Some scientists think of this ability as a kind of sixth sense, something like a number sense. One reason is that the skill appears to originate in specific parts of the brain much like our sense of touch and sight.

"When we see a small number of items visually, we don't need to count them," says Ben Harvey, of Utrecht University in the Netherlands, who lead the study. "We just know how many there are straight away."

Most people hit their limit at around five items, Harvey says. Then people get less and less accurate about estimating the quantity.

But there's definitely variation among people. "One subject we measured was just beautiful," he says. "His brain responds all the way through the number eight."