Woollett and Maguire found that the hippocampi of the qualified cabbies had grown in size, especially the back part. They were now significantly larger than those of either the failed trainees or the men who didn't take part. The cabbies also outperformed their peers on spatial memory tasks.

While acquiring "The Knowledge"—the mental map of 25,000 London streets—results in significant structural brain changes, specifically an increase in the volume of the posterior hippocampus, the overall effect on memory performance is a mix of "gain and loss." Studies, including famous research by Dr. Eleanor Maguire, show that while cabbies become world-class experts in spatial navigation and "survey" perspective memory, this intense specialization often comes at a cost to other types of memory. Specifically, licensed London taxi drivers frequently perform significantly poorer than control groups on tests of "anterograde associative memory," such as learning and recalling new, unrelated object-location pairs or complex visual figures (like the Rey-Osterreith test). This suggests that the brain "reallocates" space in the hippocampus to accommodate the massive amount of geographic data, potentially reducing the capacity for acquiring new, non-spatial information. Thus, "The Knowledge" creates a specialized "super-power" in navigation but does not necessarily translate to a generalized improvement in overall memory performance.