The ability to reason is a defining feature of human intelligence. Recently, reasoning has been redefined as a social skill, rather than an individual one, enabling people to justify and communicate their beliefs to reach joint decisions (Mercier & Sperber, 2011; Tomasello, 2014). Thus, human reasoning can best be observed through argumentation in the context of joint decision making. Here, the goal of the participants is to reach a decision that provides the greatest benefit to all while practicing the norms of reasoning (e.g., direct observation is more reliable than hearsay).

An important window into human intelligence is the reasoning of children, who are not just “little adults” but have their own sense of reason and logic (Piaget, 1929). How children engage in argumentative discourse with peers has only been documented among school-age children (ages 11-12; Kuhn, 2001). But how young children’s reasoning ability emerges and changes across development has not received much attention.

Here, we propose a series of studies that provides a developmental account of human reasoning in response to three main questions:

1) Do children as young as two justify their actions and recognize their partner’s reasons in deciding a course of action?

2) Do preschoolers engage in argumentative discourse with peers and practice the norms of reasoning to make correct joint decisions?

3) Are there cross-linguistic differences in children’s understanding of the norms of reasoning (in some languages, for instance, speakers indicate whether they have actually witnessed a past event)?

Overall, these studies will provide a window into a special form of human intelligence—children’s sense of reason and logic—and an account of ontogeny of human reasoning. The findings will be published and publicized in high-impact journals and prestigious conferences informing scientists and practitioners about how human intelligence can be observed in early ontogeny.