Tip Rates (IMAGE) American Institute of Physics Caption Left: Average reported tip rate in American restaurants over time, according to the NPD group (1982-1984) and Zagat annual surveys (1989-present). Right: For conventional tip rates below some critical threshold Tc, a rational restaurant owner would allow diners to leave gratuity to maximize profitability (black curve). Beyond that critical threshold, a rational restaurant owner would disallow tipping in their restaurant (red curve). Credit Sara Clifton Usage Restrictions This image may be used only with appropriate credit. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.