Why so many people make the theoretically irrational decision to cooperate in a one-shot Prisoner's Dilemma game remains a puzzle in game theory. Recent developments in evolutionary psychology suggest that the anomaly may be attributable to evolutionary constraints on the human brain and their interaction with general intelligence. We conduct a laboratory experiment to test three hypotheses: (a) projection of a video image of another experimental subject increases cooperation because the human brain implicitly assumes that their choice is not anonymous; (b) more intelligent individuals are more likely to defect, because they are more likely to comprehend the evolutionarily novel features of the experiment that make defection rational; and (c) the effect of the video projection on cooperation is greater among less intelligent individuals. The experiment clearly supports two of the three hypotheses.