Vital clues about how well the brain is aging may already be at our fingertips.

A new study out of Portugal found that the speed at which older adults are able to perform a basic, everyday task could be a powerful indicator of cognitive decline.

The findings suggest it may one day serve as a diagnostic tool, offering doctors a noninvasive and relatively low-cost way to monitor changes in seniors’ memory and thinking skills.

Across the country, more than 8 million Americans are believed to be living with mild cognitive impairment, yet research shows over 90% don’t know they have it.

That’s troubling, since catching the condition early is considered key to delaying the onset of dementia and Alzheimer’s disease, giving patients a chance to benefit from preventive steps and newer treatments that may slow its progression.

The alarming rate of missed diagnoses has fueled calls for better screening tools in doctors’ offices — and researchers now think the answer may be hiding in handwriting.

“Writing is not just a motor activity, it’s a window into the brain,” Dr. Ana Rita Matias, senior author of the study and a kinesiologist from the University of Évora, said in a news release.

For the study, researchers recruited 58 adults aged 62 to 92 living in Portuguese care homes. Of them, 38 had been diagnosed with cognitive impairment, while the remaining 20 were considered healthy.

Using a pen and digital tablet, the participants were instructed to complete two types of tasks.

The first measured pen control. Seniors were asked to draw 10 horizontal lines within 20 seconds and place at least 10 dots on the paper in the same amount of time.

The second focused on handwriting speed. Participants wrote two sentences of varying difficulty — one copied from a card and the other dictated.

That’s when clear differences began to emerge.

The researchers found that when writing sentences they heard spoken aloud, seniors with cognitive impairment consistently approached the task differently than their healthy peers.

They tended to pause longer between pen strokes and used more frequent, smaller strokes to write the sentence.

“We found that older adults with cognitive impairment displayed distinct patterns in the timing and organization of their handwriting movements,” Matias said.

Interestingly, the researchers didn’t see any meaningful differences between the two groups in the simpler exercises, like drawing dots and lines or copying written sentences.

The team theorized that this could be because those tasks rely mostly on basic motor skills and may not be mentally demanding enough to expose subtle differences that indicate cognitive decline.

“Dictation tasks are more sensitive because they require the brain to do multiple things at once: listen, process language, convert sounds into written form and coordinate movement,” Matias said.

“Even within dictation tasks, differences can emerge. A longer, less predictable or linguistically demanding sentence places greater strain on cognitive resources.”

In participants with cognitive impairment, researchers identified two major warning signs during the shorter dictation sentence: delayed start time and the number of pen strokes used.

For the more complicated sentence, three factors stood out: vertical letter size, start time and writing duration.

The researchers said that’s likely because not every aspect of handwriting reflects cognition in the same way.

“Timing and stroke organization are closely linked to how the brain plans and executes actions, which depends on working memory and executive control,” Matias explained.

“As these cognitive systems decline, writing becomes slower, more fragmented and less coordinated,” she continued. “In contrast, other features can remain relatively preserved, especially in the early stages of cognitive decline, making them less sensitive indicators.”

The study did have some limitations. For example, the researchers didn’t account for medications that could affect performance.

As a result, the findings still need to be confirmed in larger and more diverse populations before handwriting can be deployed as a screening tool.

Still, the team believes the approach could eventually become a simple way for doctors to monitor cognitive decline.

“The long-term goal is to develop a tool that is easy to administer, time-efficient and affordable, allowing integration into everyday healthcare contexts without requiring specialized or expensive equipment,” Matias said.

But there could be one major hurdle in the US: Americans may be losing the very skill researchers are hoping to measure.

A growing number of parents and teachers are complaining that children’s handwriting has deteriorated over the years, often starting out neat in elementary school before devolving into barely legible scribbles as they get older.

Many blame the rise of classroom technology, as students increasingly type assignments on computers instead of writing by hand. The decline of cursive lessons in schools has only added fuel to the debate.

That raises an intriguing question: If younger Americans grow up writing less — and worse — than previous generations, will handwriting still be a reliable warning sign of cognitive decline by the time they reach old age?

Only time will tell.