To explore factors that may inform teaching and learning in STEM education, we investigate individual and situational factors influencing students' cooperative versus competitive responses in a classroom, extra-credit problem social dilemma in core biology and engineering courses. We were curious how our competitive academic environment coupled with our cooperative military training impacts student decision-making. Analysis variables included class year (proxy for maturity), gender, group size, and number of exposures to the dilemma. Our results (N=2654) reveal significant main effects and interactions, with the largest effects often seen in women. To achieve more cooperative decision-making, instructors could create multiple small-group experiences and explicitly engage in and discuss cooperative decision-making. In addition to enhancing learning experiences for students, these activities could help initiate a climate shift to stimulate retention of women in STEM fields and overall achieve more cooperative decision-making.