Prosopagnosia affects around two in every 100 people in the UK and is the inability to recognise people by their faces alone. In its most extreme form, people cannot even recognise their family or friends.
Milder forms, while still distressing, can be tricky to diagnose, which is why tests are needed according to the scientists from City University London and Kings College London.
People with prosopagnosia often use non-facial cues to recognise others, such as their hairstyle, clothes, voice, or distinctive features. Many more people are unaware they have the condition, instead believing they have a "bad memory for faces" but it is entirely unrelated to intelligence or broader memory ability.
Drs Richard Cook, Punit Shah and City University London and Kings College London have come up with a 20-item questionnaire to help measure the severity of someone's face blindness which can be completed at www.troublewithfaces.org/test-yourself-1