Implicit learning theories suggest that we update syntactic knowledge based on prior experience (e.g., Chang et al., 2006). To determine the limits of the extent to which implicit learning can influence syntactic processing, we investigated whether structural priming effects persist up to 1 month postexposure, and whether they persist less long in healthy older (compared to younger) adults. We conducted a longitudinal experiment with three sessions: Session A, Session B (1 week after A), and Session C (4 weeks after B). For young adults, we found passive priming effects to persist and accumulate across sessions (1 week and 4 weeks). However, for older adults the effects persisted for 1 week but not 4. This suggests that for young adults, who unlike older adults experience no age-related decline in implicit memory, the limit to the duration of structural priming persistence is longer than 4 weeks. In a second longitudinal experiment with two sessions 1 week apart we found that priming in Session A affected syntactic processing in a different, independent task in Session B, both for young and older adults. Experiment 2 suggests that implicit persistence of the learned syntax is not limited to a specific context or task. Together, our findings give insight into how structural priming can contribute to language change throughout the life span, showing that implicit learning is a pervasive and robust mechanism that contributes to syntactic processing.

Reciprocity, responding to another one’s actions with similar actions, is central to the formation and maintenance of relationships. Reciprocity and relationship formation change with children’s development and are key aspects in human-robot interaction. So far, it is unclear how children reciprocate and build a relationship with a social robot and how reciprocity to social robots develops with age. In the current preregistered study, we collected data from 147 children aged 5 to 12 years to investigate the developmental trajectory of reciprocity towards a social robot and the formation of a relationship with this robot. To test reciprocity, children completed an Alternated Repeated Ultimatum Game with a social entertainment robot and another child. A recently validated survey on relationship formation was used that assesses trust, closeness, and social support. Results from a linear-mixed effects Bayesian analysis indicated that children reciprocated similarly to a robot as to another child. While reciprocity differed across age with lower values for 8-10-year-olds compared to younger and older children, this difference in the developmental trajectory of reciprocity was also observed when children interacted with the robot. Exploratory analysis showed differing results for positive (reciprocating positive actions with positive actions) and negative reciprocity (reciprocating negative actions with negative actions). Children’s relationship formation with a social robot changed with age but showed different developmental trajectories for trust (linear), closeness (negative quadratic), and social support (constant). No association was found between reciprocity towards the robot and relationship formation. Our findings suggest that established theories from human-human literature, such as the developmental trajectory of reciprocity, are also relevant for human-robot interaction. Children’s age is an important determinant for how children interact with and perceive robots. This therefore needs to be considered when designing robotic systems and experiments in the future as it could influence the success and effectiveness of both.