Mint is associated with fresh breath. It’s an entirely artificial association writes Joseph Stromberg:

• The first mouthwashes and toothpastes contained ingredients like gillyflower, ginger, and cinnamon.
• The person responsible for getting Americans to brush every day – the maker of Pepsodent – wanted to use behavioural conditioning to make people feel good about brushing their teeth.
• To that end he added mint oil – which generated a tingly feeling in the mouth – to toothpaste. Every time people brushed they were “rewarded” with that tingly feeling.
• That feeling soon came to be associated with freshness – even though it is entirely artificial. While mint oil makes your mouth feel cooler it doesn’t actually clean anything in your mouth by itself.
• Listerine noted the brilliance of the strategy and pumped its mouthwash full of menthol – a chemical that produced a similar cooling effect.
• Listerine also launched a marketing campaign that told people that they would be social outcasts if they had bad breath.
• The campaign was a success and sales of Literine exploded.
• Now everybody associates the feeling of mint oil and menthol with mouth freshness, a relationship that reinforces itself.

Read about the differences between mint and menthol and the history of toothpaste and mouthwash in the United States over here.

Source: Vox