An economist designs a lab experiment to study honesty. Participants, who know they are part of a study, are asked to wait in a room where they find a 'lost' wallet containing $20. The economist observes whether participants return the wallet to the front desk. A major critique of such experiments is that the artificial lab setting can produce results that do not reflect how people behave in real life. Based on this critique, which of the following changes to the experimental design would MOST effectively increase the likelihood that the observed behavior reflects real-world honesty?