Another true story. A man closes his bank account. Not because of fees, and not because the app crashed. He closes it because moving his own money made him feel afraid. To pay anyone, the bank first made him create a "beneficiary", an abstract entity buried far from the payment screen, named in jargon that could equally mean paying a government office. Every transfer became a small ordeal of doubt: is this the right thing, will my money vanish, why can I not just pay a person? The interface worked, technically. Emotionally it said you cannot trust us, and eventually he believed it.
Design theory has a precise vocabulary for this. Don Norman describes three levels of emotional response to any product:
Visceral: the instant gut reaction to how something looks, before you use it. Behavioural: how it feels during use, smooth and capable or clumsy and stressful. Reflective: the story you tell yourself afterwards, "I am an iPhone person", "I trust this bank". The levels are independent. A beautiful app can feel awful to use. An ugly tool can be loved.
The insight with real power is that the levels matter differently for different products. A tool someone uses eight hours a day lives or dies on the behavioural level, and its users will forgive an intimidating surface. An app opened twice a year gets judged almost entirely viscerally, because there is no time to build the deeper relationship. Part of the designer's job is working out which level your product competes on.
Emotion is measurable
"Feelings are data" is meant literally. Researchers measure emotional response with surveys, with facial coding, with the words people choose in interviews. And the payments example shows why the stakes are asymmetric: bad design that costs people time gets complained about, but bad design that makes people feel unsafe gets abandoned. Especially anywhere near money, health or privacy, trust is the actual product.
Find the app on your phone that annoys you most. Name which level fails. Is it ugly (visceral), infuriating to operate (behavioural), or does it just feel like a company you do not trust (reflective)? Then find your favourite app and do the same in reverse. You will never un-see the three levels again.
Don Norman, Emotional Design, opens with exactly this three-level model. His earlier The Design of Everyday Things, chapter 2, explains the psychology of why interfaces make people feel stupid when it is never the person's fault.