Impulsive unsocialized sensation seeking (ImpUSS) is a major factor discovered in factor analyses of scales used in psychobiological research. It is strongly convergent with Eysenck’s P dimension and conscientiousness in the ‘big five’. The components of the dimension and the P scale, have been validated as correlates of various kinds of disinhibited behaviors, criminality, sexuality, and substance use and abuse. ImpUSS is related to a failure in passive avoidance learning, probably as a function of impulsivity and attention to reward stimuli. Psychophysiological markers for the trait include strong orienting and weak defensive reflexes and an augmenting, rather than reducing, of cortical reaction to intense stimuli. At the neurochemical level the trait is related to low levels of monoamine oxidase (MAO) and the neurotransmitters norepi-nephrine and serotonin, and theoretically high levels of dopaminergic activity. The trait components have high heritabilities for a personality