This Black Friday stores want to know what makes people spend. Fifty shoppers in Los Angeles, Boston and Atlanta will wear sweat-sensing bracelets as part of a market research effort aimed at understanding the body’s response to buying, the Wall Street Journal reported.

The bracelet, developed by the Massachusetts Institute of Technology Media Lab, measures movement, body temperature and electrodermal activity – barely noticeable changes in skin sweat levels. If a shopper’s excited or frustrated, electrodermal activity will go up. If they’re relaxed or bored, it will go down.

The researchers, from Shopper Sciences, hope tracking sweat and spending sheds light on why people buy — whether they’re in stores or online. The team will also use cameras to monitor online shoppers’ facial reactions to finding deals and navigating websites.

Market research, a field once dominated by surveys, is increasingly turning to technology to get  more reliable reactions from consumers. By measuring biometrics like skin sweat, facial expressions and even brain activity, researchers are teasing out what really makes shoppers tick — and what ticks them off.

But skeptics stand by standard market research, saying subtle physiological changes are too complex to tie to shopping behavior. Either way, it’s safe to say someone’s paying close attention to what you buy.