The sets are basically any relevant subject matter about a person, persons, or non-persons, depending on the context. Propinquity can be more than just physical distance. Residents of an apartment building living near a stairway, for example, tend to have more friends from other floors than those living further from the stairway. The propinquity effect is usually explained by the mere exposure effect, which holds that the more exposure a stimulus gets, the more likeable it becomes. There is a requirement for the mere exposure effect to influence the propinquity effect, and that is that the exposure is positive. If the resident has repeatedly negative experiences with a person then the propinquity effect has a far less chance of happening (Norton, Frost, & Ariely, 2007).