Studies in restaurants show that the tips left by customers who pay their bill in cash tend to be larger when the bill is presented on a tray that bears a credit card logo. Consumer psychologists hypothesize that simply seeing a credit-card logo makes many credit card holders willing to spend more because it reminds them that their spending power exceeds the cash they have immediately available.
The statement 'In general, restaurant patrons who pay their bills in cash leave larger tips than do those who pay by credit card' most strongly supports the psychologists' interpretation of the studies.