Attrition, or loss of participants over the course of a study, presents a significant threat to the integrity of a longitudinal research study and theory development resulting from the study. Although there has been a recent resurgence of interest in attrition, it is an underreported and understudied phenomenon despite its potential to introduce bias. Internal and external validity, reliability, and statistical validity are all impacted by a small sample and, most likely, a non-randomness in the study sample. Development of a theory of attrition will assist researchers in development of sampling strategies that will enhance the quality of their data in longitudinal designs. An ecological theoretical model of research participation is proposed and described.