1. 1. Among the problems raised by choice behaviour is that of the mechanism of decisionmaking. Given that a chick pecks more often at a red spot than at a green one, but nevertheless sometimes pecks at the green one, what mechanism determines each individual choice? This paper presents a model called the choice threshold model to help answer this type of question. 2. 2. The model is initially expressed in terms, familiar from the ethological literature, of a fluctuating variable (cf. 'drive'), interacting with thresholds. Whether expressed in this pictorial way or not, the important assumption of the model is that for any two stimuli presented together, an animal either chooses the preferred one or chooses completely indiscriminately; the less preferred stimulus can only be chosen during periods of non-discrimination. 3. 3. A precise quantitative prediction (of what psychologists call 'strong stochastic transitivity') is deduced from the model and named prediction 1. If any three stimuli are presented in all three possible pair combinations, and if the percentage preference for the best over the worst is called PAC, the other two percentage preferences being PAB and PBC, prediction 1 is that PAC should equal 2(PAB+PBC-PAB PBC)-I. 4. 4. Experiments on domestic chicks, and data from, other workers on a variety of species including man tend to confirm prediction 1. Other predictions will be discussed elsewhere. 5. 5. Some general properties of the model, and alternative ways in which they might be realized are discussed. © 1969.
