Inflammatory Challenge in Human Aggression.

The goal of this clinical trial is to explore the differences in behavioral and cytokine response to a low dose infusion of endotoxin (vs. placebo) in individuals with histories of frequent, problematic, impulsive aggression ("aggressives") compared to similar individuals without this history ("controls"). Endotoxin is a substance that produces a reliable inflammation response in human subjects. The main questions it aims to answer are: * Do aggressive individuals have greater self-rated anger responses to low-dose endotoxin compared with controls? * Do aggressive individuals have greater analog aggressive responses (in the Taylor Aggression Paradigm) to low-dose endotoxin compared with controls? * Do aggressive individuals have greater hostile attributional and negative emotional responses (in the V-SEIP) to low-dose endotoxin compared with controls? * Do aggressive individuals have greater plasma pro-inflammatory responses to low-dose endotoxin compared with controls? * Do aggressive individuals display a greater activation of brain responses to anger-related picture during an MRI scan during low-dose endotoxin compared with controls? Researchers will compare endotoxin to a placebo (a look-alike substance that contains no drug) explore the differences in behavioral and cytokine response to a low dose infusion of endotoxin (vs. placebo) in individuals with histories of frequent, problematic, impulsive aggression ("aggressives") compared to similar individuals without this history ("controls"). Participants will: * Receive a low-dose of endotoxin and placebo on two (2) separate days. The study drugs will be given through a plastic tube inserted in a forearm vein. * Visit the laboratory on at least two (2) separate days to receive the endotoxin and placebo. * Complete rating forms, behavioral testing, and an MRI on each of the two (2) laboratory days.