Although public endorsement of conspiracy theories is growing, the potentially negative societal consequences of widespread conspiracy ideation remain unclear. While past studies have mainly examined the personality correlates of conspiracy ideation, this study examines the conspiracy-effect; the extent to which exposure to an actual conspiracy theory influences pro-social and environmental decision-making. Participants (N=316) were randomly assigned to one of three conditions; (a) a brief conspiracy video about global warming, (b) an inspirational pro-climate video or (c) a control group. Results indicate that those participants who were exposed to the conspiracy video were significantly less likely to think that there is widespread scientific agreement on human-caused climate change, less likely to sign a petition to help reduce global warming and less likely to donate or volunteer for a charity in the next six months. These results strongly point to the socio-cognitive potency of conspiracies and highlight that exposure to popular conspiracy theories can have negative and undesirable societal consequences.