Last Digit Tendency: Lucky Numbers and Psychological Rounding in Mobile Transactions

•We investigate the digit distribution of numbers in a large mobile transaction dataset.•Although the first digit of numbers in transactions follows Benford’s law, we find that the last digit has a strong tendency to be a lucky number or be influenced by psychological rounding.•This lucky number tendency is more significant in transactions that are a key part of social interactions, such as money sent as gifts or as “red envelopes” during holidays, and in transactions with potentially greater emotional needs, such as during COVID-19 outbreaks and natural disasters.•This psychological rounding tendency is more common in payments between individuals.•This tendency could be used as an indicator of public sentiment or in methods of detecting fraudulent business activity.