Some people just can't seem to keep a beat.

You know the ones: They seem to be swaying to their own music or clapping along to a beat only they can hear. You may even think that describes you.

The majority of humans, however, do this very well. We clap, dance, march in unison with few problems; that ability is part of what sets us apart from other animals.

But it is true that rhythm — specifically, coordinating your movement with something you hear — doesn't come naturally to some people. Those people represent a very small sliver of the population and have a real disorder called "beat deafness."

Unfortunately, your difficulty dancing or keeping time in band class probably doesn't quite qualify.

A new study by McGill University researchers looked more closely at what might be going on with "beat deaf" individuals, and the findings may shed light on why some people seem to be rhythm masters while others struggle.

Truly beat deaf people have a very difficult time clapping or tapping to an auditory beat or swaying to one. It's a problem that is far more severe than a lack of coordination. And it isn't attributable to motor skills, hearing problems or even a person's inability to create a regular rhythm.

Illustrating how rare the disorder really is, McGill scientists received hundreds of inquiries from people who thought they were beat deaf, but only two qualified as having truly severe problems.

Those two people — a man named Mathieu and a woman named Marjorie — were compared to a large group of 32 people who represented the general population. Among the 32 people in the "control" group, some were better than others at matching a beat.

"The range in the general population is pretty wide, but not too wide because they're really doing quite wonderfully," said Caroline Palmer, a professor of psychology at McMcGill and the lead author of the study. "If you presented with a five percent change in the rate of the metronome, the great majority in the general population will hear it and respond to it. People are pretty adept at it."

In the lab, Palmer wanted to get a baseline estimate of Mathieu's and Marjorie's beat-matching abilities. Both were able to tap their fingers to their own beat with no music, just like the control group.

"The beat deaf individuals do not differ in terms of sensing the beat and they don't differ in terms of being able to feel a beat," Palmer said. "They can clap or tap regularly when there's no sound. It's only when they attempt to synchronize that they stick out."

The beat deaf participants were put to the test when they were asked to tap to a metronome whose beat became faster or slower throughout the exercise. For most people, the sudden change in tempo would cause them to adjust their tapping — first by perhaps overshooting the beat, then undershooting it, before finally settling in on the correct tempo.

The two beat deaf individuals couldn't do that.

Both Mathieu and Marjorie were far worse at settling in on the metronome's new beat compared to the control group.

"They are too far away from the beat and they stay far away from the beat," Palmer said.

Mathieu had some musical training and Marjorie had none, but the findings weren't explained by their musical ability or lack thereof.

To understand why, you have to consider the basic principles of how humans understand rhythms.

Scientists believe that many basic biological functions — like our heartbeat, walking, and even talking — operate based on rhythms controlled by an internal oscillating function. Think of it as traveling over a cyclic pattern of peaks and valleys on a mountain.

One theory suggests that this is how humans understand the passage of time — you have a sense of how much time has passed based on where you are in the cycle. Humans can synchronize their rhythmic activities, like the pace of walking or jogging, to a beat by adapting their internal oscillators to the rate of "highs and lows" in things like music.

Palmer's research suggest that both Mathieu's and Marjorie's internal oscillators function abnormally.

"What we found is that the oscillators were not adapting correctly to the change in the metronome rate," she said.

One of the beat deaf individuals had an oscillator that was running at a completely different frequency than the general population. In the second beat deaf individual, their oscillator didn't have the "dampen" function, which allows it to settle into the pulse in the music.

"Think of it as you get thrown out of whack, you overshoot the tempo or you undershoot it, then you settle in on the tempo," Palmer explained. "This person never settled in, they kept overshooting or undershooting."

To further test the "oscillator" theory, Palmer was able to create a computer model that mimicked the pattern of the unique internal oscillators of the beat deaf people.

When they put the computer model of Mathieu and Marjorie's oscillators to the same metronome test — where it needed to adjust to the speeding up or slowing down of the tempo — it accurately predicted the rate of error that Palmer had observed between the two beat deaf individuals and the control group.

That suggests that the oscillator theory explains why some people have serious trouble moving to a beat — something that most other humans can do very easily.

The study, which was published this week in the journal Philosophical Transaction: Biological Sciences, adds to a body of research about how humans' internal timing mechanisms work to control much of what we do — for example, the circadian rhythms that influence sleep and wake cycles.

It is still unknown, however, whether beat deaf individuals also have problems with adapting to rhythms in conversational speech, for example, or visual cues.