Temperament tests for working dogs can provide substantial information about a particular dog's behavioral phenotype. When a larger proportion of the population is tested, the test results can also provide information about the effects of different environmental conditions on the phenotype because if the population is large, the social and physical environments to which the dogs are exposed differ. This means that we need to include in our evaluations the perspective that uses information about the environment in relation to the individual dog's level of development. There is substantial evidence that basic temperament traits in dogs are moderately heritable. There is also evidence that postweaning conditions have a huge effect on development, and this effect is often not assayed. Selective breeding for desired traits in combination with optimal environmental conditions, adapted to the individual dog's level of maturation, is a key point when producing outstanding working dogs.