Serotonin and Impulsivity:

"Serotonin acts very widely in the brain as a neuromodulator, with regulatory or inhibiting effects on mood, behavior, and cognition (Spoont, 1992). Serotonin not only potentiates the function of effortful control processes that allow the top-down restraint of impulses (Carver et al., 2009), but it also serves to suppress the bottom-up hypothalamic and brainstem systems (including the dopaminergic system) that generate impulses in the first place (Chambers et al., 2003; Gray & McNaughton, 2000). Serotonin acts to limit negative affect and aggression, while maintaining behavioral and motivational stability, and it has been directly linked to Conscientiousness, Agreeableness, and low Neuroticism, the traits constituting Stability (e.g., Jang et al., 2001; Manuck et al., 1998). Increasing serotonergic function thus appears to modulate both elements of impulsive action—the impulses and the lack of restraint—so as to reduce impulsivity. Individual differences in serotonergic function are therefore likely to be a key substrate of all impulsivity-­related traits, perhaps most strongly related to the dimension labeled Urgency because this dimension explicitly describes strong impulses as well as weak restraint of those impulses."

Personality and Self-Regulation, Colin DeYoung