Abigail Haslinger, Stephen Ferrigno, Steven T. Piantadosi, and Jessica F. Cantlon
Recursion is a computational method that embeds elements inside one another, allowing for long-range relations in a sequence. It is important for a wide variety of human skills, including language, counting, and motor control. To date there has been no conclusive evidence that non-human animals can use center-embedded recursion, the type of recursion necessary for language acquisition. This has led some researchers to hypothesize that it is a uniquely human skill and possibly the limiting factor of human language ability. Here we investigate whether monkeys (rhesus macaques) and children (age 3-5 yrs.) can learn to apply center-embedded recursion in a sequencing task. Subjects were given a task in which they had to touch sets of pictures of brackets in recursive orders, e.g. [ < > ]. We then tested whether subjects could transfer this center-embedded strategy to novel stimuli. We found that children naturally use center-embedded recursion to order novel lists. This suggests that mastery of this task, and thus mastery of a component of human grammar, comes easily to children and develops very early in childhood. Although monkeys did not use center-embedded recursion at first, with additional training, they were capable of learning to use a center-embedded strategy and transfer this skill to novel stimuli. This suggests that center-embedded recursion is not a uniquely human ability and is therefore not likely the limiting factor of language.