What is a double-blind trial?

Double-blinding is designed to prevent bias: if people know they're getting a real drug, their expectations can affect how they feel, and researchers' expectations can influence how they measure results. Participants are usually randomly assigned to groups, which is why the gold standard is called a randomised, double-blind, placebo-controlled trial. Only at the end is the code revealed, a process called unblinding. Blinding has roots in 18th-century investigations, including a 1784 French commission, with Benjamin Franklin as a member, that tested Franz Mesmer's claims about 'animal magnetism'. Today, most new medicines must pass such trials before being approved.