Animals are predominantly risk prone toward reward delays and risk averse toward reward amounts. Humans in turn tend to be risk-seeking for losses and risk averse for gains. To explain the human results, Prospect Theory postulates a convex utility for losses and concave utility for gains. In contrast, Scalar Utility Theory (SUT) explains the animal data by postulating that the cognitive representation of outcomes follows Weber's Law, namely that the spread of the distribution of expected outcomes is proportional to its mean. SUT also would explain human results if utility (even if it is linear on expected outcome) followed Weber's Law. We present an experiment that simulates losses and gains in a bird, the European Starling, to test the implication of SUT that risk proneness/aversion should extend to any aversive/desirable dimension other than time and amount of reward. Losses and gains were simulated by offering choices of fixed vs. variable outcomes with lower or higher outcomes than what the birds expected. The subjects were significantly more risk prone for losses than for gains but, against expectations, they were not significantly risk averse toward gains. The results are thus, in part, consistent with Prospect Theory and SUT and show that risk attitude in humans and birds may obey a common fundamental principle.
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