A new study indicates that rule-based grammatical knowledge among toddlers emerges gradually with a significant increase around the age of 24 months.
The finding, backed by statistical evidence and published in Psychological Science, is a step forward to resolve a puzzle, or a debate, over whether toddlers' grammar skills are inherent or learned with time and practice.
"The ability of humans to acquire and use language is a big difference between us and other species, and it's also one of the biggest scientific puzzles out there," said Stanford University associate professor of psychology Michael Frankx. "Studying language acquisition in children is one way for us to try to find out what makes us human."
Previous research has shown that children use articles, such as "a" and "the," early and in an overwhelmingly correct way. But, said Frank, who co-authored the study, it is difficult to sort out whether children are just imitating adults or if they actually understand that articles should be used before nouns like "dog" or "ball," and can use them appropriately with new nouns that are unknown to them.
To address that difficulty, the Stanford team created a new statistical model to measure changes in a child's grammar over time, and applied this model on data sets available for 27 toddlers. The model relies on Bayesian inference, a method that helps estimate the level of certainty in results, and takes into account the relationship between what the child says and what the child has heard from adults, separating imitation from generalization.
The researchers found that rule-based grammatical knowledge in the toddlers' speech wasn't constant and was more present in older children.
The study underscored the fact that data on language development in children under 2 years old is lacking. According to Frank, the current lack of data and the analytical challenges it presents have led to researchers on opposite sides of the grammar debate to draw contradictory conclusions. "People have very strong feelings about the question of innateness versus learning," he said. "We really didn't know what to expect because there were these conflicting reports out there."
The team hopes that its statistical model, together with new data sets, will help move the debate forward. Frank said he and his colleagues are building an online database called Wordbank to spur the gathering of data on children's vocabulary and early language development and encourage researchers to share their data with different institutions and universities.