Prior to Robert Tryon’s study of selective rat breeding, concluded in 1942, many psychologists believed that environmental, rather than genetic, differences produced individual behavioral variations. Tryon sought to demonstrate that genetic traits often did, in fact, contribute to behavior. To do so, Tryon created an experiment that tested the proficiency of successive generations of rats in completing a maze. He initiated the experiment by exposing a genetically diverse group of rats to the maze, labeling those who made the fewest errors “bright”, and those with the most errors “dull”. Tryon then mated the “bright” males with “bright” females, and “dull” males with “dull” females. After their children matured, Tryon repeated the maze test with them, and again separated the “bright” and the “dull”, again breeding “bright” with “bright” and “dull” with “dull”. Tryon continued this process for seven generations, creating two distinct breeds of “bright” and “dull” rats. In order to demonstrate that behavior had little effect on the genetically selectively bred rats, and lessen the chance of error when making his conclusions, Tryon cross-fostered the rats—that is, he had a “dull” mother raise “bright” children, and vice versa. The independent variables in his experiment were the parental pairings, the choice of environment and parents for upbringing, and number of rats put through the maze. The dependent variable was the number of errors made by the rats in 19 trials of the maze.
Implications and conclusions
While Tryon's results showed that the “bright" rats made significantly fewer errors in the maze than the “dull" rats did, the question exists of what other sensory, motor, motivational, and learning processes also influenced the results of the experiment. A common misconception of this experiment and other similar experiments is that the observed change in the performance in the maze directly correlates with general learning ability. This is not the case. Rather, it has become a widely accepted belief among behavior geneticists that the superiority of the bright rats was confined to Tryon’s specific test; thus, it is not possible to claim that there is a difference in learning capacity between the two groups of rats. Genetic variation, such as better peripheral vision, can make some rats “bright” and others “dull”, but does not determine their intelligence. Nonetheless, Tryon’s famous rat-maze experiment demonstrated that the difference between rat performances was genetic since their environments were controlled and identical.
