Working memory (WM) is a limited capacity system that permeates nearly all levels of cognition, ranging from perceptual awareness to intelligence. Through behavioral, electrophysiological, and neuroimaging work, substantial gains have been made in understanding this capacity-limited system. In the current work, we examined genetic contributions to the storage capacity of WM. Multiple studies have demonstrated a link between the serotonin transporter-linked polymorphic region (5-HTTLPR) and cognition, where better performance is observed in individuals possessing a copy of the short (s) variant of the polymorphism compared with individuals homozygous for the long (l) variant. We predicted the same profile in WM performance, such that estimated capacities of l/l carriers should be smaller than s/s and s/l carriers. To measure WM capacity, we implemented a change detection task, which requires observers to actively maintain the color and spatial location of briefly presented squares over a short retention interval. In line with our prediction, we observed similar WM performance between s/s and s/l groups, and these individuals performed better than the l/l group. We then discuss the distribution of the serotonin transporter system and parallels between WM and attention to provide insight into how variation in the 5-HTT polymorphism could lead to individual differences in the storage capacity of WM.