In intertemporal choice, decision-makers must choose between options whose outcomes occur at different times in the future and are associated with different magnitudes of gain or loss. Previous neuropsychological research on this problem is dominated by a behavioural-economic model which proposes that choice outcome is solely determined by a process of devaluing rewards with time, termed temporal discounting. This thesis investigates the veracity of this assumption by developing a new mathematical model of choice which takes into account another fundamental feature of human preference, namely the non-linearity of the relationship between the utility and magnitude of gains. Using behavioural data, methodologies are developed to demonstrate that this model is superior to previous models in accounting for human intertemporal choices. Specifically, using existing terminologies ‘impulsive’ and ‘self-controlled’ to describe preference in choices between smaller-sooner and larger-later monetary rewards, it is shown that the discounting of increasing magnitudes implied by the law of diminishing marginal utility exerts a significant effect in determining choice outcome. In addition to high rates of temporal discounting, it is shown that impulsivity can be engendered by higher rates of diminishing marginal utility and vice-versa. A neuronal account of this model is delineated using neuroimaging techniques, revealing fundamental properties of the brain’s value systems. It is shown that sub-components of value relating to time and magnitude are evaluated by distinct systems and then integrated to furnish an overall metric of utility used to guide choice – in accordance with utility theory. Finally, the ability of the neurotransmitter dopamine to modulate these features of preference and neurobiological systems is investigated using pharmacological manipulation, where it is shown that enhancing dopamine activity engenders impulsivity. These behavioural and neural findings are shown to offer a compelling account of the pathological impulsivity observed as a feature of disorders associated with aberrant dopamine function.