Experiment 1.

We began by replicating the finding that 3-month-old infants, who have received no action training and are habituated to an actor reaching over a barrier, show no differential looking to efficient versus inefficient reaching actions after the barrier is removed (27). In Exp. 1, we tested for infants’ sensitivity to action efficiency using events based directly on past research (27), featuring reaches by an actor wearing a glove rather than a mitten (Fig. 1). Three-month-old infants (n = 20; mean age = 108 d; range = 92 to 122 d; 11 female) viewed video clips of an actor who reached over a barrier, grasped and lifted a ball, and moved the ball to her side of the barrier (Fig. 1 A, H1). The height of this barrier varied across trials, and the person always adapted her reach to the barrier. After infants either habituated to these events (i.e., their attention declined by 50%), or looked for 12 trials, whichever came first, we measured their attention to alternating test events in which the person reached for the same ball as during habituation, but with no obstacles in her way (Fig. 1 B, T1). On alternating test trials, she reached on the same curvilinear path toward the ball (a familiar but newly inefficient action) or on a direct path (a novel but newly efficient action). The only differences between these events and the events from past studies (27) were that the actor in this study wore a tight-fitting white glove instead of a brown mitten, and she kept her hand in the same grasping position during the entire reach, instead of turning the ball over in the mitten after retrieving it. Thus, the shape and positions of her fingers remained visible throughout the action.

Across all experiments, we calculated the average looking time toward the efficient versus inefficient reach over 3 pairs of test events, and we analyzed these data using linear mixed-effects models (42). For details about our analysis strategy, see Materials and Methods. In light of past findings that prereaching infants fail to interpret reaching actions by a mittened hand as costly (27), we expected infants to look equally at the 2 test events in Exp. 1. Consistent with this prediction, infants looked equally to the inefficient and the efficient reach of the gloved hand (mean_ineff = 18.029 s, mean_eff = 16.844 s, 95% CI [−0.089, 0.238], standardized β-coefficient [β] = 0.155, unstandardized B coefficient [B] = 0.074, SE = 0.079, P = 0.359, 2-tailed), replicating past findings (27) (Fig. 2A). Nevertheless, looking preferences in this experiment differed marginally from those in the experiment on which this study was based (27), with relatively greater looking at the familiar but inefficient reach ([−0.015,0.464], β = 0.43, B = 0.224, SE = 0.122, P = 0.074, 2-tailed).

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Fig. 2.

Looking time in seconds toward the efficient versus inefficient reach at test across Exps. 1 to 5 (n = 152), for both (A) pick-up events (Exps. 1 and 2) and (B) state-change events (Exps. 3 to 5). Images indicate video displays used during the habituation phase (above each graph) and test phase (below each graph) for each experiment (Fig. 1). Red dots and error bars indicate means and within-subjects 95% CIs. Pairs of connected points indicate data from a single participant. Horizontal bars within boxes indicate medians, and boxes indicate the middle 2 quartiles of data. Upper whiskers indicate data up to 1.5 times the interquartile range above the third quartile, and lower whiskers indicate data up to 1.5 times the interquartile range below the first quartile. Beta coefficients (β) list effect sizes in SD units for each condition. *P < 0.05, **P < 0.01, ***P < 0.001, 2-tailed, except for the causal condition in Exp. 5, which was preregistered as a 1-tailed test.

Experiment 2.

Do 3-month-old infants struggle to represent the cost of mittened and gloved reaches because of the gloves and mittens themselves? In Exp. 2, infants (n = 20; mean = 108 d; range = 93 to 120 d; 12 female) were presented with the same actions from Exp. 1, except that the person performing the actions wore no gloves, further clarifying the contact relation between her hand and the object (Fig. 1, H2 and T2). Infants looked longer at the inefficient than the efficient reach of the bare hand, in the familiar context of a bare-handed reach (mean_ineff = 9.715 s, mean_eff = 8.036 s, [0.008,0.331], β = 0.429, B = 0.170, SE = 0.078, P = 0.043, 2-tailed). Performance in Exp. 2 differed significantly from performance in the original study on which it was based (27) ([0.047,0.547], β = 0.539, B = 0.297, SE = 0.124, P = 0.022, 2-tailed). However, performance in Exps. 1 and 2 did not differ from each other ([−0.128,0.319], β = 0.167, B = 0.095, SE = 0.111 P = 0.396, 2-tailed). Collapsing across both Exps. 1 and 2, infants looked marginally longer at the inefficient than the efficient action (mean_ineff = 13.872 s, mean_eff = 12.440 s, [−0.004,0.227], β = 0.185, B = 0.112, SE = 0.058, P = 0.060, 2-tailed) (Fig. 2A).

These experiments, together with past research (26, 27), suggest that untrained 3-month-old infants have weak and inconsistent looking preferences for direct versus indirect reaching and grasping actions. Nevertheless, the significant difference between Exp. 2 and the experiment presenting a mittened hand (27) calls into question the conclusion, from past research, that 3-month-old infants need action training in order to appreciate the physical costs of reaching actions. An exploratory analysis comparing the 3 experiments that used this method revealed that the magnitude of infants’ looking preference for the indirect reach increased with increases in the visibility of the form of the reaching hand, from a mitten that obscured its shape and texture (27), to a glove that revealed its shape but obscured its color and texture (Exp. 1), to a fully visible hand (Exp. 2) ([0.007,0.053], β = 0.416, B = 0.03, SE = 0.011, P = 0.011, 2-tailed). SI Appendix presents a full report of this exploratory analysis, which raises the possibility that the use of mittens obscuring the hand in all past research with 3-month-old infants underestimates the infants’ sensitivity to natural, bare-handed acts of reaching. Further research is needed to test this possibility.

What makes reaching for, grasping, and lifting objects problematic actions for 3-month-old infants to understand? Although infants frequently see people lifting objects, the mechanism by which this action serves to displace an object depends on variables that are opaque to vision, such as the weight of the object and the force of the actor’s grasp. Without understanding how the posture of the hand and the forces it exerts allow an actor to lift and move an object, infants may have difficulty distinguishing pick-up actions from hand movements that are guided by different intentions. If this is correct, then infants may more robustly represent the causal powers of other people who engage in simpler, albeit less familiar, efficient, object-directed actions. The next experiments test this possibility.

Experiment 3.

Drawing inspiration from past studies of infants’ and adults’ causal perception (10, 11, 35, 36, 43), in Exp. 3 we tested infants’ responses to displays similar to those of Exp. 1, except that the person reached for and touched the ball with the tips of her gloved fingers, causing it to illuminate and emit a soft sound on contact, and then withdrew her hand, causing the ball to return to its initial state (Fig. 1, H3-H4, T3). Because this event has not been used in previous research, infants were randomly assigned to 1 of 2 habituation conditions (n = 40; 20 per condition; mean age = 108 d; range = 91 to 122 d; 23 female). In the experimental condition, infants watched the person reach over a barrier that prevented direct access to the goal object (H3), as in Exps. 1 and 2. In the control condition, infants watched the person perform the same reaches with the barrier behind the goal object, out of the actor’s way, as in the control condition of previous research with mitten-trained infants (H4) (27). Across both conditions, all barriers were added digitally to the same videos: Thus, the actor performed identical actions in the 2 conditions, but only in the first condition did the actor appear to reach efficiently on the habituation trials. After habituation, infants viewed the efficient, direct reach and the inefficient, indirect reach, as in Exps. 1 and 2, both of which activated the object (T3). These 2 conditions allow us to test whether infants differentiate efficient from inefficient reaches at test only when prior curved reaches were efficient.

In Exp. 3, infants responded differently to the test events across the 2 habituation conditions ([0.273,0.732], β = 0.781, B = 0.502, SE = 0.114, P < 0.001, 2-tailed) (Fig. 2B). When the actor’s reaches were initially constrained by a barrier (H1) in the experimental condition, infants looked longer, at test, at the inefficient than the efficient action (mean_ineff = 15.448 s, mean_eff = 12.368 s, [0.159, 0.486], β = 0.501, B = 0.322, SE = 0.081, P < 0.001, 2-tailed). Their preference for the inefficient test action cannot be attributed to low-level preferences for the curvilinear reach, because infants in the control condition (H2) showed a small preference in the opposite direction (mean_ineff = 8.788 s, mean_eff = 10.104 s, [−0.343,−0.017], β = −0.28, B = −0.18, SE = 0.081, P = 0.032, 2-tailed). Infants’ preference for the inefficient action was stronger in this experiment than in Exp. 1, which presented the same reaching trajectories ending in object pick-up ([0.029,0.467], β = 0.457, B = 0.248, SE = 0.112, P = 0.032, 2-tailed). Exp. 3 therefore provides evidence that infants are sensitive to the physical constraints on object-directed reaching when these reaches terminate in a simple, causally transparent contact event.

Experiment 4.

In Exp. 4, preregistered at https://osf.io/a5byn/, we tested whether this sensitivity depends on infants’ construal of the actor as a causal agent who changes the states of objects on contact. We introduced digital manipulations to the habituation and test events from Exp. 3 to create a small spatial and temporal gap between the termination of the actor’s reach and the activation of the object, thereby removing the key condition that elicits causal perception in older infants and adults (10, 11, 35, 36, 43). Infants (n = 20; mean age = 107 d; range = 93 to 121 d; 12 female) saw videos identical to those from the experimental condition of Exp. 3, except the actor’s hand never contacted the object (her fingers paused 50 pixels, or 2 cm, above it), and the object changed state 0.5 s after the hand came to rest in midair (Fig. 1, H5, T4). In contrast to Exp. 3, infants looked equally at test trials showing the inefficient and efficient actions (mean_ineff = 15.306 s, mean_eff = 16.38 s, [−0.301,0.191], β = −0.096, B = −0.055, SE = 0.119, P = 0.649, 2-tailed) (Fig. 2B). Across Exp. 4 (H5, T4) and the experimental condition of Exp. 3 (H3, T3), infants responded differently to the test events, depending on whether or not the person acted on the object on contact ([0.003,0.623], β = 0.547, B = 0.313, SE = 0.154 P = 0.049, 2-tailed). Therefore, Exp. 3 provides initial evidence that infants appreciate the physical constraints on goal-directed reaching if this action causes a change in its goal object on contact, but not if the change in the object occurs after, and at a distance from, the end of the action.

Experiment 5 (Direct Replication).

To evaluate this suggestion further, we conducted a preregistered direct replication of Exps. 3 and 4. In Exp. 5, preregistered at https://osf.io/f2hvd/, we randomly assigned infants to events that differed only in spatiotemporal continuity: The object either activated on contact with the agent’s hand, or after a small gap in space and time (n = 52, 26 per condition; mean age = 107 d; range = 92 to 121 d; 21 female). This design allowed us to compare infants’ responses to causal (H3, T3) versus noncausal (H5, T4) actions, under testing conditions where all researchers were blind to condition as well as test events. We fully replicated the findings from Exps. 3 and 4: Infants again responded to the test events differently depending on whether or not the activation of the object occurred on contact with the hand ([0.184,0.815], β = 0.729, B = 0.5, SE = 0.158, P = 0.003, 2-tailed) (Fig. 2B). As in Exp. 3, infants looked longer at the inefficient than the efficient reach when the person appeared to cause a change in the object (mean_ineff = 12.166 s, mean_eff = 7.791 s, [0.211,0.66], β = 0.635, B = 0.436, SE = 0.112 P < 0.001, 1-tailed); as in Exp. 4, infants looked equally to the inefficient and efficient reaches when she did not appear to cause this outcome (mean_ineff = 11.395 s, mean_eff = 12.888 s, [−0.289,0.160], β = −0.094, B = −0.064, SE = 0.112, P = 0.567, 2-tailed). Although 3-month-old infants have limited experience acting on objects themselves, they understand that other people intend to cause changes in the world through their actions. Infants exhibited this ability in Exps. 3 and 5, both of which presented clear information that a change in the goal object occurred on contact with the actor’s hand.

See SI Appendix for a metaanalysis over these 5 experiments and 5 previous experiments using similar methods at the same age (27), which compare different conditions of mittens-training, object manipulation (grasping and entraining vs. touching and activating an object), and causal information. Overall, we found that knowledge of the causal intentions behind and physical constraints on reaching actions arises without training, but it is more robust when infants view causally transparent actions or receive mittens-training.