1. the biological processes involved in an organism’s becoming functional or fully developed.

2. naturally occurring time-related changes in a participant (e.g., growth, aging, fatigue, boredom, attention shifts) that pose a threat to the internal validity of a study, particularly a longitudinal one. These processes—as opposed to the specific treatment or intervention—may explain any changes in participants during the experiment. For example, a researcher may study substance use in a set of individuals from early adolescence to late adulthood. In the study, substance use may naturally decline as a function of the development of the participants rather than because of the influence of an experimental intervention. Thus, the investigator would want to assess and possibly control for this maturation effect in order to maintain the internal validity of the study.