Numbers above three are incomprehensible to people who don't speak a language that uses symbols for counting, say researchers at the University of Chicago.
A study of deaf people in Nicaragua who never learned formal sign language showed that those who communicate using self-developed gestures, called homesigns, couldn't comprehend larger numbers, unlike those who learned conventional sign language as children.
"It’s not just the vocabulary words that matter, but understanding the relationships that underlie the words - the fact that 'eight' is one more than 'seven' and one less than 'nine'," said Susan Goldin-Meadow.
"Without having a set of number words to guide them, deaf homesigners in the study failed to understand that numbers build on each other in value."
Scholars have previously found that people in isolated cultures don't learn the value of large numbers when they aren't part of the local language. Two groups studied in the Amazon, for instance, lack words for numbers greater than five and can't match two rows of checkers containing more than five items.
For the new study, scholars gave homesigners a series of tasks to determine how well they could recognize money. They were shown 10-unit and 20-unit bills and asked which had more value. When asked if nine 10-unit coins had more or less value than a 100-unit bill, they were all able to determine the money’s relative value.
However, "The coins and bills used in Nicaraguan currency vary in size and color according to value, which give clues to their value, even if the user has no knowledge of numbers," said researcher Elizabet Spaepen.
To see if the homesigners could express numerical value outside of the context of money, the scholars showed them animated videos in which numbers were an important part of the plot. They then asked the deaf individuals to retell the story to a friend or relative using homesigns. As the numbers grew, the homesigners became less and less able to indicate the number with their fingers.
They were then shown cards with different numbers of items on them, and asked to give a gesture representing that number - and were accurate only up to the number 3.
They also had difficulty making a second row of checkers match a target row when there were more than three checkers involved, despite the fact that this didn't require any comprehension or production of number gestures.
Their difficulty in understanding large numbers therefore did not stem from an inability to communicate about large numbers, but rather from an inability to think about them, the researchers concluded.