Developmental psychologists argue that children's moral reasoning abilities develop through a sequence of stages, stimulated by interactions with the social environment. This thesis attempts to extend the theory of human moral reasoning development to interpret ethical thinking about animal treatment. The study addresses the following questions: (1) Are boys capable of using consistent patterns of reasoning about animal treatment dilemmas? (2) Does each boy reason at different levels about humans and animals and about different types of animals? (3) Do boy's animal ethical reasoning abilities advance during maturation? (4) Do environmental interactions influence boy's levels of animal ethical thinking?

Interviews were conducted with 81 eighth and twelfth grade boys from suburban and rural Connecticut. Each boy was asked to resolve hypothetical human moral dilemmas and animal dilemmas involving three animal types: chimpanzees, dogs, and turkeys. Each boy was also questioned about his animal-related activities and knowledge.

The results support the conclusion that moral development theory can be extended to interpret boys' ethical reasoning about animal treatment. First, the findings show that boys were capable of using consistent reasoning patterns about animal dilemmas, indicating that animal reasoning, like human reasoning, is organized by stage structures. Second, it was discovered that human reasoning levels tended to be more advanced than animal reasoning levels and that chimpanzee and dog reasoning tended to be more advanced than turkey reasoning. Third, older boys tended to use more advanced animal reasoning levels than younger boys, indicating that animal reasoning abilities, like human reasoning abilities, improve with age. Fourth, although no relationship was found between animal reasoning levels and animal-related activities or knowledge, suburban boys often tended to have more advanced animal reasoning abilities than rural boys. The findings suggest that environment may influence animal ethical reasoning progress just as social interactions encourage human moral reasoning development.