Explicit social pressure has been shown to be a powerful motivator of prosocial behavior-like voting in elections. In this study, I report the findings of a randomized field experiment designed to study the impact of more subtle, implicit social-pressure treatments. The results of the experiment, conducted in the October 2011 municipal elections in Key West, Florida, demonstrate that even subtle, implicit observability cues can effectively mobilize citizens to vote, perhaps as much as explicit surveillance cues. The findings speak more broadly to our understanding of human decision making, and even evolution, and provide fodder for the claim that humans are evolutionarily programmed to respond to certain stimuli. I interpret the evidence to support the notion that evolutionarily charged impulses, like exposure to images that implicitly signal the potential for surveillance and observability, are sufficient to overcome powerful collective action incentives to free ride.