A tip can turn a worker’s income into a referendum on a stranger’s mood.

Tipping makes someone’s income depend on the moods of strangers. A good service can be poorly rewarded because the customer had a bad day, and the worker absorbs the variance while having no way to predict or contest it. It looks like a reward for quality and functions as a transfer of risk.

Tip consistently rather than expressively, and toward the top of the range where you can. If you are deciding based on how you feel about the interaction, notice that the interaction was not the whole job. Consistency is worth more to the person than generosity that arrives unpredictably.