FOR eight months up to this April, a French bookstore chain had video in a Paris shop fed to software that scrutinises shoppers’ movements and facial expressions for surprise, dissatisfaction, confusion or hesitation. When a shopper walked to the end of an aisle only to return with a frown to a bookshelf, the software discreetly messaged clerks, who went to help. Sales rose by a tenth.

The bookseller wants to keep its name quiet for now. Other French clients of the Paris startup behind the technology, Angus.ai, are testing it in research shops that are not open to the public. They include Aéroports de Paris, an airport owner; LVMH, a luxury conglomerate; and Carrefour, a chain of hypermarkets. In a test at a Mothercare shop in Tallinn, Estonia, software from Realeyes, an emotion-detection firm based in London, showed that shoppers who entered smiling spent a third more than others.

Simple video yields a lot of insight. But there are far more sophisticated and initmate ways of learning about emotions of shoppers. Thermal-imaging cameras can detect the heart rate. Wirelessly captured data from smartphone accelerometers can suggest when shoppers become fascinated (movement often stops) or are fretting over prices (a phone is repeatedly raised to search for cheaper products online).

For even more insights, shoppers are sometimes asked to don special kit, typically in exchange for a discount or other reward. Wearable “galvanometer” gadgets, for example, measure moisture and electrical resistance on hand skin to reveal arousal.

All of this could be a chance, some say, for bricks-and-mortar retailers to trim the advantage that data have long given online sellers. A race is on to work out how best to collect and use emotions data, be it to improve packaging, displays, music, or the content and timing of sales pitches, says Rana June, chief executive of a firm in New York called Lightwave. It measures shoppers’ emotions for retailers, for malls, and for consumer-goods firms such as PepsiCo, Procter & Gamble and Unilever.

Not everyone is impressed. Some find it all a little creepy. Nielsen, a consumer-research giant, deems using technology to work out shopper emotions en masse too “avant-garde” for now, says Ricardo Gutiérrez, head of shopper insights at Nielsen Colombia in Bogotá.

But it is much cheaper than old-fashioned interviews. Nielsen charges roughly $10,000 to interview 25 shoppers about three products. Angus.ai’s service costs just €59 ($66) a month per camera. For $15,000 or so, iMotions, based in Copenhagen, gives retailers an EEG cap that detects brain activity, an eye-tracking headset that notes when an attractive object dilates pupils, and a galvanometer. iMotions’ 150 or so consumer-goods clients include Mondelez International, Nestlé and Unilever, which use them in mock-up stores and real ones.

What’s more, conventional market research can mislead. People typically “edit” verbal responses to make themselves sound rational, when purchases are often driven by subconscious emotions. The key is in tracking the unconscious things that shoppers do, says Jeff Hershey of VideoMining, a firm in Pennsylvania whose software also analyses store video. And surveys can also ask the wrong questions—such as how much people like a product when what really matters, notes Simon Harrop of BrandSense, a consultancy in Britain, is whether, say, it makes them feel attractive.

The notion of “retail therapy”, consumers driven to spend when they are feeling blue, is an obvious example of shopping’s emotional side. Whichever store is first to work out how to spot mildly depressed customers could make a bundle.