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Original Articles
Introduction
Impulsivity is a core feature of borderline personality disorder (BPD) and is one of nine diagnostic criteria in the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV; American Psychiatric Association, 1994). It is seen in behaviours such as substance misuse, unsafe sexual activity, disordered eating and impulsive self-harm (Soloff et al. 2000; Trull et al. 2000; Dougherty et al. 2004; Rosval et al. 2006). These impulsive behaviours are the strongest predictor of borderline psychopathology on follow-up after 7 years (Links et al. 1999), indicating the importance of understanding this trait.
There is no unitary idea of 'impulsivity', which is instead considered to consist of several independent factors. There is little agreement as to what these are (for reviews, see Evenden, 1999a , b ). Barratt developed a self-report scale that highlighted three second-order factors of impulsivity, specifically: (1) attentional impulsivity which refers to poor cognitive control, concentration and attention; (2) motor impulsivity which represents acting without thinking, and (3) choice or non-planning impulsivity which is being focused on the present with little regard to the future (Patton et al. 1995). Measurement of impulsivity is frequently achieved by using rating scales completed by participants (Lecrubier et al. 1995). These have limitations; some patients may be poor at assessing their own status or may be at variance with external raters (Mattila-Evenden et al. 1996), or they may be influenced by the setting. Thus there is a need to use objective, valid and reliable measures to assess impulsivity in conditions such as BPD.
One of the main factors of impulsivity is choice impulsivity (Lecrubier et al. 1995). This can be objectively assessed by delay discounting tasks. In such tasks, the participant is given two choices: they can choose a small immediate reward or a large reward for which they have to wait. Less valuable, immediate rewards are often chosen over more valuable delayed rewards. Greater tendency to discount the value of the reward according to delay in this way reflects greater impulsivity (Reynolds, 2006). Typically, value is discounted precipitously over relatively short delays but discounting slows as delay length increases (Kirby, 1997), following a hyperbolic discounting function (Mazur, 1987). Reward-based decision making in BPD tends...