Some women were more effective than others at making that flirtatious face, the researchers found, and some men were better at recognizing it. Overall, the researchers described this combination of traits as "highly recognized flirtatious expressions".
"We found that most men were able to identify a certain female facial expression as flirting," study co-author and psychology professor Omri Gillath, Ph.D., said in a press release. "It has a unique morphology and is different from expressions that have similar characteristics – for example, smiling – but are not identified as a flirtatious expression by men."
Notably, the flirtatious look didn't necessarily include a Duchenne smile, the kind of smile that shows in your eyes and is known to be more authentic. This type of smile can be interpreted as "too happy", which may lead to the expression being interpreted as purely friendly, "write the researchers in the paper about their results.