Psychology of the Scientist: X. Observer Bias in Classical Conditioning of the Planarian

Three groups of undergraduate volunteers were given differential expectancies about S's behavior prior to their observations of planaria undergoing conditioning. Group HE ( n = 5) received a high response expectancy, Group LE ( n = 5) a low response expectancy, and Group HLE observed one planarian under each expectancy condition. Group HE reported 18% contractions and 49% head turns in 100 trials whereas Group LE reported but 9% and 9.9%. Group HLE reported 15.4% contractions and 30% head turns under high expectancy instructions, but only 4.8% and 15.4% under low expectancy instructions. Analyses of the effect of instructions between Groups HE and LE and within Group HLE both yielded significant F ratios ( P < .001). Although it is unwise to generalize from naive volunteers to sophisticated investigators, it is clear that response recording in planaria should be made less ambiguous, perhaps by taking photographic records.