Some of Tolman's early researches were early developments of what is now called behavioral genetics. Tolman would selectively breed rats for the ability to learn the mazes he constructed. Despite his major research focus involved instinct and purpose, he was open to the idea of researching innate abilities in the rats. Tolman's study was the first experiment to examine the genetic basis of maze learning by breeding distinct lineages of rats selected for their maze performance. Tolman started and continued this research project until 1932, where, after coming back from Europe on a sabbatical leave, his interest started to decrease. Tolman's theoretical model was described in his paper ""The Determiners of Behavior at a Choice Point"" (1938). The three different variables that influence behavior are: independent, intervening, and dependent variables. The experimenter can manipulate the independent variables; these independent variables (e.g., stimuli provided) in turn influence the intervening variables (e.g., motor skill, appetite). Independent variables are also factors of the subject that the experimenter specifically chooses for. The dependent variables (e.g., speed, number of errors) allows the psychologist to measure the strength of the intervening variables.