This study will examine the effects of a single low dose of the 5HT2A agonist LSD (Lysergic Acid Diethylamide) (13 g) or placebo in individuals who are or are not explicitly told what drug they will receive. Although it is known that expectancies strongly influence subjective responses to most drugs, no studies have examined expectancies on response to a very low dose of LSD. This is especially important in the context of 'microdosing' of drugs. People who practice microdosing typically do so with strong expectations of positive effects, making it difficult to determine whether there is a pharmacological effect. To minimize expectancies in the laboratory, participants are usually not told exactly what drug they will receive (i.e., double-blind), but given a range of possibilities. In the present study, the study team will test half the subjects under single-blind conditions, where the participants (but not the research assistant) will know exactly what they are receiving. Other subjects will receive the usual instructions. Healthy volunteers will receive either a marginally detectable dose of LSD (13 micrograms) or placebo, under conditions where they i) know for sure what drug they are receiving or ii) where the identity of the drug is uncertain. Four groups of subjects (N=12 each) will attend single 4-hour laboratory session. The study team will examine subjective and behavioral responses to the drug in each of four conditions (Known-Drug; Known-Placebo; Uncertain-Drug; Uncertain-Placebo).