deficits in attention has generally been lacking (Barkley, 1997). Barkley (1997) argued that this disorder rests on deficits in executive functions and consequent deficits in motor control (for evidence that problems labeled attention problems reflect primarily difficulties with response inhibition, even in unselected children, see Friedman et al., 2007).

A more recent review led Nigg (2001, 2003) to a similar conclusion. Indeed, Nigg explicitly utilized a two-mode approach in analyzing ADHD. Inhibition by anxiety, uncertainty, or fear he termed motivational inhibition. What he termed executive inhibition resembles the concept of effortful control (though he applied it only to inhibitory tendencies). His review led him to conclude that ADHD reflects executive deficits. Consistent with this, an association has been found between the 5-HTTLPR polymorphism and ADHD (Beitchman et al., 2003).

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