Children from birth to age 5 engage in making sense of the world on many levels: language, human interactions, counting and quantification, spatial reasoning, physical causality, problem solving, categorization. Indeed, even preverbal infants show surprisingly sophisticated understandings in each of these areas. Complex human reasoning is thus rooted in early childhood. For example, infants less than a month of age can imitate others' gestures that are no longer in view, such as sticking out their tongues or opening their mouths (Meltzoff and Moore, 1989). By 9-12 months of age, infants can learn new behaviors simply by watching others, such as remembering how to unlock a container up to 24 hours after observing a peer do it (Bauer and Wewerka, 1995; Mandler and McDonough, 1995; Meltzoff, 1988). Six- to eight-month-olds can represent numbers: they match the number of objects visually depicted on a display with the number of drumbeats emanating from a loudspeaker (Starkey et al., 1983), and, when shown first one toy and then another hidden behind a screen, 5-month-olds expect to see two toys when the screen is lifted (Wynn, 1992). By the second half of the first year of life, infants have already learned about the properties of physical objects (Baillargeon et al., 1995). They know, for example, that objects cannot pass through one another and that objects fall when they are not supported.

Within the first year of life, infants become highly attuned to causal relations between objects. They distinguish events involving a causal sequence from other, noncausal events. For example, babies are more surprised when a video of one object colliding into another is run backward than when an object changing color is run backward (Leslie and Keeble, 1987). Furthermore, babies are aware of the effects of their own behaviors, in that they prefer consequences that they control directly over those that are uncontrollable (e.g., Parritz et al., 1992). For example, a child interacting with a noisy mechanical monkey perceives it as mildly threatening when it moves unpredictably, but enjoys it when he himself controls the toy's movements (Gunnar-vonGnechten, 1978). Similarly, infants 12 and 18 months old respond more positively to strangers who act in predictable ways that allow them more control than to strangers who are less predictable (Mangelsdorf, 1992).

In addition to distinguishing cause from effect, infants can distinguish accidental from intentional actions (Leslie and Keeble, 1987; Oakes and Cohen, 1990; Tomasello et al., 1996). In one study, 18-month-old children viewed an adult attempting to perform a series of target actions (e.g., pulling the ends off a tube) (Meltzoff, 1995). The adult was shown trying, but failing, to perform the target acts. When children imitated the event, they imitated the intended action —not the observed behavior. In a control experiment, children viewed a machine performing the same failed target acts. In this case, children did not attempt to perform the target acts at all.

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