Abstract Use of behavior evaluations for shelter dogs has progressed despite their lack of scientific validation as reliable diagnostic tools. Yet results of these evaluations are often used to make life-and-death decisions. Despite acknowledging the significant limitations of evaluations, most authors suggest that the solution is to continue to attempt to remedy deficiencies. We take a contrary position and use existing data and principles of diagnostic test evaluation to demonstrate that reliably predicting problematic behaviors in future adoptive homes is vanishingly unlikely, even in theory, much less under the logistical constraints of real-world implementation of these evaluations in shelters. We explain why it would be difficult, if not impossible, to calculate robust values for sensitivity and specificity of a shelter canine behavior evaluation as required for any valid diagnostic test. We further explain the consequences of disregarding the effect of prevalence on the predictive value of a positive test (e.g., eliciting biting or warning behavior from the dog in the behavior evaluation). Finally, we mathematically demonstrate why, for any plausible combination of sensitivity, specificity, and prevalence of biting and warning behaviors, a positive test would at best be not much better than flipping a coin, and often be much worse, because many of the dogs who test positive will be false positives. Shelters already screen from adoption obviously dangerous dogs during the intake process. Subsequent provocative testing of the general population of shelter dogs is predicated on an assumption of risk that is far in excess of existing data and relies on assumptions about dog behavior that may not be supportable. We suggest that instead of striving to bring out the worst in dogs in the stressful and transitional environment of a shelter and devoting scarce resources to inherently flawed formal evaluations that do not increase public safety, it may be far better for dogs, shelters, and communities if effort spent on frequently misleading testing was instead spent in maximizing opportunities to interact with dogs in normal and enjoyable ways that mirror what they are expected to do once adopted (e.g., walking, socializing with people, playgroups with other dogs, games, training). In conjunction with a thorough and objective intake history when available, these more natural types of assessment activities will help identify any additional dogs whose behavior may be of concern. Engaging in the normal repertoire of activities familiar to pet dogs has the additional benefit of enriching dogs' lives and minimizing the adverse effect of being relinquished and confined to a shelter, will be more indicative of the typical personality and behavior of dogs, and may help make dogs better candidates for adoption.