What I Think I Want: Belief-Desire Formation in Young Children 

How do people make sense of actions, both others’ and their own? To explain your own action, reasoning begins from an evaluation (“playing with my friends would be fun”) and a fact (“if I learn the rules of the game and practice, I can play with my friends”), to intention (“I will practice playing the game”), and finally to action (“practicing”). To explain someone else’s action, you must proceed, in a reverse order, by inferring the reasoning behind their action (“Why does she keep practicing how to kick the ball?” “Because she wants to be good enough to play with her friends.”). Thus, to successfully interpret someone else’s actions, you must explain both their representation of the situation (i.e., beliefs), and their motivations (i.e., wants, desires). For this reason, our folk psychology—or theory of mind, the process of ascribing mental states to oneself and others—is essentially a belief-desire psychology (Moses et al., 2000; Fodor, 1975). 

A longstanding debate in theory of mind research is the development of belief-desire psychology, particularly how both components emerge relative to each other. On the one hand, the asymmetry view holds that the understanding of desires precedes the understanding of beliefs (Rakoczy et al., 2007; Wellman & Wooly, 1990). Conversely, the symmetry view proposes that beliefs and desires develop almost simultaneously, because they share the same underlying cognitive capacity (Perner et al., 2018). 

Evidence for the asymmetry view comes largely from the study of language through text, or corpus analyses. Children express desires even before they acquire the language to do so. For instance, they make grabby hands to reach for a toy or use rudimentary verbal expressions like “water!” to indicate that they are thirsty. Corpus analyses show that of the several desire verbs, children use “want” the most often and start to produce it around age 2 (Ferres, 2003), likely due to being exposed to the word by their parents who ask them about their wishes. In fact, children seem to produce desire verbs (e.g., want, like, and love) even before belief verbs (e.g., know, think, and forget). Belief verbs, on the other hand, first emerge around 3.5 to 4 years of age (Jenkins et al., 2003). Not only do children use and understand desire verbs before belief verbs, they also use desire verbs more frequently than all other mental state verbs (Ferres, 2003). 

Researchers speculate that this preference for desire verbs may have a utilitarian purpose: children may discover that verbally asking for what they want makes it easier to seek food and comfort from their caregivers, while talking about their beliefs may yield no immediate gratification or goal achievement (Harner & Khelmani, 2024). Desires may also be superior in terms of explanatory power (Steglich-Petersen & Michael, 2015). If a child were asked why another child keeps practising how to kick a ball, she would most likely answer, “Because she wants to be good enough to play with her friends.” Philosophically speaking, this explanation omits the belief that connects the desire to the action (that is, that practising will make her a better player and therefore allow her to play with her friends). In everyday conversation, however, this omission rarely causes confusion. The belief is typically shared as common knowledge, so mentioning the desire alone is often enough for the explanation to make sense. 

In contrast, the symmetry view assumes that understanding both beliefs and desires requires the same capacity to ascribe subjective attitudes and mental states to others (Perner & Roessler, 2012). If this is indeed true, one would expect children to understand them at roughly the same time. The clearest evidence for this claim comes from false-belief tasks, which assess whether children understand that other people can hold beliefs that differ from reality and from their own knowledge. In the classic Sally-Anne task (Baron-Cohen et al., 1985), a child watches Sally place a marble in a basket and leave the room. In Sally’s absence, Anne moves the marble to a box. The child is then asked where Sally will look for her marble when she returns. Children younger than 4 often answer that Sally will look in the box, the marble’s actual location. Around 4 years of age, however, children begin to answer correctly: Sally will look in the basket because she falsely believes the marble is still there. This shift suggests that children are beginning to understand beliefs as mental representations that can differ from reality. 

Interestingly, a similar developmental pattern is found for desires when the task requires children to represent perspectives different from their own. Consider a child who loves cookies but knows that her friend prefers carrots. To predict what her friend will choose, she must set aside her own preference and reason about someone else’s desire. Studies show that by around 18 months, children can already appreciate that other people may want things different from what they want themselves (Repacholi & Gopnik, 1997; Nichols et al., 2010). However, representing more complex desires, especially when they conflict with one’s own perspective or depend on another person’s beliefs, develops gradually throughout the preschool years. 

Given that evidence on the development of belief-desire psychology is mixed, it is possible that the distinction between the development of desires and beliefs may not be so straightforward. Children talk about desires earlier and more often, but this may not necessarily mean that they possess a qualitatively different understanding of desires from beliefs. More recent work (e.g., Schünemann et al., 2021) has therefore shifted away from pitting the two accounts against each other to understanding how children learn to coordinate beliefs and desires together. For instance, a child may know that her friend wants to find her toy, but to predict where she will search, she must also know what her friend believes about its location. 

Seen in this light, belief-desire psychology is less a sequence of isolated milestones than a gradual reorganisation of how children understand the minds of others. They learn that people act not simply because they want things, nor because they believe things, but that they act on what they believe will satisfy their wants. This important insight allows children to explain behaviour in various contexts, to predict how others may act, and to navigate increasingly complex social relationships as they grow older.