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Journal of Experimental Child Psychology 103 (2009) 87–107

Contents lists available at ScienceDirect

Journal of Experimental ChildPsychology

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Causal perception of action-and-reaction sequencesin 8- to 10-month-olds

Anne Schlottmann a,*, Luca Surian b, Elizabeth D. Ray a

a Department of Psychology, University College London, Gower Street, London WC1E 6BT, UKb Dipartimento di Scienze Cognitive e della Formazione, Università di Trento, Corso Bettini 31, 38068 Rovereto (Trento), Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 March 2008Revised 8 September 2008Available online 29 October 2008

Keywords:Infant cognitionCausal perceptionGoal perceptionHabituation

0022-0965/$ - see front matter � 2008 Elsevier Indoi:10.1016/j.jecp.2008.09.003

* Corresponding author.E-mail address: [email protected] (A. Sch

Four experiments with 202 8- to 10-month-old infants studiedtheir sensitivity to causation-at-a-distance in schematic eventsseen as goal-directed action and reaction by adults and whetherthis depends on attributes associated with animate agents. InExperiment 1, a red square moved toward a blue square withoutmaking contact; in ‘‘reaction” events blue moved away while redwas approaching, whereas in ‘‘delay” events blue started afterred stopped. Infants were habituated to one event and then testedon its reversal. Spatiotemporal features reversed for both events,but causal roles changed only in reversed reactions. Infants disha-bituated more to reversed reaction events than to reversed delayevents. Squares moved rigidly or in a nonrigid animal-like fashion.Infants discriminated these, but motion pattern did not affectresponses to reversal. Infants also discriminated reactions fromlaunching and dishabituated to reversed reactions lacking self-ini-tiated motion. These results suggest that sensitivity to causation-at-a-distance depends on the event structure but not pattern oronset typical of animal motion.

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Introduction

People see each other as psychological agents acting or reacting to reach their goals and guided bywhat they intend and believe. People are also material bodies subject to physical constraints, withphysics and psychology providing different causal systems for understanding behavior (Carey &Spelke, 1994; Dennett, 1987). Infants employ basic principles of both contact mechanics (e.g., Baillar-

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88 A. Schlottmann et al. / Journal of Experimental Child Psychology 103 (2009) 87–107

geon, Kotovsky, & Needham, 1995; Leslie & Keeble, 1987; Spelke, Breinlinger, Macomber, & Jacobsen,1992) and psychology (e.g., Gergely, Nádasdy, Csibra, & Bíró, 1995; Onishi & Baillargeon, 2005; Surian,Caldi, & Sperber, 2007; Woodward, 1998) from early on. Here we investigate an aspect of causalunderstanding originally discussed by Duncker (1945) and Michotte (1946/1963; see also Yela,1952), namely, the perception of causation-at-a-distance. To study this in infants, we showed thema simple event with minimally contingent motions seen as goal-directed action and reaction from 3years of age (Kanizsa & Vicario, 1968; Schlottmann, Allen, Linderoth, & Hesketh, 2002).

Our work extends previous work on contact causality (Leslie & Keeble, 1987). Infant sensitivity tothis may be linked to early physical understanding (e.g., Leslie, 1994; Mandler, 1992). Causation-at-a-distance, in contrast, is typical of the social domain. Humans and animals can perceive each other andoften interact from afar. Thus, infant sensitivity to causation-at-a-distance may be linked to early psy-chological understanding.

Causation-at-a-distance in the reaction event

In the reaction event, a shape moves toward a second shape, which moves away prior to contact(Fig. 1). Both shapes move simultaneously for a while and then the first one stops. Adults describe thisas A chasing B and B running away. Kanizsa and Vicario (1968) argued that observers see an inten-tional reaction—an attempt to avoid contact—but because it is unclear whether mental state attribu-tion occurs, we merely refer to perceiving goal-directedness. Control events with a delay between themotions, in contrast, appear to be noncausal.

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A B

Fig. 1. Schematic reaction event. Shape A moves toward Shape B, which is stationary. B begins to move before contact, slightlybefore A has reached its final position.

A. Schlottmann et al. / Journal of Experimental Child Psychology 103 (2009) 87–107 89

Kanizsa and Vicario’s (1968) studies were inspired by Michotte’s (1946/1963) seminal work thatfocused mainly on physical causality in launch (collision) events. Reaction and launch events differin whether B moves before contact can occur or on contact. This difference signals that the events be-long to different ontological domains. Thus, studies of launch and reaction events offer the opportu-nity to study domain-specific perception in events of comparable minimal complexity. Lack ofspatial contiguity typically weakens or destroys the causal impression in launch events, but Kanizsaand Vicario’s (1968) studies, as well as work by Michotte (1946/1963) and Yela (1952) on relatedevents, show that lack of contact does not always have this effect; action at a distance can appearto be causal in appropriate event configurations.

The perception of launch causality is well established in adults (see review in Scholl & Tremoulet,2000) and infants (e.g., Leslie & Keeble, 1987; Oakes, 1994), but there has been less work done on reac-tion causality. A few studies confirmed it with adults (Schlottmann, Ray, Mitchell, & Demetriou, 2006)and children from 3 years of age (Schlottmann et al., 2002; Thommen, Dumas, Erskine, & Reymond,1998), and one study found it in 9-month-olds (Schlottmann & Surian, 1999). This infant study useda reversal paradigm following Leslie and Keeble (1987). Earlier work had shown that infants see theinternal structure of simple motion events, discriminating launching from single motions and non-causal events with temporal delay or spatial gaps at impact (Leslie, 1982, 1984). However, these dis-crimination studies left open whether infants see causal or merely spatiotemporal sequences, and thereversal paradigm can separate these options.

In the reversal paradigm, infants are habituated to causal or noncausal delayed events. At test, in-fants see the reversed habituation event; that is, left–right becomes right–left motion, or the sequenceof Fig. 1 now runs from bottom to top. Thus, spatiotemporal features change equally in both groups,and both groups should recover equally if infants perceive only these. However, cause and effect alsoreverse, but only in causal events and not in delayed events. More recovery of looking in the causalgroup therefore indicates sensitivity to the causal structure, and not just the spatiotemporal structure,of the event. Leslie and Keeble (1987) used this approach to show that 6-month-olds are sensitive tothe causal structure of launch events. Schlottmann and Surian (1999) used it to show that 9-month-olds are also sensitive to the causal structure of reaction events.

This result then raises the issue how infants perceive the causal structure of the reaction event. Dothey already construe it as ‘‘psychological” causality in goal-directed reactions, similar to older observ-ers? The alternatives are a physical understanding or a domain-general understanding. Here we ad-dress the issue by considering the conditions under which infants perceive causation-at-a-distance,in particular, whether this depends on the reaction event involving perceptual attributes associatedwith animate agents.1

Early perception of goal-directed actions and their agents

The psychological interpretation is plausible because infants perceive goal-directedness (e.g.,Woodward, 1998) and because they—like adults (Heider & Simmel, 1944)—do so even in simple eventswith schematic shapes. Woodward’s (1998) study showed that infants habituated to a hand reachingfor one of two toys react more to a switch in goals than to a switch in location, but infants as young as5 months also react to a goal switch when the agent is a block or even an animated shape (Luo & Bail-largeon, 2005; Schlottmann & Ray, in press; Shimizu & Johnson, 2004). Similarly, 9-month-olds, habit-uated to a circle jumping a barrier to reach another, dishabituated to the familiar curved path whenthe barrier was removed but not to a novel more direct path, suggesting that only ‘‘rational” actionsappear to be goal directed (Csibra, Gergely, Biro, Koos, & Brockbank, 1999; Gergely et al., 1995; see alsoCsibra, 2008; Kamewari, Kato, Kanda, Ishiguro, & Hiraki, 2005; Sodian, Schoeppner, & Metz, 2004). Itseems clear from this work that infants can interpret individual actions of schematic shapes as goaldirected. Little is known, in contrast, about children’s ability to understand links between actions,as in action causing a reaction—the topic here.

1 Some consider the important distinction to be between agents and inert objects, whereas others consider it to be betweenanimates and inanimates (see Gelman & Opfer, 2002; Rakison & Poulin-Dubois, 2001). It is doubtful that infants acquire abiological category of animates, but the typical agent they encounter is animate. We use the terms interchangeably.


Many of these studies also show that infant goal attribution does not depend on the presence of hu-man agents. Infants identify the agents of goal-directed actions from perceptual features correlated with,but not limited to, humans (Baron-Cohen, 1994; Biro & Leslie, 2007; Leslie, 1994; Luo & Baillargeon,2005; Mandler, 1992; Premack, 1990). Just how much agent information infants need may depend onthe event. Most strikingly, Csibra and colleagues’ (1999) infants saw shapes stripped of all agent attri-butes (e.g., self-initiated motion, nonrigidity) but rational variation in behavior as goal directed, so theevent configuration itself can suffice for goal attribution. In contrast, in Woodward’s paradigm, wherethe agent simply approaches the target, infants need further cues such as extensive self-initiated or bio-logical motion or a communicative contingency (Guajardo & Woodward, 2004; Johnson, Shimizu, & Ok,2007; Luo & Baillargeon, 2005; Schlottmann & Ray, in press; Shimizu & Johnson, 2004). Such cue inter-dependence lends additional support to the interpretation of goal-directedness.

Following this line of work, here we consider whether causal perception of the reaction event de-pends only on its spatiotemporal configuration or whether it requires further agent cues. The latterresult would support a view that infants may already see psychological causality in action-and-reac-tion sequences.

Contingency-at-a-distance as an agent cue

The psychological interpretation of the reaction event is also plausible because its spatiotemporalconfiguration—simultaneous motion without contact—involves a minimal contingency-at-a-distance,and contingency-at-a-distance is generally discussed as an agent cue. Mandler (1992) used the term torefer both to infants’ experience that agents react systematically to them and to perceptual contin-gency not involving the self but rather ‘‘such factors as one animate following another. . ., avoidingbarriers and making sudden shifts in acceleration” (pp. 595). The latter becomes apparent if the pat-tern of one action resembles that of the other action, either over time (one person talks, then the sec-ond person talks, etc.) or through the spatiotemporal configuration. Thus, when a cat chases a mouse,the second trajectory follows the first trajectory. In all such cases, the contingent behaviors are caus-ally linked to each other; that is, an action causes a reaction. But contingency need not involve causal-ity and can appear between inert objects (e.g., lights blinking in sequence). Accordingly, infants (andadults) need to tell causal contingencies from noncausal contingencies (Russell, 1948), a preconditionfor seeing contingency-at-a-distance as goal directed. From this point of view, our study considerswhether infants see our contingency per se as causal or only in conjunction with other agent cues.

Most existing evidence merely shows sensitivity, typically to contingencies involving the self, invery young infants (see reviews in Muir & Nadel, 1998; Watson, 1994). Only data from 12-month-oldsare more conclusive because they show a conjunction of agent cues. Infants follow the ‘‘gaze” of a no-vel furry object shaped like a body with head if it reacts contingently to them or has a face, but not if itacts noncontingently and lacks a face (Johnson, Slaughter, & Carey, 1998). Infants can also use purelyobserved communicative contingencies for goal attribution in Woodward’s (1998) paradigm (Shimizu& Johnson, 2004).

The current study also concerns the perception of contingencies external to the self, but with a fo-cus on motion rather than on primarily communicative actions. In Heider and Simmel (1944), complexmotions of triangles and squares appeared as actions and reactions of agents with goals and emotions(for neuroimaging data, see Castelli, Happe, Frith, & Frith, 2000; for data from preschoolers, see Berry &Springer, 1993). At 5–6 months, infants already distinguish complex correlated motion of two dotsfrom uncorrelated motion (Rochat, Morgan, & Carpenter, 1997), showing infant sensitivity to motioncontingencies as well. Schlottmann and Surian’s (1999) study went one step further and suggestedthat infants interpret a simple form of contingency-at-a-distance as causal, as they should if theysee it as psychological action and reaction. The next step, then, is to investigate whether this dependson further perceptual attributes of animate agents, that is, whether there is cue interdependence as inJohnson and colleagues (1998) or whether such motion contingencies can appear to be causal evenwithout such cues.

To address this, we do not use complex pursuit movements with multiple trajectory adjustments,as did Heider and Simmel (1944) and Rochat and colleagues (1997), because these involve a confound;it would be unclear whether infants react to the contingency per se or to its particular form. For exam-


ple, a cat does not follow all intricacies of a mouse’s trajectory but rather takes the shortest path to itsprey, approximating its future positions. In Gergely and colleagues’ (1995) sense, complex pursuit in-volves rational or efficient variation in the motion of the chaser with respect to the target’s motion,and this might lead to interpretations of goal-directedness rather than the contingency itself. The reac-tion event, a minimally contingent cat-and-mouse sequence without rational path adjustment, doesnot have this confound.

The current study: The role of nonrigid and self-initiated motion

Schlottmann and Surian’s (1999) study contained agent cues besides contingent simultaneous mo-tion. First, both shapes engaged in self-initiated motion. Many argue that infants initially distinguishobjects that start on their own from those that are passively set in motion (Baron-Cohen, 1994; Leslie,1994; Mandler, 1992; Mandler, 1998; Premack, 1990; Premack & Premack, 1995), and although in sev-eral studies self-initiated motion was not necessary (Csibra et al., 1999, Experiment 3) or sufficient(Johnson et al., 1998; Johnson et al., 2007; Movellan & Watson, 2002; Schlottmann & Ray, in press;Schlottmann & Surian, 1999; Shimizu & Johnson, 2004) for agent identification, Luo and Baillargeon(2005) recently reported that repeated self-initiated motion is effective.

Second, the shapes in Schlottmann and Surian (1999) did not move rigidly (Kanizsa & Vicario,1968); rather, they moved by rhythmic expansion/contraction (Fig. 2). Adults and children see this‘‘caterpillar” motion as self-generated and animate (Michotte, 1946/1963; Schlottmann & Ray,

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Fig. 2. Schematic caterpillar motion. A square expands from the right edge and then contracts from the left edge. The resultingtranslation appears to be self-generated and animal-like.


2004; Schlottmann et al., 2002), and it amplifies adults’ impressions of psychological causality in reac-tion events (Michotte, 1950/1991a, 1954/1991b; Schlottmann et al., 2006); that it does not affect chil-dren’s causal impressions could reflect processing limitations (Schlottmann et al., 2002). We do notknow how infants see caterpillar stimuli, but we know that they are sensitive to nonrigid biomechan-ical constraints in point light animations (Bertenthal, 1993). They also rapidly learn that an object canself-initiate motion if it has independently moving body parts like an animal, but not if it lacks these orhas only wheels (Markson & Spelke, 2006). Thus, infants’ grasp of causation-at-a-distance might de-pend on the contingency, or on the conjunction of contingency, with nonrigid and self-initiatedmotion.

Accordingly, the current experiments address which agent cues—contingency-at-a-distance, non-rigid motion, and/or self-initiated motion—contribute to perception of causation-at-a-distance. Theoutcome of these experiments will bear on the issues of whether infants already see psychologicalcausality in action-and-reaction sequences and on the role of contingency-at-a-distance as an agentcue, as outlined above. Experiment 1 tested for perception of causality in contingent reaction eventsor delayed control events with a rigid or nonrigid caterpillar motion. Experiments 2 to 4 addressedalternative interpretations by considering how infants perceive reaction events without self-initiatedmotion and whether they discriminate rigid motion from nonrigid motion and launch events fromreaction events.

Experiment 1

Method

ParticipantsThe final sample consisted of 132 babies with no known health problems who were recruited by

advertisement. There were 67 8-month-olds and 65 10-month-olds, with younger babies ranging inage from 8 months 1 day to 9 months 1 day (mean age = 8 months 17 days, 31 girls and 36 boys)and older babies ranging from 10 months 1 day to 11 months 6 days (mean age = 10 months 13 days,29 girls and 36 boys). We excluded 77 additional infants: 43 for fussing, parental interference, or tech-nical problems; 33 because they failed to habituate in 12 trials; and 1 for ceiling-level looks on bothtest trials (more than 4 standard deviations away from the group means).

StimuliEach movie involved a red square and a blue square, approximately 22 � 22 mm in size, initially

stationary, with red on the left and blue in the middle (left-to-right version). Red moved toward blue,stopping approximately 9 mm to its left. Blue began to move right either before red stopped, withapproximately 420 ms overlapping movement at a distance of 7.1 cm between centers in the ‘‘reac-tion” event, or approximately 1220 ms after red stopped in the ‘‘delay” event. To equate sequenceswith and without delay, the stationary periods at the beginning and end were adjusted, with eachcycle symmetrical around the midpoint of the sequence.

Movies were made with MacroMedia Director animation software. One cycle took approximately4.8 s and repeated up to 10 times, with a 750-ms interval between cycles during which the screenwent gray. Right-to-left movies were mirror-reversed, with blue moving first and red moving second,from right to left; this was the only spatiotemporal difference between the two versions. Infants sawone version on habituation and the alternate version on reversal.

In the rigid motion condition, each square moved at a constant rate of approximately 9.4 cm/s tocover a 113-mm distance in 1.2 s. In the nonrigid motion condition, corresponding to Michotte’s(1946/1963) caterpillar stimulus, the squares moved by expansion–contraction. Each square ex-panded for 200 ms at approximately 18.8 cm/s to a rectangle of roughly 4.1 cm length, with the rightedge stationary, and then contracted in the same way, with the left edge stationary until the originalshape was recovered. These steps were repeated twice, separated by a 40-ms delay. Thus, the averagetranslation speed for both rigid and nonrigid motions was the same. The shapes also had the samestart and end positions for both rigid and nonrigid motions and in both reaction and delay events.


Design and procedureInfants of each age were randomly assigned to one of four groups. Half saw reaction events with

either rigid or nonrigid motion and half saw delay events with either rigid or nonrigid motion; direc-tion of motion was counterbalanced within these groups.

Each infant was tested in a semidark room sitting on his or her caretaker’s lap approximately 90 cmaway from the monitor (viewable area 19 inches diagonally). All other equipment was hidden. Thecaretaker had no knowledge of the purpose or design of the study, was told not to interfere withthe infant, and was instructed to close his or her eyes on the reversal test. A camera above the monitorwas centered on the infant’s face; the experimenter observed the infant on video. A MacIntosh G3PowerPC was used to control stimulus display and record looking times.

Trials began with sounds and a flashing screen to attract attention. When the baby looked, theexperimenter hit a key to start the movie and record onset of a look. When the baby looked away,the experimenter hit another key. If the baby looked away prior to the half-point of the first cycle,the trial was abandoned; otherwise, it ended if the baby looked away for 2 s (consecutive) or after10 cycles. Habituation continued until the average looking time on 3 consecutive trials fell below halfof the average on the first 3 trials; the minimum number of trials was 6 and the maximum was 12.After habituation, the infant saw a familiar test (the habituation movie), followed by a novel test(the reversal movie) after a 30-s break. The break was to ensure some attention to the novel event.It was used for all groups and so cannot account for group differences in recovery. These proceduraldetails follow Leslie and Keeble (1987).

A second observer without knowledge of the purpose or design of the study checked videos for arandom third of the babies. Interobserver reliability was high; the correlations of looking times mea-sured on- and offline were r = .87 and r = .98 for the familiar and reversal tests, respectively. When alltrials, habituation, and test were considered, the mean correlation across 44 babies was r = .98(range = .78–1.00).

Results and discussion

HabituationLog looking times were used in Experiment 1 to normalize error variances and reduce the influence

of extremely long looks. Looking time in the reaction group decreased from 1.555 (38.85 s) on the firsthabituation trial to 0.926 (9.16 s) on the last habituation trial. In the delay group, the decrease wasfrom 1.548 (37.88 s) to 0.883 (8.38 s). The groups were also similar in the number of habituation trials,7.52 and 7.37, and in the mean looking time over the first and last 3 habituation trials, 1.138 (18.08 s)and 1.128 (17.44 s). An analysis of variance (ANOVA) on the change in looking over the first and last 3habituation trials, with age, event, and type of motion as between-participants factors, found asignificant decrease over the 6 trials, F(4.05,502.24 [Greenhouse–Geisser]) = 198.46, MSE = .05,p < .01, gpartial

2 = .62, with no other effects.Because infants were run to a criterion, spontaneous recovery should and did occur from the last

habituation to the subsequent familiar test trial due to regression toward the mean,F(1,124) = 13.48, MSE = .08, p < .01, gpartial

2 = .10. This recovery was more pronounced for delay eventsthan for reaction events, F(1,124) = 5.41, MSE = .08, p = .02, gpartial

2 = .04. In the delay group, lookingtime increased from 0.883 to 1.027 (from 8.38 to 12.26 s), but in the reaction group, it increased onlyfrom 0.926 to 0.958 (from 9.16 to 10.35 s). However, the decrease in looking from first habituation tofamiliar test trial remained significant, F(1,124) = 576.90, MSE = .07, p < .01, gpartial

2 = .82, with nogroup differences. Moreover, the difference in looking at reaction and delay events on the familiar testtrial did not reach significance, F(1,124) = 3.14, MSE = .05, p = .08, gpartial

2 = .03. Overall, it seems thatthe groups behaved similarly during habituation and that attention to reaction events was comparableto attention to delay events prior to reversal.

Reversal testFig. 3 presents our central result, showing looking times on the familiar and reversal test for infants

watching reaction or delayed events with rigid or nonrigid motion. Infants recovered interest in rever-sal of all events; that is, they attended to the change in spatiotemporal direction and order common to

4

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12

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familiar reversal familiar reversalreaction event delay event

r igidnon- r igid

meanlooking

time(s)

Fig. 3. Looking times and standard errors to familiar and reversal tests in babies seeing reaction or delay events with rigid ornonrigid motion. The raw means are shown to facilitate comparison with later experiments, but log 10 looking times were usedin the ANOVA. Infants recovered significantly to reversal of all events, but (as predicted) infants in the reaction groups recoveredsignificantly more, with recoveries of .1895 and .2718, than did infants in the delay groups, with recoveries of .0960 and .1429.


the events. As predicted, looking time increased more for the two groups of infants seeing reversedreaction (left) than for the two groups seeing delayed events (right), in line with the argument thatin reversed reaction events infants also perceived a change in causal roles. This corroborates the ear-lier findings of Schlottmann and Surian (1999).

It may appear from Fig. 3 that infants looked particularly long if reaction events with nonrigid mo-tion were reversed, but the statistical analysis does not bear this out. The overall Age � Event � Typeof Motion ANOVA on the amount of recovery (i.e., the difference between infants’ looking times on thefamiliar and reversal tests) showed only an event main effect, F(1,124) = 5.62, MSE = .07, p = .02,gpartial

2 = .04, reflecting the predicted and obtained differences between reaction and delay events.Thus, infants seeing reaction events recovered by .231 (7.61 s), whereas infants seeing delayed eventsrecovered by only .120 (4.05 s). More recovery on reversal of reaction events than on reversal of delayevents also reached marginal significance nonparametrically, Mann–Whitney U = 1782, p = .07. Thesame pattern of results is found if nonhabituators are included in the analysis.2

2 If direction of motion is included in the analysis, the effect of event is qualified by an Event � Type of Motion � Direction ofMotion interaction, F(1,116) = 11.33, MSE = .06, p = .001, gpartial

2 = .089. More recovery to reversed reaction events than to delayevents appeared for nonrigid left–right and rigid right–left motion, with recovery of .349 versus .055 (13.57 and 1.09 s),F(1,30) = 14.99, MSE = .05, p < .01, gpartial

2 = .33, and .192 versus �.030 (7.04 and �0.80 s), F(1,28) = 5.83, MSE = .07, p = .02,gpartial

2 = .17, respectively. The other two groups did not show this, with F < 1 for the difference between reaction and delay events.Why the predicted effect appeared in one direction with nonrigid motion and in the other direction with rigid motion is unclear.No consistent pattern emerged when we analyzed for direction effects throughout all studies here and in other work with thisparadigm: First, direction effects in test and habituation data here did not correspond. Second, direction effects in Experiments 1and 2 did not correspond. Third, no direction effects appeared in Experiments 3 and 4, in previous work with 9-month-olds(Schlottmann & Surian, 1999), or in subsequent work with 6-month-olds (Schlottmann et al., 2002) using the same design andstimuli as used here. Thus, direction effects in our paradigm seem neither robust nor meaningful. Further details are available onrequest.


In contrast, the ANOVA found no effects involving age, F(1,124) < 1.69, p > .20, or type of motion,F(1,124) < 1.86, p > .18. In fact, more recovery to reversal of reaction events than to delay events ap-peared regardless of age and for both rigid and nonrigid motions. Table 1 shows that the predicted pat-tern is obtained across all subgroups of the design. Recovery on reversal was significant in all fourreaction groups but was not significant in three of four delay groups (except for 8-month-olds watch-ing delay events with nonrigid agents), F(1,17) = 17.58, MSE = .25, p < .01, gpartial

2 = .51. However, fromTable 1 it is clear that even in this case infants in the corresponding reaction group showed substan-tially more recovery of looking.

A potential source of concern may be that attention to the familiar event in reaction and delaygroups was not identical in Fig. 3 and that this may complicate interpretation of the event effect,which reflects looking at both the reversed and familiar events. We use the amount of recovery asthe dependent variable here precisely because it controls for individual differences between thegroups and so allows a more sensitive test of that part of the group difference attributable to the dif-ference between the events themselves. Nevertheless, if looking at the familiar event differed grosslybetween the groups, one would worry that processing of reaction events and processing of delayedevents were not comparable in the first place. But again, the difference in looking on the familiar trialdid not reach significance and, more important, there were no systematic group differences duringhabituation.

To address any residual concern about this, we can even out looking times on the familiar test byusing the average of the last habituation and the familiar test trial as the dependent measure. Withthis new measure, the group difference largely disappeared: Infants looked for 0.958 (9.76 s) at thefamiliar reaction and for 0.983 (10.32 s) at the familiar delayed event, bringing recovery to the re-versed events to .230 (8.20 s) and .163 (5.99 s), respectively. An ANOVA on this recovery, after remov-ing 4 outliers (box plot) on the new dependent measure, found the same pattern as before, with asignificant effect of event, F(1,120) = 4.44, MSE = .05, p = .04, gpartial

2 = .04, and no other differencesdue to age or type of motion. It is clear, therefore, that the marginal group difference on the familiartest trial poses no problem for interpretation.

In sum, Experiment 1 shows that 8- and 10-month-olds are sensitive to causation-at-a-distance inthe reaction event. This occurred not only with nonrigid motion, taken as animate by older observers,but also with rigid motion. These data extend previous results (Schlottmann & Surian, 1999) and com-plement previous work on infant sensitivity to contact causality in the launch event (Leslie & Keeble,1987).

Further experiments were run to clarify these findings: Because infants perceived causality withboth rigid and nonrigid motions in Experiment 1, Experiment 2 tested whether they actually distin-guish between these motions. In Experiment 3, we considered whether self-initiated motion is neces-

Table 1Mean looking times in the familiar and reversal tests, numbers of babies, percentages of babies looking longer at reversal, andrecovery scores

Familiar Reversal n % Showing recovery Recovery score

8-Month-oldsReaction, nonrigid 11.99 (2.41) 24.19 (3.64) 16 81 .3154* (.0912)Delay, nonrigid 9.92 (1.20) 14.41 (1.47) 18 83 .1666* (.0397)Reaction, rigid 10.45 (1.19) 14.86 (2.18) 16 75 .1491* (.0578)Delay, rigid 14.02 (1.61) 18.85 (3.14) 17 53 .0828 (.0703)

10-Month-oldsReaction, nonrigid 8.82 (1.02) 15.49 (2.01) 16 88 .2282* (.0546)Delay, nonrigid 14.70 (2.23) 17.77 (2.05) 17 53 .1179 (.0729)Reaction, rigid 10.13 (1.23) 17.30 (2.53) 16 81 .2300* (.0726)Delay, rigid 10.42 (1.10) 14.20 (1.94) 16 63 .1101 (.0632)

Note. Looking time values are in seconds (s). Standard errors are in parentheses. Recovery scores are difference between meanlog looking times on familiar and reversal tests as used in the ANOVA.

* The recovery differs significantly from 0 at p < .05.


sary for perception of causation-at-a-distance. Finally, Experiment 4 tested whether infants discrim-inate between causal launch and reaction events.

Experiment 2

Experiment 1 found that infants perceive causation-at-a-distance even with rigidly movingsquares. Neither human agents nor motion of a type characteristic of animate agents is required.3

However, the result leaves open the possibility that animate motion might be able to amplify such a per-ception—as it does for adults (Schlottmann et al., 2006), although not for children (Schlottmann et al.,2002)—but in contrast to older observers, the infants in the current study did not see the nonrigid motionof our artificial stimuli as animate. Experiments to determine this fully go beyond the scope of this study,but we certainly need to establish whether infants discriminate between our rigid and nonrigid motionsat all. This was the objective in Experiment 2.

Method

ParticipantsParticipants were 16 infants (9 girls and 7 boys, mean age = 9 months 17 days, range = 8 months 1

day to 10 months 29 days). Half were 8-month-olds and half were 10-month-olds. An additional 2 in-fants were excluded for failure to habituate in 12 trials; 11 were excluded for fussiness, parental inter-ference, or experimenter error; and 1 was an outlier (box plot) on the familiar test trial.

Stimuli and procedureStimuli and procedure were as before. Of the 16 infants, 8 were habituated to shapes with rigid mo-

tion and then tested for discrimination on nonrigid motion, whereas 8 were habituated on nonrigidmotion and then tested on rigid motion. Direction of motion was counterbalanced within groups.All infants saw reaction events on habituation and test trials. No break was given between the familiarand novel test stimuli so as to avoid renewal of attention being simply due to this break rather than tointrinsic features of the test stimulus.


In this experiment and the subsequent experiments, the data were normally distributed, so the rawdata could be used for the analyses. Mean looks decreased from 32.53 to 6.91 s from first to last habit-uation trials, and the mean length of looking over the first and last 3 habituation trials, 15.25 s, did notdiffer between the two ages or stimulus orders. Infants looked for 7.38 s on the familiar test trialinvolving the habituated motion and for 12.24 s on the novel test presenting the alternate motion. Thisrecovery was significant, F(1,12) = 7.11, MSE = 26.57, p = .02, gpartial

2 = .37. This was also confirmednonparametrically, Z = �2.53, p = .01 (Wilcoxon). Thus, infants indeed distinguish between a squareshape moving rigidly or nonrigidly in a manner considered as animal-like by older observers. The lackof type of motion effects in Experiment 1, therefore, does not simply reflect lack of discrimination.

Our result adds to Bertenthal’s (1993) data that infants distinguish nonrigid dot pattern motion de-rived from realistic biomechanical models from randomized or upside-down versions. The exact rela-tionship between our artificial caterpillar motion displayed by a geometrical shape and perception of

3 The conclusion is also supported by a further control condition in which 17 infants were habituated and tested on reversal ofreaction events in which the objects moved nonrigidly in a way not considered as animate by adults (Schlottmann & Ray, 2004).The stimulus involved a square that expanded and contracted orthogonal to its direction of translation, rather than in this directionas in the nonrigid stimulus of Experiment 1, but the amount of nonrigid deformation was the same in both stimuli. Infants lookedfor 13.15 s on the familiar test and for 24.26 s on reversal, showing as much recovery as to the reaction event and far more than tothe control event in Experiment 1. The recovery was significant, F(1,15) = 8.31, p = .01, gpartial

2 = .36, and the direction of motionhad no effects. Recovery was also confirmed nonparametrically, Z = �2.30, p = .02 (Wilcoxon). Accordingly, infants may perceivecausality with this stimulus as well. To be conclusive, this condition would need its own control group with delayed verticalmotion, but the data are offered here merely to supplement the conclusion from Experiment 1 that babies’ response to role reversalin reaction events does not depend on the type of motion.


realistic biological motions without shape cues in point light displays remains to be determined, butchildren of talking age (Schlottmann et al., 2002) and adults (Schlottmann & Ray, 2004) consider thecaterpillar motion as animal-like. Adults are clearly amused by the contrast between inanimate shapeand animal-like motion; this contrast may also contribute to the enduring appeal of Slinky toys thatmove in a similar fashion.

Discrimination data cannot show, of course, that infants already see our nonrigid motion as ani-mate. However, the same nonrigid motion selectively triggered infant goal attribution in an animatedversion of Woodward’s (1998) goal switch paradigm, whereas rigid motion in the absence of othercues did not (Schlottmann & Ray, in press). This suggests that even infants may well appreciate theimplication of our nonrigid motion for animacy.

Experiment 3

Even if perception of causation-at-a-distance does not depend on animal-like motion, it may dependon other agent cues such as self-initiated motion. Self-initiated motion is a crucial difference betweencontact and noncontact causalities; it is integral to canonical reaction events, in which the reacting shapeB starts to move when A comes close, whereas launch events have contact of A and B. Therefore, Exper-iment 3 studied whether self-initiated motion is necessary to perceive a causal reaction.

To eliminate self-initiated motion from a causal reaction, we presented a nonstandard event inwhich first B and then A moved in from the side with motion onset off-screen, similar to the wayin which Csibra and colleagues (1999) tested the role of self-initiated motion in their barrier event.If the motion of A and B extends across the whole screen, however, the event involves far more con-tingent motion than does the standard event. This in itself might provide additional agent informationand make up for lack of self-start. To avoid this, we showed infants only part of the event, containingthe same amount of simultaneous motion as the standard event. This was done through the use of anoccluder with an opening in the middle, giving the impression of seeing the events through a window(Fig. 4). In Experiment 3, we habituated infants to such reactions with the same minimally contingentmotion as the standard event but without self-initiated motion and then tested them on reversal tosee whether self-initiated motion is needed for infant causal perception.

Older observers treat such noncanonical reaction events just like standard reaction events, with 77,87, 97, and 96% psychological responses for 3-, 5-, and 7-year-olds and adults (n = 30 per child group,n = 22 adults), respectively (Watts, Schlottmann, & Ray, 2007). The stimuli in Watts and colleagues’(2007) study were the same as those used here except that they were scaled for a smaller computerdisplay.

In the reversal paradigm, the corresponding control event is an identical event except for a delaybetween the motions; that is, B moves across the screen, and sometime after it has disappeared, Amoves across the screen. To reduce memory demands, however, we went from simultaneous to con-tiguous motion rather than delayed motion. Therefore, in our control event, B moved across thescreen, followed immediately by A. The shapes were never visible simultaneously, but their motionswere temporally (although not spatially) contiguous.

Method

ParticipantsExperiment 3 involved 38 infants (18 girls and 20 boys, mean age = 9 months 4 days, range = 7

months 28 days to 10 months 19 days). Infants across the age range participated, but for comparabilitywith the other experiments, we did a median split on age for the analyses. An additional 18 infantswere excluded for failure to habituate in 12 trials, 7 were excluded for fussiness or parental interfer-ence, and 3 were outliers on the familiar test trial (box plot).

Stimuli and procedureA total of 21 infants were habituated to a modified reaction and 17 to a control event. The shapes

moved at the same speed and distance from another as in the standard, moving either rigidly or non-rigidly. In the reaction event, the screen was initially blank, with the shapes moving in from the side

t0

AB

t1

t2

t3

t4

A

Fig. 4. The noncanonical reaction used in Experiment 3. Shape B moves in from the side toward its standard start position. Onceit reaches this, Shape A moves out from behind the occluder and both move in parallel, as in the standard event. At its standardstopping position, B disappears from view and A continues on its own.


rather than starting from rest on-screen. The blue shape appeared first (left-to-right version) from be-hind an occluder with a window-like aperture sized so that just as blue reached its usual starting po-sition in the center, red also became visible. The shapes then moved in the standard simultaneousmotion. On reaching its usual stopping point, blue disappeared behind the occluder, with red contin-uing until it also reached the edge. In the control event, the only difference was that red did not appearon the left until blue had disappeared on the right. Piloting showed that babies quickly lost interest instimuli of the same duration as the standard because for some time both shapes were hidden behindthe occluder. To increase interest, we shortened these periods to an overall sequence length of approx-imately 3.3 s. Of the 38 participants, 9 babies in the reaction group and 8 in the control group werehabituated to the left–right version of the event, with the remainder being habituated to the right–leftversion. Then all babies were tested with the alternate event.


From the first to last habituation trials, mean looks decreased from 25.00 to 6.86 s in the reactiongroup and from 18.67 to 6.50 s in the control group. The average length of looks across the first andlast 3 habituation trials was 14.56 s for the reaction event and 11.71 s for the control event. This dif-ference was the only significant effect, F(1,30) = 6.37, MSE = 9.34, p = .02, gpartial

2 = .18, in an ANOVAwith event, age, and type of motion as between-participants factors.

The main results are shown in Fig. 5. Infants recovered by 9.30 s in the reaction group but by only3.29 s in the control group. The ANOVA on the amount of recovery with age, event, and type of motion

4

6

8

10

12

14

16

18

20

22

24rigidnon-rigid

meanlooking

time(s)

familiar reversal familiar reversalreaction event control event

Fig. 5. Mean looking times and standard errors to familiar and reversal tests in babies seeing the modified reaction or controlevent without self-start, with either rigid or nonrigid motion, in Experiment 3. Infants in the reaction groups show morerecovery of looking than did infants in the control groups.


as between-participants factors showed a corresponding effect of event, F(1,30) = 6.97, MSE = 52.16,p = .01, gpartial

2 = .19. The only other effect in this analysis was the Age � Event � Type of Motion inter-action, F(1,30) = 5.12, MSE = 52.16, p = .03, gpartial

2 = .15. The looking time of 10-month-olds seeingnonrigid delayed events decreased from familiar to reversal tests, when all other groups increased.This group, however, had only 1 participant (due to our post hoc assignment of the age variable; allother groups had more participants). This was the only age effect in any of the experiments here.

The event difference was also confirmed nonparametrically, Mann–Whitney U = 101.5, p = .02. Whenreaction and control events were considered separately, there was significant dishabituation only forreaction events, F(1,17) = 24.25, MSE = 34.17, p < .01, gpartial

2 = .59, and F < 1, and the number of babieslooking longer at reversal exceeded chance only for reaction events, p < .01 and p = .14 (sign test).

The main result here, at any rate, was that infants reacted to role reversal in a noncanonical reac-tion event with simultaneous motion but without self-initiated motion onset. They did not react toreversal if the event lacked both features. Infant perception of causation-at-a-distance seems to de-pend on the simultaneous motion component of the reaction event but not on self-initiated motion.

Our result may seem odd because the event without self-initiated motion would seem ambiguous;it could be mechanical, with two blocks being pushed out of sight, or it could be a reaction, with self-initiated motion onset behind the occluder. However, the data from older observers are very clear inthat all ages, from 3 years to adulthood, see our occluded event as a reaction (Watts et al., 2007). Theimpression, if anything, is stronger than in the standard event, with no sign of a domain ambiguity. Itis not entirely clear at this point what factors bias the impression in the direction of a reaction. Thisdoes not seem to depend on the presence of nonrigid motion in either older observers or infants (seeFig. 5), but it may be affected by the shapes appearing to float in midair, like birds or airplanes, as asecondary cue to self-propulsion that has not received much attention in the literature. A strongercue to the interpretation, however, would seem to be the presence of motion that is contingent at dis-tance. It should be noted that the same contingency exists in the control group, but it is presumablymore difficult to detect when the objects are not visible at the same time than when they engage insimultaneous motion as in the reaction group.


This difference between the two groups also gives rise to a potential alternative interpretation ofour result, namely, that the simultaneous motion provides prolonged spatiotemporal information thattwo objects moving in order are involved. Without simultaneous motion, in the control group, ordermust be determined from the objects’ different colors which infants may find more difficult (Wilcox,1999; Xu, Carey, & Quint, 2004). If so, differential recovery might reflect that the experimental groupreacts to changed order and direction, whereas the control group reacts to changed direction only, incontrast to Experiment 1, where the experimental group reacts to a change in causality, order, anddirection, whereas the control group reacts to changed order and direction.

Although we cannot rule out this spatiotemporal alternative account, it is unlikely given theamount of recovery. Recovery of 9.30 s in the experimental group here is similar to 7.61 s for the re-versed causality group in Experiment 1. Recovery of 3.26 s in the control group here is similar to 4.05 sfor the reversed order control group in Experiment 1. This observed data pattern does not fit with theview that infants in the experimental group here react only to order, like the control group but unlikethe causal group of Experiment 1, a view that predicts less recovery in Experiment 3 than in Experi-ment 1. Accordingly, in conjunction with Experiment 1, Experiment 3 suggests that infants perceivecausality in reaction events even without self-initiated motion.

Experiment 4

If infant perception of causality in reaction events is possible even in the absence of agent cues suchas self-initiated or animal-like motion, the question arises as to whether infants distinguish contactand noncontact causality at all. Several proposals hold that infants, from early on if not from birth, di-vide the world into broad domains based on salient differences in object motion and interaction (Bar-on-Cohen, 1994; Leslie, 1994; Mandler, 1992; Premack, 1990). All argue that an initial distinctionbetween contact and self-initiated motion provides a blueprint for what eventually become the phys-ical and psychological domains. Distinct percepts for contact and noncontact causality could help in-fants to carve up the world in this way. But perceptual causality could also be an undifferentiateddomain-general ability to see relationships between events (‘‘A does something to B”) that become do-main specific later on.

Our study shows perception of noncontact causality parallel to Leslie and Keeble’s (1987) percep-tion of contact causality. Infants’ sensitivity to both does not mean, however, that infants distinguishthe two. If perception of causation-at-a-distance had required nonrigid or self-initiated motion, itwould have implied such a distinction because contact causality does not require either, but we didnot find this. Nevertheless, infants may still distinguish the two forms of causality. If so, at least theyshould discriminate launch events from reaction events.

We already know that infants distinguish launch events from contiguous events without contact(also described as gap or no-collision events [e.g., Leslie, 1982; Leslie, 1984]), but the latter differ fromreaction events in that they lack simultaneous motion of the two shapes and are typically not seen ascausal by older observers (Schlottmann et al., 2006; Watts et al., 2007). It is also clear that infants dis-tinguish different types of noncausal events (Leslie, 1982; Leslie, 1984). Therefore, Experiment 4 ad-dressed whether infants also distinguish the two types of causal event.

Method

ParticipantsExperiment 4 involved 16 infants (7 girls and 9 boys, mean age = 9 months 15 days, range = 8

months 2 days to 10 months 23 days). Half were 8-month-olds and half were 10-month-olds. An addi-tional 5 infants were excluded: 3 for fussing and 2 for failing to habituate in 12 trials.

Stimuli and procedureHalf of the infants of each age were habituated to reactions with either rigid or nonrigid motion and

then tested for discrimination on launch events with the same motion. The other half were habituatedto launch events (in which A stopped and B began to move on contact) and then tested on reactionevents. Again, there was no break between familiar and novel test trials.



Mean looks decreased from 30.42 to 6.27 s from the first to last habituation trials, and the meanlength of looking on the first and last 3 habituation trials, 16.30 s, did not differ for different age groupsor types of motion. Infants looked for 7.34 s on the familiar test and for 10.92 s on the novel test. Therecovery was significant, F(1,12) = 5.17, MSE = 19.90, p = .04, gpartial

2 = .30. There were no other effectsin the Age � Type of Motion ANOVA. The result was also confirmed nonparametrically, Z = �1.97,p = .05 (Wilcoxon). Thus, infants do distinguish launch and reaction events. Although they may dis-criminate these events based on spatiotemporal features, this is a first step toward showing that in-fants distinguish contact causality from noncontact causality. One way to address this issue furthermight be work correlating contact and noncontact causality with other tasks with already establishedvalidity in assessing physical or psychological cognition.

General discussion

This study found that infants from 8 months perceive causation-at-a-distance in Kanizsa and Vica-rio’s (1968) reaction event regardless of whether the motions are nonrigid and self-initiated. Thus, in-fants see the minimal contingency between two spatially separated motions instantiated in thereaction event as more than just a spatiotemporal relation. At a minimum, our results show an earlycase of perceptual causality that does not easily map onto a simple mechanical event. Below we re-view the extent to which our results show infant perception of causality and how domain-specificand domain-general views of causal understanding may account for them. We start with the view thatinfants, like older observers, see psychological causality here and end with the other view we considerplausible, namely, that domain-general and domain-specific processes combine developmentally.

Domain-specific perception of reaction-at-a-distance?

The view that infants, like older observers, see goal-directed causal action and reaction in our eventsits easily with the literature, but our data alone are inconclusive. The interpretation would be moredefinite if causal perception here had required self-initiated or nonrigid motion; such interplay ofagent cues would be unlikely if they were not linked via an early psychological understanding (John-son et al., 1998). The finding would also have implied a distinction from contact causality, which doesnot require animal-like motion, further supporting the domain-specific view. Some might have ob-jected that nonrigid or self-initiated motion merely increases saliency, leading to better chances ofa causal encoding without this being domain specific. However, contact causality is encoded as causalwithout such cues, so one would also need to assume that noncontact causality is weaker than contactcausality and needs buttressing. With older children, this is clearly not the case (Schlottmann et al.,2002). Moreover, this in itself would imply a distinction between the two forms of causality. Thus, sal-iency effects do not seem to detract from the logic of our approach.

Our studies, however, did not find such cue interdependence. Infants in Experiments 1 and 3 de-tected causal structure even when the shapes moved rigidly without self-initiated motion. The lackof correlation between causal contingency-at-a-distance and other agent cues means that we haveno direct evidence that infants, like children from 3 years (Schlottmann et al., 2002), see not just cau-sality but also psychological causality in reaction events.

Absence of evidence, however, is not evidence of absence. Nothing in the current data rules out thatinfants here see goal-directed action and reaction. This view fits with Csibra, Biro, Koos, and Gergely(2003) and Wagner and Carey’s (2005) findings that 12-month-olds see complex contingent reactionsequences with multiple rational path adjustments as goal directed. Our results also fit with the viewthat agents are identified from features typically found in, but not limited to, humans so that unfamil-iar artifacts/shapes can be agents (Biro & Leslie, 2007; Luo & Baillargeon, 2005); they fit especially withwork showing that some event configurations correlated with humans appear to be goal directedwithout any other agent cues (Csibra et al., 1999). In addition, infants distinguished causal reactionfrom launch events in Experiment 4, a precondition for a distinction between two kinds of causality


for the events. The view that infant causal perception of reaction events is domain specific, like that ofolder observers, remains empirically viable and theoretically plausible.

On this view, our results also highlight a potential difference between goal attribution to individualactions and to object interactions as studied here. In studies of individual actions, slight variability ofgoal approach during the habituation trials, or some indication that the object ‘‘chose” its path, seemsto be important (Biro & Leslie, 2007; but see Kamewari et al., 2005; Csibra, 2008; Johnson et al., 2007;Luo & Baillargeon, 2005). In the current study of action and reaction, in contrast, infants were habit-uated to exactly repeating motions. Thus, criteria for goal attribution may differ slightly for objectinteractions, perhaps due to the relatively modular nature of infant causal perception. Alternatively,it might not be the nature of the agency cues that matters in both cases; rather, it may be their overallstrength, with the contingency information of the reaction event compensating for its lack of approachvariability.

Note that even if infants see the reaction event as goal directed—or as lying on a continuum towardthis—they still need not interpret it like older observers. Adults typically report that A causes B to runaway; that is, they see B’s motion as a goal-directed reaction, with variability in whether A’s motion isgoal directed as well (Schlottmann et al., 2006). A different possibility is that B’s departure causes A tostop or that multiple relations appear at once; that is, A chases B, B runs away, and then A gives up.Without the possibility of verbal data, the exact correspondence between infants’ and adults’ experi-ences must remain unknown.

The role of contingent motion-at-a-distance

Although our data fall short of establishing that infants recognize psychological goal-directed cau-sality in the reaction event, they have implications for another facet of early psychological under-standing. Our results go further than previous work regarding the role of contingency-at-a-distance.If causal perception of reaction events had depended on self-initiated motion or nonrigid motion, thiswould have meant that this type of minimal motion contingency is not sufficient on its own to triggera causal interpretation. The causal interpretation, however, is a precondition for a psychological inter-pretation, as discussed in the Introduction. Our finding that the reaction event appeared to be causalwithout help from other agent cues establishes this precondition. Thus, the current data support Man-dler’s (1992) and Johnson’s (2000) claim that contingency-at-a-distance serves as an agent cue.

Ultimately, contingency refers to an observed dependency between two events/features. Contin-gencies may or may not involve causation or agents. Causal contingency-at-a-distance typically in-volves agents but appears in various behavioral forms, simultaneous motion in reaction events, orthe pattern of a communicative exchange (Shimizu & Johnson, 2004) or variation in the action thatrationally fits environmental variation (Csibra, 2008; Csibra et al., 1999; Csibra et al., 2003; Gergelyet al., 1995). Formal indexes of statistical contingency ignore such differences in event type and con-sider only the pattern of occurrence/nonoccurrence (e.g., Watson, 1994). For humans, however, thesedifferences are crucial. Some contingencies are coincidence or spurious correlation, and what mattersto adults, at least, is not so much that there is contingency but that it can be interpreted as causal, ra-tional, communicative, or the like. That in turn depends very much on the particulars of the behavior.Infants seem to be no different: They pick up on causal or rational contingencies, but not on noncausalor irrational contingencies in similar control events. The general concept of contingency may in theend be of limited use in debates of infant reasoning because it is too broad. We might be better offfocusing on how specific forms of contingent behavior indicate causally interacting or rationally actingor communicating agents. In the current case, the crucial factor seems to be that the contingency in-volves simultaneous motion-at-a-distance.

Perception of causal structure or merely of spatiotemporal structure?

Skeptics might try to argue that infants do not see causality here at all but rather see only spatio-temporal structure. Babies might simply react more to reversal of continuous motion than to discon-tinuous motion. This is not plausible, however, because infants dishabituate little to reversal of thecontinuous motion of a single object (Leslie, 1984). In the same study, they distinguished causal


launching from the motion of a single object, changing color halfway (Leslie, 1984). So, babies clearlyanalyze the internal structure of continuous motion events.

Could one argue that infants merely react to a change in the order of the motions rather than tocausal roles? Order reverses in reaction and control groups alike, so one would need to assume thatinfants react to order only in continuous two-motion events, such as launch and reaction, but notin control events. However, 4-month-olds are sensitive to temporal order of three nondelayed motions(Lewkowicz, 2004), and 3-month-olds react to order of series of up to four pictures presented 1000 msapart (Canfield & Haith, 1991), so there is no reason to suspect that our much older infants could notdetect order of only two objects with a 1200-ms delay.

In addition, infants clearly see more than ordered sequences in continuous two-motion events, dis-tinguishing launch from reaction (Experiment 4) and contiguous gap events (Leslie, 1984). Also, forlaunching at least, there is independent evidence that infants perceive causality obtained with a tech-nique that involves causal categorization rather than reversal perception (Cohen, Amsel, Redford, &Casasola, 1998; Oakes, 1994). All in all, although alternative interpretations are conceivable for indi-vidual studies, convergent results help to validate the view that the reversal paradigm assesses infantsensitivity to changes in causal roles.

Note that the percepts of nonverbal organisms, even if structurally identical to those of verbalobservers, could still differ in phenomenal—here causal—content. Such views provide leaner interpre-tations of infant data (e.g., Haith & Benson, 1998) but then require additional processes to explain howthe content is eventually acquired. Thus, their parsimony is debatable (Onishi & Baillargeon, 2005;Surian et al., 2007), especially if, as in this case, 3-year-olds’ percepts clearly are domain specific. How-ever, calling infant percepts causal does not imply equivalence to adults. On the contrary, the earlypercepts are taken as tools to help children acquire causal knowledge and produce developmentalchange (Leslie, 1994; Schlottmann, 2001). Such changes appear at older ages for contact and noncon-tact causality (see below), and they occur throughout infancy for contact causality (Cohen et al., 1998).

Perception of physical causation-at-a-distance?

If infants see causality here but not necessarily psychological causality, mechanical causality, as inlaunch events (Leslie, 1994), is another option. In this case, infants might see reaction events as akin tolaunching-at-a-distance. However, many studies show that infants’ understanding of collisions closelydovetails with physical constraints. Kotovsky and Baillargeon (1994), Kotovsky and Baillargeon(1998), for instance, found detailed expectations about collision trajectories related to object size.Such work makes it difficult to believe that infants’ notion of a collision would be so broad as toinclude simultaneous motion-at-a-distance. One might argue that even adults sometimes seelaunching-at-a-distance (Yela, 1952) despite physical knowledge to the contrary. However, adultsdo not do this if alternative causal interpretations are allowed, nor do they do this for reaction events(Schlottmann et al., 2006).

To be sure, repelling magnets may move simultaneously without contact. However, it is unlikelythat infants would come across this, and so learned or built-in sensitivity to magnet motion seemsto be unlikely. Finally, there is the issue that an initially physical view of causation-at-a-distanceshould make it difficult to arrive at a psychological interpretation later, yet 3-year-olds have no doubtsabout this (Schlottmann et al., 2002). All in all, the notion that infants interpret reaction events asphysically causal does not seem to be very plausible.

An undifferentiated domain-general origin of perceptual causality?

Yet a different possibility is that perceptual causality is initially separate from domain-specificknowledge of mechanical or psychological events. Two versions of such a view might be considered.One sees perceptual causality as akin to standard forms of domain-general causal learning, possiblyrelying on Bayesian nets (e.g., Gopnik et al., 2004), arguing that even young children have sophisti-cated mechanisms for inferring causality from complex statistical contingency data. But althoughadults and children clearly make such inferences, work with infants merely shows sensitivity to rel-evant data patterns, leaving open whether these are interpreted as causal (Sobel & Kirkham, 2007).


Moreover, for adults at least, causal launch perception dissociates from their ability to infer causalityfrom statistical contingencies among the same events (Huber, Schlottmann, & Daum, 2004; Schlott-mann & Shanks, 1992).

Our study did not set up a standard causal learning situation because infants here merely saw tworepeatedly co-occurring motions during habituation; contingency learning paradigms require morevariation in the data such as information on whether the effect occurs without the cause. Moreover,the experimental and control groups did not differ in this. If perceptual causality reflects domain-gen-eral causal inferences, one would also need to assume that our infants did their causal contingencylearning prior to the experiment, with the reversal paradigm merely testing whether this preexistingunderstanding is causal.

Note that a view under which infants learn about perceptual causality outside of the experimentseems less plausible for reaction events than for launch events. Infants may observe collisions of balls,blocks, and the like, but it is not clear whether they similarly observe chasing and running awayevents. These tend to involve much larger scale movements that are difficult to track for still largelyimmobile infants. All in all, it is not so clear how a domain-general ability to infer causality from sta-tistical contingencies could account for the current findings.

Another version of the domain-general view of perceptual causality may have greater plausibility.Perceptual causality is typically taken to involve simple, potentially innate event templates (Schlott-mann, 2001; Scholl & Tremoulet, 2000). Instead of separate templates for reaction and launch events,one might consider a single, initially undifferentiated template for co-occurring motions. Thus, younginfants might have an unspecific causal percept of ‘‘A does something to B” that later differentiatesinto physical and psychological causality. This is not to say that infants do not attend to the contactfeature differentiating the events, only that this is not part of their causal template. Undifferentiatedperceptual causality in infants is compatible with domain-specific launch and reaction causality inobservers from at least 3 years of age, merely positing a common developmental origin for both. Itis also compatible with domain-specific reasoning from infancy onward (e.g., Saxe & Carey, 2006),merely arguing that perceptual causality is not yet integrated with infants’ frameworks for reasoning.

Either way, undifferentiated, domain-general perceptual causality would require modification ofexisting theories of infant ontology; for instance, it would then be difficult to characterize their causalperception of launch events as mechanical. To further investigate domain specificity in infant percep-tion of causality, future work might correlate reaction causality and tasks with established links tophysical or psychological cognition.

Conclusions

Perceptual causality for reaction events as well as launch events operates from infancy. Kanizsa andVicario (1968) saw launch and reaction perception as similar, but for Michotte (1950/1991a, 1954/1991b) the latter involved interpretation because he found reaction reports only when giving observ-ers hints of animacy. More recent work found that nonrigid motion enhanced reaction and interferedwith launch causality in adults (Schlottmann et al., 2006). Children were affected less, perhaps due toprocessing limitations (Schlottmann et al., 2002). Our results with preverbal infants concur. Thus, thedevelopmental course so far suggests similar processes, and perhaps a common origin, with some rolefor interpretation later on.

The current data emphasize the importance of specific event configurations in perceptual causality.Such potentially built-in causal blueprints may aid or combine with initial broad divisions of the worldinto its physical and psychological domains. Nuances and borderline cases are worked out later whenother cues begin to modulate the impression. However, to describe these event configurations as con-tact and noncontact causality is oversimplified. Causal events have both distinctive spatial featuresand distinctive temporal features. Launch events involve contiguous sequential motion, whereas reac-tion events involve simultaneous motion.

Causality in the reaction event is a Gestalt effect relying on simultaneous motion and lack of con-tact. Neither one alone suffices for reaction perception in children and adults. Simultaneity is not en-ough because simultaneous contact motion appears as mechanical entraining. It may be necessary,


however, in simple motion contingencies because contiguous noncontact motion does not appear as areaction (Michotte, 1946/1963; Schlottmann et al., 2006; Watts et al., 2007; Yela, 1952). Self-initiatedmotion, as argued earlier, would also seem to be neither necessary nor sufficient. It is important, how-ever, to separate the absence of self-initiated motion from the presence of contact motion becausecontact destroys reaction causality.

In sum, by 8 months, infants are sensitive to causation-at-a-distance in reaction events seen by old-er observers as goal directed. The perception appears to depend on the event configuration rather thanon nonrigid or self-initiated agent motion. Our data contribute to work on how humans come tounderstand the causal texture of the world (Tolman & Brunswik, 1935) and to the growing literatureon early social cognition (e.g., Leslie, 1994; Luo & Baillargeon, 2005; Surian et al., 2007; Tomasello,1999). Perceptual causality in launch and reaction events may provide one mechanism for causal anal-ysis that requires little or no physical knowledge or mind-reading skills.

Acknowledgments

This work was supported by Economic and Social Research Council (ESRC) Project Grant Nos.R000237058, R000223481, and RES000230198. Thanks go to Sarah Hesketh for testing many of thebabies in this study.

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