Reasoning during joint decision-making by preschool peers

•This study investigated preschoolers’ argumentation for joint decisions with their peers.•Preschoolers can reason with one another and justify their proposals based on appropriate common ground assumptions.•5-year-olds produced more justifications and reached mutual agreement in their joint-decisions more than did 3-year-olds. Reasoning with a peer to make a joint decision involves making a proposal (e.g., “Polar bears go here”) and justifying it with relevant facts (e.g., “This is ice”) based on common ground assumptions or warrants (e.g., polar bears need ice). Twenty-four dyads of 3- and 5-year-olds built a zoo with toy items that were either conventional (e.g., animals, cages) or unconventional (e.g., piano). For conventional items, both participants in both age groups used justifications that relied on implicit warrants (e.g., stating only the fact “This is ice”, assuming that both partners know that polar bears need ice). For unconventional items, they more often articulated the warrant explicitly, arguably to create the necessary common ground. Five-year-olds made warrants explicit more often, produced more justifications, and reached mutual agreement more often than did 3-year-olds. These results suggest that preschoolers can reason with one another appropriately, specifically in justifying their proposals based on appropriate common ground assumptions.