When someone looks at a familiar face, it takes less than half a second for their brain to match the nose, eyes, mouth, chin, and cheeks to an identity.

That superpower-like instinct happens so effortlessly for most of us that we never even think about it. But not everyone has that luxury.

Some people struggle their whole lives with a mystifying condition known as developmental prosopagnosia, in which known faces look unfamiliar, or the faces of strangers look tantalizingly recognizable. Some who have so-called face blindness can't even recognize themselves in a mirror.

Today, most estimates predict about 2 to 2.5 percent of the world's population has some form of this cognitive disorder, and yet according to new research from Harvard University, it may not be as rare as we first thought.

As the condition has gained greater media attention in recent years, more people have come forward to express their own personal struggles with face blindness.

More than half of those who think they were born with the condition don't meet the most common diagnostic standards. These milder cases are not included in research, and yet they clearly stand out at a population level.

When researchers at Harvard gave a variety of tests and questionnaires on facial recognition to more than 3,100 adult participants in the United States, they found a cluster of people who scored quite poorly.

Depending on which diagnostic cut-offs were used for face blindness (of which there are many), the authors found the condition ranged from a prevalence of 0.13 percent all the way up to 5.42 percent.

Today, the most common diagnostic standards are quite strict, typically including a combination of self-reports and objective tests. In the Harvard study, this strict threshold produced a diagnostic rate of nearly 1 percent.

Interestingly enough, however, this group of patients did not necessarily score the worst on facial recognition tests.

Some who were not diagnosed with prosopagnosia under this strict criteria actually performed worse than those who were.

The findings suggest face blindness exists on a spectrum, like many other developmental disorders, such as autism and multiple sclerosis.

In total, researchers at Harvard identified 31 individuals who had major prosopagnosia and 72 individuals who had mild prosopagnosia. Together, that represents 3 percent of the entire sample size.

Expanded to a population level, that's roughly 10 million Americans who may suffer from face blindness, millions of whom are currently left out of the picture.

"This is important on several levels," explains psychiatrist Joseph DeGutis from Harvard.

"First, the majority of researchers have used overly strict diagnostic criteria and many individuals with significant face-recognition problems in daily life have been wrongly told they do not have prosopagnosia."

If scientists working on face blindness loosen these parameters, more people who struggle with facial recognition might seek out workarounds and tricks to help them identify faces. And as long as these milder cases are included alongside more severe cases in research, they do not seem to significantly dilute the overall pool of patients.

"This provides support for [developmental prosopagnosia] existing on a continuum rather than representing a discrete group," scientists at Harvard write.

"This finding provides preliminary support for the assertion that using more relaxed diagnostic criteria does not appreciably change the nature of the disorder being studied," they add.

Another paper published at the end of 2022 makes a similar argument. Loosening the diagnosis for face blindness is more inclusive, the authors argue, and will ultimately expand our limited knowledge of the disorder.

According to scientists at Harvard, those who are studying developmental face blindness should use two standardized disorder cut-offs from now on, one for major cases and one for mild cases. At least, that is, "until more mechanistically grounded cutoffs can be identified."

"Expanding the diagnosis is important because knowing that you have real objective evidence of prosopagnosia, even a mild form, can help you take steps to reduce its negative impacts on daily life, such as telling consequential coworkers, or seeking treatment," says DeGutis.

There's even a chance that mild forms of face blindness could actually benefit more from cognitive training and treatment.

It's about time we took those cases into account.

The study was published in Cortex.