It is often thought that people perceive the passage of time in steady increments. This is known as the theory of the universal internal clock. A new study suggests that this theory is incorrect. The study, conducted by three psychologists at George Mason University in the United States, involved 170 volunteers. The volunteers were asked to look at images. These images varied in size and content. Some were full of objects, while others seemed almost empty. The volunteers were then asked to estimate how long they had been looking at each image. In some cases, they were asked to rate how memorable they found each image. The volunteers often overestimated how long they had been looking at the messy images. Alternatively, they underestimated how long they had been looking at the emptier images. In many cases, their estimates were more accurate with the images they felt were memorable. These results suggest that there is no universal internal clock. Rather, people may perceive time based on their environment.