Recent research indicates that social trust of those who manage a hazard is strongly correlated to judgments about the hazard's risk and benefits. The present study investigates the more specific question of "For which hazards is this?" It was postulated that when an individual lacks knowledge about a hazard, social trust of authorities managing the hazard determines perceived risks and benefits. On the other hand, when an individual has personal knowledge about a hazard and therefore does not need to rely on managing authorities, social trust is unrelated to judged risks and benefits. Participants (N = 91) assessed risks, benefits, and trust in managing authorities and personal knowledge associated with 25 hazardous technologies and activities. As expected, strong correlations between social trust and judged risks and benefits were observed for hazards about which people did not possess much knowledge. No significant correlations between social trust and judged risks and benefits were found for hazards about which people were knowledgeable. Results suggest that the lay public relies on social trust when making judgments of risks and benefits when personal knowledge about a hazard is lacking. Replicating findings of other studies, the present study also found negative correlations between perceived risks and perceived benefits. When social trust was controlled for, correlations between perceived risks and benefits diminished. Implications of the results for risk management are discussed.