A theoretical comparison of the consumer surplus and the elasticities of demand as measures of motivational strength

• Published in: Animal behaviour 2003, Vol.65 (1), p.157-178
• Main Authors: Kirkden, R.D, Edwards, J.S.S, Broom, D.M
• Format: Article
• Language: English
• Subjects: Animal behavior; Animal ethology; Biological and medical sciences; Comparative analysis; Consumers; Elasticity of demand; Fundamental and applied biological sciences. Psychology; General aspects; Psychology. Psychoanalysis. Psychiatry
• ISSN: 0003-3472; 1095-8282
• DOI: 10.1006/anbe.2002.2035

The price and income elasticities of demand have been used by ethologists to estimate motivational strength. The consumer surplus is an alternative measure of motivation, deriving from microeconomic theory. We made a theoretical assessment of the validity and versatility of these indices. Two factors are expected to compromise the internal validity (veracity) of the elasticity of demand indices: failure to take into account the amount that an animal is required to pay to maintain some level of consumption; and a tendency to confuse its readiness to defend a preferred consumption level with a propensity to become satiated. A third factor, expected to compromise the external validity (usefulness) of these indices, is the unrealistic assumption that a single value can be assigned to each resource. None of these problems applies to the consumer surplus index. One further factor, expected to compromise the internal validity of both the consumer surplus and price elasticity indices, is their failure to account for the effects that income has upon consumption. Overall, we conclude that the consumer surplus should be more valid, both internally and externally, than the price elasticity index. The consumer surplus should also be more externally valid than the income elasticity index, but it is unclear, on balance, which of these indices is the more internally valid. Finally, we show that both elasticity indices are considerably less versatile than the consumer surplus, owing to the assumption that a single value can be assigned to each resource. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.