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Background. Separate lines of research link lowered serotonin tone to interpersonal submissiveness and bulimia nervosa (BN). We explored the impact of co-morbid avoidant personality disorder (APD), as a proxy for submissiveness, on behavioural inhibition and serotonin function in women with BN. Method. Participants included women with BN with co-morbid APD (BNA+, N=13); women with BN but without APD (BNA-, N=23), and control women with neither BN nor APD (N=23). The women were assessed for psychopathological tendencies and eating disorder symptoms, and participated in a computerized laboratory task that measured behavioural inhibition and disinhibition. Participants also provided blood samples for measurement of serial prolactin responses following oral administration of the partial 5-HT agonist meta-chlorophenylpiperazine (m-CPP). Results. The BNA+ group had higher scores than the other groups on self-report measures of submissiveness, social avoidance, restricted emotional expression, affective instability and self-harming behaviours. Compared with the other groups, the BNA+ group tended to be more inhibited under cues for punishment on the computerized task and to have blunted prolactin response following m-CPP. The bulimic groups did not differ from each other on current eating symptoms or on frequencies of other mental disorders. Conclusions. Findings indicate that women with BN and co-morbid APD may be characterized by interpersonal submissiveness and avoidance, affective instability, self-harm, behavioural inhibition in response to threat and lower sensitivity to serotonergic activation. These findings may indicate common, serotonergic factors, associated with social submissiveness, behavioural inhibition to threat and BN. [ABSTRACT FROM AUTHOR]