Behavioural Economics: The Inferior-Good Effect
These experiments used 15 domestic hens to investigate the inferior-good effect, a decrease in consumption of a commodity as income increases. Experiment 1 investigated plain and salted wheat to serve as superior goods (opposite to inferior goods) and inferior goods. Hens consumed mostly plain wheat when given ad-libitum access so it was the intended superior good and salted wheat was the intended inferior good. In the next experiments, 3 s of plain wheat and 10 s of salted wheat were available for single responses on 2 keys during discrete trials. Income changed by changing the inter-trial interval (ITI) of fixed-length sessions or the total number of trials. Experiment 2 partially replicated the Silberberg et al. (1987) procedure, using the ITI income analogue for 6 hens. When income increased, 4 hens sometimes responded less on the salted-wheat key (demonstrating the inferior-good effect), 2 of these hens and a 5th hen sometimes responded more on both keys but proportionally more on the plain-wheat key (termed here as a relative inferior-good effect). Experiment 3a partially replicated the Hastjarjo et al. (1990a) procedure using the total-trials analogue for 7 other hens. The inferior-good effect occurred across some conditions for 2 hens while other hens tended to respond on the plain-wheat key, suggesting lack of contingency contact. When a 60-s ITI was added in Experiment 3b, variability increased for most hens, but only 1 hen showed the effect, 1 of the hens that did so in Experiment 3b. These 6 hens' (1 died) 80% bodyweights were re-assessed in Experiment 4 and hens were below 80% during Experiments 3a and 3b, suggesting that the lack of the inferior-good effect was not due to some hens being at high weights. The ITI analogue was used for these 6 hens in Experiment 5 and the effect occurred for 2 hens. Experiment 6 added forced-choice trials to the total-trials analogue (with 60-s ITI) to guarantee contingency contact. The inferior- and/or relative-inferior-good effect occurred for 3 hens. Across Experiments 2 through 6, body weights were usually heavier in high-income conditions and a within-session pattern of early-salted-late-plain responding occasionally occurred. Crop capacities of 5 Experiment-2 hens and a new hen (1 died) were assessed in Experiment 7 and there was no relation between this measure and inferior-or relative-inferior-good effects. Experiments 8 and 9 examined effects of pre- and post-feed in low-income conditions using the ITI analogue. When hens were pre-fed, responding for 5 of 7 hens resembled responding in high-income conditions of Experiment 2 with more plain-wheat responding and similar or less salted-wheat responding in some conditions (behaviour similar to the inferior- and relative-inferior-good effects, but without the income change). A similar pattern was found for 4 of 5 hens when hens were post-fed in Experiment 9, suggesting that food in the digestive tract may have played a role, and perhaps not the income manipulations themselves, where it (or other component of body weight) may have abolished quantity (i.e., the intended-inferior-good) as a reinforcer. Although these experiments occasionally demonstrated inferior- and relative-inferior-good effects, but less convincingly than published studies, the effects of income may have been non-specific. The usefulness of the inferior-good concept and other income-related economic concepts are thus challenged.
behavioural economics, inferior good, income, choice, consumption, body weight, crop, gallus domesticus
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