INFANCY,9(3).313-325Copyright © 2006. Lawrence Eribaum Associates. Inc.

BRIEF REPORTS

Two-Month-Old Infants' Sensitivityto Social Contingency in

Mother-Infant and Stranger-InfantInteraction •

Ann E. BigelowDepartment of Psychology

St. Francis Xavier University

Philippe RochatDepartment of Psychology

Emory University

Two-month-old infants (A'= 29) participated in face-to-face interactions with theirmothers and with strangers. The contingent responsiveness for smiles and vocaliza-tions, while attending to the partner, was assessed for each partner in both interac-tions. For smiles and for vtx-alizatJons. infants were less responsive to the strangerrelative to the mother when the stranger's contingent responsiveness was either morecontingent or less contingent than thai of the mother. Results are supportive of thehypothesis that young infants develop sensitivities to levels of social contingencypresent in their maternal interactions, which influence their responsiveness to others.

Infants' early awareness of the contingency between self-actions and externalconsequences occurs most readily in parent-infant face-to-face social interactions(Neisser, 1991). Parents tend to respond contingently to infants' vocalizations, ges-tures, and facial affect. For infants under 6 months, parental responses to infant

Correspondence should be addressed U) Ann E. Bigelow. Depanment of Psychology, St. FrancisXavier University. Antigooish. Nova Scotia. B2G 2W5, Canada. E-mail: abigelow@stfx.ca

3 1 4 BIGELOW AND ROCHAT

behavior are primarily modified imitations of infants' actions that parents perceiveto contain emotional content (Stem. 1985; Stern. Hofer. Haft. & Dore. 1985).Individual differences in parental contingent responsiveness to infant behaviors arethought to be stable within mother-infant dyads from the time infants are 2 to3 months of age. but vary across the population {Stern et al., 1985; Watson. 1985).

Parental responsiveness within parent-infant interactions tnay create preferredcontingency levels through familiadzatioti. which are reflected in infants' respon-siveness to new social partners {Bower, 1982; Watsoti. 1985). Watson {1985)hypothesized that infants will be most responsive to people whose level of contin-gency is similar to that of their parents. Bigelow {1998) found support for thishypothesis. Four- and 5-month-old infants were most similar in their responsive-ness to mothers and strangers when the strangers' contingent responsiveness tothem resembled that of their mothers, and infants were iess responsive to strangers,relative to their mothers, when strangers' contingent responsiveness wa.s eithermore contingent or less contingent than that of their mothers.

This study investigated whether 2-month-old infants show this sensitivity tofamiliar contingency levels. The beginning of bidirectional social exchangesbetween infants and other people is marked by infants manifesting socially elicitedsmiles in face-to-face interactions around 2 months of age (Rochat. 2(X)I). Thesereciprocal exchanges involve attentional. vocal, and postural transactions as wellas facial expressions. Infants at 2 months of age are responsive to disruptions insocial contingency as evidenced by their reactions to the still-face and replay pro-cedures (Adamson & Frick. 2003; Murray & Trevarthen. 1985; Nadei. Carchon.Kervella. Marcelli. & Reserbat-PIantey. 1999; but see Bigelow & DeCoste, 2003;Rochat. Neisser. & Marian. 1998). Yet how sensitive infants this young are todifferences in others' social responsiveness to them remains an open question.

In this study. 2-month-old infants participated in face-to-face interactions withtheir mothers and with strangers. Contingent responsiveness was assessed foreach partner in both interactions. Watson's (1985) hypothesis predicts that infantswould be more similar in their contingent responsiveness to mother and strangerwhen the stranger's contingent responsiveness to the infant resembled themother's contingent responsiveness to the infant, and infants would be lessresponsive to the stranger relative to mother when the stranger was either morecontingent or less contingent than the mother.

METHOD

Participants

Participants were 29 infants (M age = 68 days. SD = 9 days; 12 males and17 females) and their mothers. Five additional infants were excluded. 1 for excessive

TWaMONTH-OLD INFANTS' SENSITIVITY 315

crying and 4 for equipment or procedural error. The infants were from a universitytown and the surrounding area in eastern Canada. They were located throughnewspaper birth announcetnents.

Socioeconomic status (SES) of the infants' families was measured by a Canadianindex (Blishen. Carroll. & Moore. 1987) based on education, income, and occu-pational prestige. In the index, occupations me divided into 514 groups, rangingfrom SES scores of 17.81 to 101.74 (M = 42.74, SD = 13.28). The scores of thehigher status parent in the participants' families yielded an SES mean of 50.50(SD ^ 16.69). Most of the participants' parents had a college degree or mure(47%). 30% had some postsecondary education. 21% had only a high schooldegree, and 2% had not finished high school. Racial and ethnic composition ofthe infants" families was 100% non-Hispanic White.

Materials and Procedure

The study took place in a laboratory room. The mother-infant dyads were sched-uled in pairs so that the mother of one infant was the stranger to the other infant. Afemale experimenter took demographic infonnation from the mothers andexplained the procedure. Then one infant and one adult were taken to a three-sided booth made from room dividers, while the other infant and adult waited inanother room. In the booth, the infant was placed In an infant seat attached to atable (55 cm |H| x 75 cm IW] x 150 cm [LI). A chair for the adult faced the intantseat 1 m away. Three video cameras were used. One. positioned above and to theside of the adult when seated in the chair, recorded the frontal view of the infant.A second, positioned above and to the side of the infant seat, recorded the frontalview of the adult. A third, mounted on the wall, recorded a side view of the dyad.The cameras were connected to a four-way split-sereen monitor located outsidethe booth (the fourth screen was blank).

The infants engaged in two 5-min face-to-face interactions with the adults,who were told to interact normally with the infants. In the mother condition,mothers were the social partners. In the stranger condition, mothers of otherinfant participants were the social partners. The order of the mother and strangerconditions was counterbalanced. v ,,-

M e a s u r e s - • - ' ' > ' r

The videotapes of the mother and stranger conditions were scored for attention,smiles, and vocalizations, using a real-time microcomputer ctxling program. REAL-TIME, developed by Sytnons. Acton, and Moran (1990). Attention was scored asthe presence or absence of looking at the partner. Smiles were scored as upwardlip movements with or without vocalizations or sudden noticeable increases in

31 6 BIGELOW AND ROCHAT

such Upward lip movements with or without vocalizations. Vocalizations excludedfussing, crying, and digestive sounds (e.g.. burps, hiccups). For each 5 min of adult-infant interaction, six passes of the videotape were scored: attention, smiles, andvocalizations for the infant and attention, smiles, and vocalizations for the adult.Behaviors were scored for their onset and offset. After scoring each of the part-ners, the data files were merged using Symons et al.'s (1990) TMERGE program.

For reliability purposes, two coders independently scored 14% of the adult-infant interactions. The proportion of agreement (Roberts, 2005). within a timetolerance of 1 sec between coders, for infant behaviors was .86 for attention, .74for smiles, and .75 for vocalizations and for adult behaviors was .95 for attention,.71 for smiles, and .82 for vocalizations.

Stuiles that cooccurred with attention and vocalizations that cooccurred withattention were the critical behaviors. Of particular interest was each partner's crit-ical behavior that followed within a 1-sec window the onset of a similar criticalbehavior by the other partner. A 1-sec window was chosen because, in mother-infant face-to-face interactions, smiling responsiveness tends to occur within1 sec (Bigelow. 1998; Bloom. Russell. & Wassenberg, 1987; Symons & Moran,1994). and infants' vocalizations in face-to-face interactions tend to occur mostfrequently during pauses in adults" speech (Mayer & Tronick. 1985). which lastapproximately 1 sec (Papousek, Papousek, & Bornstein, 1985). Measures of con-tingent responsiveness were adjusted residuals (Bakeman & Gottman. 1997)using Watson's (1979, 1985) contingency tnagnitude scores. Contingency magni-tude for each partner's critical behavior was calculated from the merged real-timedata as the difference between observed conditional probabilities (e.g.. probabil-ity of smile onset by partner A follows, within a I-sec window, smile onset bypartner B) and expected unconditional probabilities (e.g., probability of smileonset by partner A).

RESULTS

Four contingent responsiveness scores (adjusted residuals) were calculated foreach infant for smiles and vocalizations: mother's contingent responsiveness toinfant (Mi), infant's contingent responsiveness to mother (Im). stranger's contin-gent responsiveness to infant (Si), and infant's contingent responsiveness tostranger (Is). Table 1 shows the means and standard deviations for the contingentresponsiveness scores of each of the partners and for the number of smiles andvocalizations by each of the partners. Table 2 shows the correlations among thecontingent responsiveness scores, which were all nonsignificant.

Preliminary analyses of variance (ANOVAs) for order effects (mother condi-tion vs. stranger condition first) and sex of infant indicated no significant effectsin any of the dependent variables. Thus, order and sex were excluded as variablesin subsequent analyses.

TABLE 1Contingent Vocal and Smiling Responsiveness Scores of Each

Partner and Number of Vocalizations and Smiles byEach Partner With Partial Eta Squared for Analyses of Variance

ContingenI responsivenessAdtill to infant

SmilesVocalizalions"

Infanl to adultSmilesVocalizaltons''

Number of behaviorsAdult to infant

SmilesVtKalizations

Intant to adultSmilesVocalizations

Fnim or to Moiher

M

-211

.920'-.376

23.9132.7

9.825.5

SD

1.8621.104

1.433.999

9.220.0

8.614,6

From or to Stranger

M

.316"-.672"

.074"

-.145"

22.7I32.i

8.821.5

,50

1.6071.436

1.190

1.139

9.827.5

8.216.3

.024

.057

.251,029

.012

.000

.013

.050

Note. ^ = 29."Contingent responsiveness scores require both adull and infant to have pertbnned the critical

behavior. Only 28 of the 29 infants performed the crilical behavior in these instances. "Mean contingentresponsiveness scores for vocali/.ations are negative because they included cases in which vocalizationsby the responding panner did not follow within 1 sec the onset of"any vwalizations by the other partner,although both partners vocalized. Contingent responsiveness scores for these cases were negative.

••n- ' . • • , ' '

TABLE 2Correlations Among Contingent Responsiveness Scores

SmilesMiSiImIs

VocalizationsMiSiImIs

Mi

-.04.16.00

-,OI-.08

.27

.11

Smiies

Si

.24

.01

.03-.03.22

-.08

Im

.31

.03-.25

.11

.15

Is

.05-.10

.13

.09

Mi

.14-.28-.02

Si

-.12-.32

,no..

Im

-•

-.03

h

Note. Mi - mothers" contingent responsiveness scores to infants; Si = stratigers' contingentresponsiveness scores to infants: Im = infants' contingent responsiveness scores to mothers; Is = infants'contingent responsiveness scores to strangers,

317

318 BIGELOW AND ROCHAT

Repeated measures (adult partners) ANOVAs were conducted on the contingentsmiling and voca! responsiveness scores of adulls to infants and infants to adultsand on the number of smiles and vocalizations made by adults to infants andby infants to adults. As indicated by the partial etas-squared in Table 1, only thecontingent responsiveness of infants' smiles to adults" smiles was significant,F{\. 26) = 8.71, /; = .007. Infants, as a group, smiled more contingently to theirmothers' smiles than to the strangers' smiles.

Repeated measures ANOVAs comparing mothers' behavior to their owninfants and to unfamiliar infants were nonsignificant for number of smiles andvocalizations, and contingency of smiles and vocalizations. Mothers were simi-larly expressive and contingent in each behavior across infants.

The correlation between mothers' contingent smiling responsiveness and contin-gent vocal responsiveness was nonsignificant, indicating that mothers' contingencyin one behavior was not predictive of their contingency in the other behavior.When mothers' contingent smiling and vocai responsiveness scores were ranked.24% of the mothers were in the top half of the contingency ranking for bothbehaviors. 79% were in the top half in at least one of the behaviors, and 21 % didnot rank in the top half in either of the behaviors.

To measure how similar mothers and strangers were in their contingent respon-siveness to infants, strangers' contingent responsiveness scores to infants (Si) weresubtracted from the mothers' contingent responsiveness scores to infants (Mi).Likewise, to measure how similar infants were in their contingent responsivenessto mothers and strangers, infants' contingent responsiveness scores to strangers (Is)were subtracted from infants' conlingent responsiveness scores to mothers (Im).

Watson's (1985) hypothesis would predict a U-shaped curve when infants'contingent responsiveness to mothers minus infants' contingent responsiveness tostrangers (Im-Is) is plotted against mothers' contingent responsiveness to infantsminus strangers' contingent responsiveness to infants (Mi-Si). Infants' respon-siveness to mother and stranger would be most similar when mother's andstranger's contingent responsiveness to the infant is similar. However, if themother's and stranger's contingent responsiveness differed in either a positivedirection (mother was more contingent than stranger) or a negative direction(stranger was more contingent than mother), infants would be less contingentlyresponsive to strangers than to mothers.

Figure I for smiles and Figure 2 for vocalizations show that for such plots theresultant curve is U-shaped, corresponding to a significant quadratic trend. Trendanalyses were conducted on the data using a multiple regression approach(Pedhazur. 1982). Arbitrarily Im-is was used as the dependent variable and thepredictors were Mi-Si (linear trend), (Mi-Si)^ (quadratic trend), and (Mi-Si)^(cubic trend). The linear trend was entered first in the regression equation, fol-lowed by the quadratic trend, and then the cubic trend. This procedure allowed anassessment of the percentage of variance accounted for in Im-Is by each trend,

TWO-MONTH-OLD INFANTS' SENSITIVITY 3 1 9

Smiles

mT

-6

FIGURE 1 The relational plot of the difference between the infants' contingenlresponsiveness to mothers and strangers (Im-Is) and ihe difference between mothers' andstrangers" contingent smiling responsiveness to the infants (Mi-Si).

over and above the contribution of the previously entered trends. Table 3 showsthe R- for the linear trend and the changes in /?' for the quadratic and cubic trendsfrom the previous trend(s) for smiles and vocalizations. Only the quadratic trendswere significant. For smiles, the quadratic trend. F{\. 24) = 6.94. /J < .025.accounted for 22% of the variance in Im-Is. For vocalizations, the quadratictrend. /•( 1.25) -4.80. p < .05, accounted for 15% of the variance in Im-Is.'

'To determine whether the significant quadratic trends were driven by a few participants, trendanalyses (Pedha/ur. 1982) were conducted on the smiling data wiihoui the four ca,ses with Mi-Siscores greater than 4 absolute and on the vocalizations data without the two cases with Mi-Si scoresgreater than 2 SDs from the mean. For smiling, only the quadratic trend was significant. f ( l . 20) =6.11. /)< .02.^. accounting for ly/r of the variance in Im-Is when the linear trend was p;irtialcd out,R^ was .004 for the linear trend; changes in fi' from the previous trends were .233 for the quadruiic Irendand .038 for the cubic trend. For v(x.'aiizati(>ns. only ihc quadratic trend was signilicani. Fi\. 23) =5.66, p < .05, accounting for 19% of the variance in Im-Is when the linear trend was partialed out. R^was .017 for the linear trend; changes in R' from the previous trends were . 194 for the quadratic trendand .051 for the cubic trend.

3 2 0 BIGELOW AND ROCHAT

Vocalizations

FIGURE 2 The relational plot of Ihe difference between the infants' contingent vocairesponsiveness to mothers and strangers (Im-Is) and the difference between mothers' andstrangers' contingent voca! responsiveness to the infants (Mi-Si).

TABLE 3R̂ for Linear Trend and Changes in R̂ for Quadratic and Cubic Trends From Previous

Trend(s) for Smiles and Vocalizations

SmilesVocalizations

Linear

.001,062

Mi-Si Trend

Quail ral ic

.224

.151

Cubic

.011

.078

Note. Mi-Si - mothers' contingent responsiveness to infants minus strangers' contingenl respon-siveness to infants.

DISCUSSION

By 2 months of age, infants' responsiveness to new partners is influenced by howsimilar tho.se partners* contingency levels iire to the contingency levels with whichthe infants are most familiar. The quadratic trends depicted for smiling contingencyin Figure I and for vocal contingency in Figure 2 indicate that infants are less

TWO-MONTH-OLD INFANTS' SENSITIVITY 3 2 1

responsive to strangers relative to mothers when strangers' contingency levelswere either more contingent or less contingent than those of mothers., supportingWatson's (1985) hypothesis that infants become sensitive to the contingency levelsexperienced in maternal social interactions.

The similarities and differences, shown in Table 4. between the performance of2-month-old infants and their adult partners in this study and the pertbrmance of4- to 5-month-old infants and their partners in Bigelow's (1998) study revealdevelopments in infants' sensitivity to familiar contingency levels. Familiar contin-gency levels are embedded in mother-infant routines in face-to-face interactionsthat become established over the infants' first months of life, piuiicularly between2 months, when infants" interest in face-to-face interactions emerges (Rochat.20t)l), and 4 to 5 months, when their interest in face-to-face interactions is at itsheight (Lamb. Morrison. & Malkin. 1987). Both younger and older infants smiledand vocalized as much to strangers as to mothers. Yet the older infants smiled andvocalized more contingently to mothers than to strangers: the younger infants weremore contingent to mothers than to strangers only with their smiles. Infants maybecome more contingently responsive to mothers than to strangers across differentmodalities as their experience with mother-infant interactions increases. Theyounger infants and their adult partners had higher numbers of smiles and vocal-izations but lower contingent responsiveness scores in smiles and vocalizationsthan the older infants and their adult partners, indicating that at 2 months the dyadswere expressive but the interactions were less reciprocal and responsive than at4 to 5 months. Indeed, in Bigelow's (1998) study, strangers smiled more often thandid mothers, which was interpreted as strangers' attempts to engage infants in theab.sence of established reciprocal mother-infant routines.

The younger infants were more variable in their contingent responsivenessdifference scores (Im-Is). suggesting that although infants showed sensitivityto familiar contingency levels at 2 months, this sensitivity becomes more stableby 4 lo 5 months. The variability in younger infants' contingent responsivenessdifference scores was particularly prevalent when mothers' and strangers' contin-gency levels were similar (when Mi-Si scores clustered around zero). Olderinfants may be more sensitive to familiar contingency levels, responding tostrangers whose contingency levels are familiar as they do in their maternal inter-actions, whereas younger infants may be reactive to strangers' contingency levelsparticularly when the levels are dissimilar to those that are becoming familiar,responding to the dissimilarity with less contingency. That some 2-month-oldinfants were more responsive to mothers than strangers even when mothers" andstrangers' contingency levels were similar is not surprising given that mothers'face and voice as well as emerging mother-infant face-to-face routines were morefamiliar. Yet some young infants were more responsive to strangers than motherswhen the adults' contingency levels were similar. In other studies where infants'responsiveness to strangers was greater than to mothers, the strangers were

TABLE 4Comparisons Between Performance in 2-Month-Old and

4- to 5-Month-Old Infant-Mother and Infant-StrangerFace-to-Face Interactions

Similarities Differences

Adult Behaviors

Withiti each age group, mothers and strangerswere similar in the iitimher of vocalizationsmade to infants.

Within each age group, mothers and strangerswere similar in their contingency to infantswith both smiles and v(K'ali/,ations.

Mothers' contingent responsiveness inone behavior (vocalizations, smiles) wasnot predictive ot" mothers' contingentresponsiveness in the other behavior.

When mothers' contingenl sn:iiling and vocalresponsiveness scores were ranked, mostmolhers were in the top half of the contingencyranking iti only one of the t>ehaviors.

When interacting with older infants, strangersprtKluced more smiles than mothers; withyounger infants, strangers and mothers weresimilar in the number of smiles made lo infants.

Adult partners of younger infants had highermean number of smiles and vocalizations butlower mean contingent smiling and vwalresponsiveness scores than adult partners of olderinfants.

Infant Behaviors

Within each age group, infants did not differ inthe number of smiles or number ofvocalizations made to mothers and strangers.

Older infants smiled and vocalized morecontingently to mothers than to strangers;younger infants smiled more contingently tomothers than to strangers hut did not differ intheir contingent vocalizations to mothers andstrangers.

Younger infants had higher mean ntimber ofsmiles and vocalizations but lower meancontingent smiling and vocal responsivenessscores than older infant.s.

Relation Between Contingent Responsiveness Difference Scores

For both smiles and vocalizations, significantquadratic trends resulted when lni-Is wasplotted against Mi-Si, indicating infants weresimilar in contingent responsiveness tomothers and strangers when strangers' andmothers' conlingent responsiveness scoreswere similar and infants were less contingentlyresponsive to strangers, relative Eo mothers.

For both smiles and vocalizations, youngerinfants' contingent responsiveness differencescores (Im-Is) were more variable than those ofolder infants, particuhirly when mothers' andstrangers' contingent responsiveness sctires weresimilar (Mi-Si clustered around zero). Thevariance of Im-ls for the younger infants whointeracted with mothers and strangers who were

(continued)

322

TWO-MONTH-OLD INFANTS' SENSITIVITY 323

TABLE 4 (Continued)

Simliaiitit's Diffcremes

when strangers' coniingeni responsiveness scores similar in contingent responsiveness (Mi-Siwere eithermore contingent or less contingent within .5 SD from the mean 1 was 2.61 tor smilesthan those uf mothers. and 2.03 tor vocalizalions; lor older infants the

vatiance was 1.17 for smiles and .62 forvocaH/ations.

Nolf. The pertbrmance of the 4- to 5-month-old infants and their adult partners is reported inBigelow (1998), with the exception of the variance for Im-Is scores of infants whose adult partners'Mi-Si scores were within .5 SD of the mean. lm-Is = infants' contingent responsiveness to mothersminus infants' contingent responsiveness to strangers; Mi-Si - mothers' contingent responsiveness toinfants minus strangers' contingent responsiveness to infants.

* • ' • .• • \.y\ t .

trained or selected for their sensitivity in interactions with infants. Jaffe. Beebe,Feldstein, Crown, and Jasnow (2001) found vocal interactions between 4-month-old infants and their mothers to be less bidirectionally coordinated than vocalinteractions between the infants and strangers who were trained to be responsiveand sensitive to infant behavior. When investigating interactions of depressedmothers and their infants. Field (1987) found 3-month-oki infants mirrored theirmothers' depressed affect, but when paired with nondepressed social partnerswbo were selected for tbeir sensitivity to infant behavior, the infants became moreresponsive (Field et al.., 1988). Interestingly, during the slranger-infant interac-tions, these infants elicited what appeared to be depressed behavior from theirnondepressed partners. Sensitive partners, with or without awareness, may adjusttheir contingency levels to levels where infants are most engaged, which are thelevels with which the infants are most familiar. In this study, strangers were mothersof other infant participants. When the stranger's contingency level was similar tothat of the mother, the familiarity ofthe contingency level coupled with Ibe noveltyof the new person may have made the interactions particularly engaging for someyoung infants. However, when novelty ofthe stranger was paired witb unfamiliarcontingency levels, infiints were less engaged witb strangers than with mothers.Although the novelty of interaction witb strangers can be interesting to younginfants, infants' experience with maternal contingency levels influences tbeirinteractions with new people.

Mothers were similarly expressive with their own and unfamiliar infants, yetthe means that they used were individually distinctive, which supports previousfindings that mothers have individual patterns in tbe bebaviors they most fre-quently use to respond to tbeir infants (Bigelow. 1998; Stern et al., 1985) and thatthese patterns persist across infants (Kaye. 1982). Approximately a quarter of tbemothers were high in contingent responsiveness for both smiles and vocalizationsbut most were only high on one behavior, and 2 1 ^ were low on botb. although

3 2 4 BIGELOW AND ROCHAT

Other modes of maternal contingency, such as tactile contingency, were notmeasured. As in Bigelow's (1998) study, mothers' smiling responsiveness andvocal responsiveness were unrelated. Yet in each behavior, infants were sensitiveto the familiar levels of contingency experienced with their mothers.

Limitations to this study are several. The sample was small and homogeneous.Although the significant trends obtained with the small sample suggest the findingsare robust, whether infants from diverse populations would demonstrate similarsensitivities to familiar social contingency remains to be tested. Tbe contingencylevels were modality specific and. therefore, conservative. Adults may contingentlyrespond to infants" behavior in cross-modal ways, although in the first 6 months oflife, adults" responses to infants tend to be imitative ofthe infants' behavior (Stem,1985). The data were obtained from interactions in a laboratory setting. Jaffe andcolleagues (2001) showed differences in mother-infant and stranger-infant interac-tion synchrony in laboratory and home environments, indicating that the generaliz-ability from laboratory data may be limited. Nevertheless, the findings suggest that2-month-old infants are sensitive to familiar contingency levels.

Young infants' intimate relationships impact tbe levels of engagement theybring to .social encounters witb others. By 2 months of age. infants' experiencewith maternal contingency levels affects their responsiveness to others. Althoughinfants" responsiveness to diftering contingency levels can expand, particularly asthey enlarge their social networks, the ease with which new relationships areformed may be influenced by the similarity between others" interactive contin-gencies and the contingencies witb wbicb infants are most familiar.

ACKNOWLEDGMENTS ' •

This research was aided by a Natural Sciences and Engineering Researcb Councilof Canada grant to Ann E. Bigelow. Gratitude is expressed to tbe infants andmothers who participated in the study: to Michelle Power. Andrea Beck, MaryRankin. and Cindy DeCoste for their research assistance; and to Bonnie Quinn forher programming assistance.

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