On the Psychology of Watching Birds: The Problem of Observer-Expectancy Bias

seen it if I hadn't believed it" (Foster et al. 1975). The influence upon data of an observer's preconceived notions or wishes has been recognized and investigated at least since the beginning of this century. Much of the research has been in educational and social psychology and has focused upon the psychological environment in which experiments are conducted. Typical of such investigations is Cordaro and Ison's (1963) study of observers who documented the activity of planaria. Some observers were given to believe that their planaria would move and turn frequently, whereas others expected their animals to move and turn infrequently. The high-activity-expectancy group recorded an average of 18 moves and 49 turns during 100 trials, whereas the low-activity-expectancy group recorded an average of only 1 move and 10 turns in 100 trials. The authors con-