Anxiety and depression are leading causes of mental illness, and 25% of adults will be diagnosed with mental illness at some point in their life. The causes of these disorders remain unknown, but studies reveal a connection between early life stress (ELS) and psychopathological development. Some reports suggest that ELS is sufficient to induce depression- and anxiety-like behavior in mice. However, substantial evidence shows that traditional models of ELS inconsistently induce significant alterations in commonly used behavioral tests of depression- or anxiety-like behavior. This project tested a novel behavioral model and analyzed the effects of maternal separation in later post-natal days (PND) compared to previous studies, aiming to develop a more consistent and realistic understanding of the relationship between maternal separation and its long-term effects. This study tested 73 subjects in a 2x2 design (sex x ELS). Maternal separation occurred on PND 10-20. On PND 21, all subjects were weaned. Once 8 weeks old, all subjects underwent the light-dark emergence test (LDE) followed by an active versus passive primary reward preference test (sucrose water vs. running wheel) (AvP). The LDE test showed that stressed females took less time to enter the light portion of the light-dark box compared to stressed males. Females had increased entries and distance traveled in the light box under stressed and unstressed conditions compared to males. Following stress, females displayed impulsive-like behavior relative to males, who appeared to exhibit more of an anxious phenotype. The results of the AvP test show that control mice increased their running each day of the test, unlike stressed mice. There were no significant differences in the amount of sucrose drank. These findings suggest that the running test is a more sensitive measure of stress effects than the sucrose preference test. Future work will analyze the molecular mechanisms behind these findings.