People who pay in cash tend to make healthier food choices, according to a 2011 study from the Journal of Consumer Research. The study suggests that foods that are considered unhealthy are more likely to be impulse buys, which result from the anticipation of visceral factors, such as pleasure. The desire to impulse buy appears to be weakened by aversive visceral factors, such as pain. Most people find it more difficult to part with cash than to use a credit or debit card, so they're more likely to resist unhealthy choices if they pay in cash.
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