You walk up to a self-checkout kiosk, scan your own groceries, bag them yourself, and then - right before you can escape - a screen lights up asking if you'd like to leave a 20% tip. For what, exactly ...

A study tracked purchases at grocery stores during the rollout of self-checkout between 2008 and 2011. Shoppers were significantly more likely to buy stigmatized items—especially condoms and pregnancy ...