In the Game of Thrones universe, confusing a photo of actor Jack Gleeson, who played the popular HBO TV show's sadistic Joffrey Baratheon, for one of Maisie Williams, the beloved Arya Stark, is an egregious case of mistaken identity.

It's true Gleeson has dark hair rather than Joffrey's usual blond, but still. If nothing else, Gleeson is male and Williams female.

This is just one of the failures we have seen when we used characters from Game of Thrones to develop a new test of human face recognition. Thanks to the series, our research shows that becoming familiar with a face and reliably recognising a person are complex processes.

Facial recognition feels easy

Humans are a social species and recognising faces is a crucial skill.

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We seem to do it effortlessly, but there are huge individual differences. Some people are practically "face-blind" and can't identify people in their own family, while "super-recognisers" claim they never forget a face.

Security and law enforcement agencies are now seeking to employ the latter, but it is critical to understand their limits, too.

As researchers, we noticed significant limitations in face recognition tests. Some rely on recognition of famous faces — politicians, athletes, actors, musicians. But people vary widely in how often they might encounter these people so it is difficult to know if mistakes are the result of poor recognition ability, limited exposure or a mere lack of interest.

Other tests use strictly controlled faces of strangers — bare of hair, glasses and other adornments — that people study in the lab. But this seems very unlike the way we become familiar with faces in the real world, where we meet people in many different situations and without any real intent to study their features.

Which brings us to Game of Thrones. Fans have been exposed to hundreds of characters over several years, while they aged, changed hairstyles or became disfigured. One of us (Christel Devue) is good with faces according to the standard lab-based tests, but noticed she kept mixing up similar-looking characters while watching the show.

GoT as a research tool

When we conducted our test, Game of Thrones had been running for six seasons and had featured many hundreds of actors. Because the show has such a dedicated following, the internet provided us with a wealth of information to develop a very well-controlled experiment.

For example, some fans had calculated how long each actor was visible on screen. As well as being able to gauge actors' exposure levels, we also knew how much time had elapsed since characters were last seen (before facing a gruesome death).

We tested 32 fans who had watched all six seasons only once, as each one was released. This way, we were sure everyone had had the same exposure.

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Participants had not read George R.R. Martin's original novels, so would not know the characters from a different source.

We showed participants 90 head shots of actors (not in character) with four different levels of exposure in the show (as main heroes, lead characters, support characters and bit parts), mixed with 90 strangers. They judged whether each face was familiar, rated their confidence in that judgment, and tried to identify and name the character or the actor.

Half were shown pictures in which actors' head shots were similar to their character, while the others saw pictures in which the actors' appearance differed (for example, different hairstyle, facial hair, makeup, glasses).

Surprisingly, we found that simple changes in hairstyle or the passing of time throw off recognition of actors in all exposure levels, even those who fans saw repeatedly for years.

Limitations in human recall

No one, even the best recognisers, spotted all faces. The best hit rate was about 80 per cent of the actors, but from someone who also falsely recognised about 50 per cent of strangers. Good recognisers were good not because they recognised more faces but because they were able to reject novel faces as unfamiliar.

Some people were extremely confident despite reporting very inaccurate information. Others were uncertain but were spot-on most of the time. Some could not remember many names and gave really convoluted descriptions of characters.

Joffrey and Arya were not the only cross-gender mix-up. A photograph of a short-haired Sibel Kekilli (Tyrion Lannister's mistress Shae) was identified by several fans as Isaac Hempstead Wright (Bran Stark). Such misidentifications and others were often based on superficial features like hair colour or style, facial hair or the shape of a head, showing how important these are.

Although recognition rates rose with more exposure, performance was far from what you'd expect of fans of a show. Identification errors occurred at all exposure levels. Only the most prominent actors were correctly identified and named more often than they were just recognised. As for bit parts, they were never correctly identified and only rarely recognised, even if their brief appearance was sometimes shattering (one of them was the cause of the Red Wedding). Familiarity judgments for them were barely higher than for strangers.

This research has important practical implications. Criminals know simple disguises make it possible to escape prosecution. Many innocent people are convicted based on errors in eyewitness testimony.

Our research confirms recognition based on brief encounters is likely to be wrong. Confidence of a witness in these conditions is not a good indicator of whom they have actually seen. And while super-recognisers have made useful contributions to law enforcers, they are not infallible.

As for people planning to watch the final season of Game of Thrones, perhaps a refresher viewing is in order to avoid any confusion and ensure you really do know who's who.

Christel Devue, Lecturer in Cognitive Psychology, Victoria University of Wellington and Gina Grimshaw, Associate Professor, Victoria University of Wellington

- The Conversation