At the end of the laboratory session, which included a series of psychophysiological and behavioral tests, the participants were told that they had earned an additional bonus for their cooperation and good performance. They were asked to choose between two options: to receive $7 in cash immediately (the bills were placed on the desktop in front of them) or to receive $10 in cash by mail in 7 days. The test was administered individually to each twin, such that the twins were unaware of their co-twin’s choice. If the subjects chose the immediate reward, they were handed the $7 bonus, and if they chose a delayed reward, a $10 bill was mailed to them in a regular envelope on the 4th day following the lab visit (the local mail delivery takes 2–3 days, with larger delays being extremely rare). During a routine follow-up call, the receipt of the reward was confirmed and the subject was queried about their experience. No complaints about failed delivery or delays were received.
