There is a widely recognised national trend for girls to outperform boys at all levels of compulsory schooling. With few exceptions, however, most recent research has reported that, in relation to academic performance at university, men are proportionately over-represented at the First Class level. A number of general hypotheses have been put forward to explain this phenomenon, including those that assume gender-linked differences in cognitive and/or personality traits. A smaller proportion of research has given explanatory primacy to the broad subject area studied. More specifically, it has been alleged that the over-representation of men within the First bracket is largely a function of a "compositional effect" whereby men achieve proportionately more Firsts as there are more of them within the First-rich Sciences. Based upon analysis of 1,707,408 students graduating between 1995 and 2002, this paper seeks to provide the most comprehensive exploration, to date, of this effect. It confirms that a substantial proportion of the "gender gap" can be explained with reference to the male propensity to take degrees in First-rich disciplines.