The punch cards stuffed in your wallet know next to nothing about you, except maybe how many frozen yogurts you still need to buy to get a free one.

But loyalty programmes, as they shift from paper and plastic to apps and websites, are increasingly tracking a currency that can be more valuable than how much you spend: personal data. As a result, the programmes know things about you that some of your friends may not, like your favorite flavour (mango), when your cravings strike (early afternoon) and how you pay (with your Visa), in addition to billing details and

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