which focus on easily measured aspects of the physical environment. Others have argued that the performance of the animals should be used to assess psychological well-being. There has been considerable debate over the use of engineering versus performance standards (Novak and Drewson 1989; Sackett 1991).

The USDA, the agency with primary authority for enforcing the Animal Welfare Act, initially preferred engineering standards, which had been the cornerstone of all previous regulations. In the absence of performance measures to validate the efficacy of physical manipulations, however, it proved very difficult to identify the environmental features that promoted psychological well-being. For example, USDA initially proposed to alter cage sizes, assuming that larger cages would promote psychological well-being.

However, results of studies of the effects of cage expansion did not indicate that the change reduced abnormal behavior (Bayne and McCully 1989; Line and others 1990c); tension and aggression actually increased in some cases in which group-housed monkeys were given more floor space (Erwin 1979; Novak and Drewsen 1989). USDA also suggested that captive primates be given access to enrichment devices. However, it is often not clear what specific benefits enrichment devices provide or which devices actually provide enrichment, inasmuch as there are enormous individual and species differences in responses to such devices.

As a consequence of the inadequacy of an engineering approach to psychological well-being, the committee believes that the focus should be on the primates themselves and on their reactions to various features of life in captivity. This emphasis on performance standards presupposes that explicit criteria are available for the evaluation of psychological well-being. Although psychological well-being implies a subjective mental state or private experience, the practical need for observable criteria as a basis for assessment is imperative. As a first step toward developing such criteria, assessments of psychological well-being should look for signs of chronic distress as manifested in maladaptive or pathological behavior.

The essential next step is to decide on the signs of distress and on what constitutes maladaptive or pathological behavior (see also discussions of causes and signs of distress in NRC 1992). Meaningful judgments will involve some subjective component based on the observer's perceptions, experiences, and values, as well as reflect knowledge about the animal whose psychological state is being evaluated—ideally, knowledge based on experience with the species. Human perceptions and values, unless tempered by an understanding of the other species, can be a poor basis for judgments about psychological well-being. Many animals not only live their everyday lives under conditions that humans would find unendurable, but prosper in these circumstances. The natural lives of rats, lions, alligators, and giraffes differ from each other and from ours in myriad ways. So do the requirements of those species when they are maintained in captivity. Clearly, the criteria used to assess the psychological well-being of any species should be based on the best available information about that species.

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