Recently, the personality factor of Honesty-Humility has attracted substantial attention. In particular, it has not only been shown to predict several criteria, but also to entail a specific pattern of interaction with situational factors. Herein, we extend previous research on both findings in linking Honesty-Humility to the quintessential social dilemma, the prisoner's dilemma. In three investigations, we found support that those high in Honesty-Humility were more likely to cooperate - so long as this was sensible in any way. Those low in Honesty-Humility, by contrast, tended to defect - especially when this behavior was very tempting but not risky. Thus, Honesty-Humility shows promise as a crucial basic personality variable predicting social dilemma decision making, especially in interaction with situational factors.