While corpus analysis has long been useful for developing genre-based teaching materials in English for Specific Purposes (ESP), somewhat less attention has been paid to how well people actually learn to use the genre features identified. This study shows how individuals' use of genre features changes over time, as a function of growing disciplinary experience. Using a measure of standard deviation, the study examines how five geologists show increasing discoursal expertise in their writing over a ten-year period. The method of analysis used allows for comparison of individual and collective uses of the generic features authors use to construct their disciplinary voice.