Patterns of Attachment: A Psychological Study of the Strange Situation - Mary D. Salter Ainsworth 2015
Normative Trends Across Episodes
Measures and Methods of Assessment
Introduction
Whereas the previous chapter provided a detailed account of behaviors that commonly occur in each separate episode of the strange situation, the present chapter is concerned with the pattern of changes in a given behavior or class of behaviors from one episode to another. The intrinsic design of the strange situation was dictated by the hypothesis that 1-year-olds who are attached to their mothers will use her as a secure base from which to explore an unfamiliar environment when she is present. Consequently, it was expected that the array of toys would elicit exploratory behavior in the preseparation Episode 2, while at the same time the infant would display weak attachment behavior, if any. Although it was expected that the entrance of the stranger in Episode 3 would attract the baby’s attention and therefore lead to a decrease in exploratory behavior directed toward the toys, no hypothesis was formulated in regard to normative trends in attachment behavior in Episode 3, because no advance prediction was made about the extent to which the presence of the unfamiliar person would activate fear (or wary) behavior and/or attachment behavior directed toward the mother. It was expected, however, that attachment behavior (crying and search) would be activated by the mother’s departure and/or absence in the separation episodes, at the expense of exploratory behavior, which would thus decline. It was further expected that relevant forms of proximity- and contact-seeking behavior would be activated in the reunion episodes (at least initially), also at the expense of exploratory behavior.
The behaviors that are traced from Episode 2 through Episode 8 are all of those for which we have measures of frequency or strength. The frequency measures are: three measures of exploratory behavior (locomotor, manipulative, and visual), a measure of orienting behavior (looking), three measures of attachment and/or social behavior (crying, smiling, vocalization), and, finally, a measure of oral behavior. The measures of strength are the “interactive-behavior” scores. These are of two major classes: (1) attachment behaviors (or, in the case of the stranger, sociable behaviors)—namely, proximity and contact seeking, contact maintaining, search, and distance interaction; and (2) behaviors antithetical to attachment or sociability—namely, avoidant and resistant behaviors.
To supplement these measures of frequency and strength, we make appropriate reference to differences between episodes in the percentages of subjects showing the particular behavior at all (i.e., with minimal frequency or more). Full details of these percentage comparisons are given in Appendix VI, Tables 30 and 31. Finally, the “negative” behaviors—avoidant and resistant—seem best represented by percentage figures of infants at each score point, rather than by comparisons of mean scores across episodes.
Intersample and Sex Differences
ANOVA tests, supplemented where relevant by t tests, were made for each of the behavioral measures of significance of differences among the four component samples. Of 26 ANOVAs, only four yielded significant intersample differences—exploratory locomotion, looking at the mother, total vocalization, and resistant behavior to the stranger. None of these behaviors is critical to the classification system; and the intersample differences in exploratory locomotion seem attributable to a change in a physical aspect of the room in which the strange situation was conducted, rather than to “true” intersample variabilty. The relative lack of significant intersample differences justifies combining the samples.
ANOVA tests were also made for each behavior of the significance of sex differences. No significant differences were found. Hence, the sexes are not considered separately in the cross-episode comparisons.
Exploratory Behaviors
In the home situation, it is difficult to ascertain whether it is the infant’s mother or the whole familiar environmental context that provides him with the security necessary for him to be able to explore. In an unfamiliar situation, however, it is possible to determine the potency of the mother’s role in supporting exploratory behavior. Will the baby explore a new environment readily with his mother present, but less readily when she is absent? How will his exploratory behavior be affected by the entrance of a third person who is benign but totally unfamiliar? An examination of changes in exploration from one episode of the strange situation to another is germane to these issues.
Exploratory Locomotion
A variety of attractive toys was provided in order to give strong instigation to active exploration. To test whether an infant would leave his mother to explore, the toys were placed at the maximum feasible distance from her. The infant could examine them from a distance, but to actively manipulate them, he had to leave the starting point where he had been put down and move toward them, by creeping, crawling, or walking. Of course, he might also move about the room, exploring its furniture or fixtures, or move in pursuit of a toy that had itself been moved (e.g., chasing a ball that was rolled by his mother, by the stranger, or by himself).
An analysis of variance shows a highly significant episode effect for exploratory locomotion [F (6,612) = 15.072, p < .0001]. Figure 2 shows the changes across episodes. Exploratory locomotion was most frequent in Episode 2, when the infant was alone with his mother. The stranger’s entrance in Episode 3 sharply reduced locomotion, and this behavior remained at a low level of frequency in Episode 4 after the mother had departed. The mother’s return in Episode 5 activated some increase in exploratory locomotion, although not nearly to the level of Episode 2. The second separation, beginning in Episode 6 and continuing through Episode 7, led to another decrease. The slight recovery in Episode 8 did not bring the frequency of exploratory locomotion up to the level of the first reunion episode, number 5.
FIGURE 2 Mean incidence of exploratory behavior in each episode.
These findings confirm the hypothesis that 1-year-old infants tend to move away from the mother to explore an unfamiliar environment as long as she is present and presumably perceived as readily accessible to him. The stranger’s entrance dampened the vigor of exploratory locomotion even in the mother’s presence—so much so that the subsequent separation episodes served to reduce it very little more.
There was a significant difference among the four component samples [F(3, 102) = 7.13), p < .0005] in the frequency of exploratory locomotion. This was almost entirely due to the fact that Sample 1 showed more exploratory locomotion than any of the other three samples. This finding is attributable to a change in the experimental conditions. A rug was on the floor for the first 13 subjects of Sample 1, but was subsequently removed so that the floor might be marked into squares to facilitate the recording of locomotion. In retrospect it is clear that the rug provided traction that made creeping, crawling, and early unsteady walking easier.
Exploratory Manipulation
An analysis of variance yielded a significant episode effect for exploratory manipulation [F(6,612) = 4.47, p < .001]. Figure 2 shows that the changes across episodes are similar to those for exploratory locomotion, although the frequency of manipulation is greater than locomotion in every episode. The highest frequency was in Episode 2. There was a substantial drop in Episode 3 and a further slight drop in Episode 4, the first separation episode. Manipulation increased in the reunion Episode 5—when indeed the mother intervened, if necessary, to reinvolve the baby in play—but it decreased to new lows during the second separation Episodes 6 and 7. It recovered little in Episode 8, despite the mother’s presence.
Visual Exploration
Visual interest in the toys and in other features of the inanimate environment is both the most frequent form of exploratory behavior and the least active. It may be seen from Figure 2 that infants spend 10 or 11 of the 12 15-second intervals in Episode 2 visually examining one or another aspect of the physical environment. Although this behavior was less vulnerable to stress than the other two forms of exploratory behavior, its cross-episode trends are similar and significant [F(6, 612) = 6.61, p <.0001>
<.0001>
FIGURE 3 Mean incidence of crying in each episode.
Crying
In general, distress behavior is incompatible with exploratory behavior, and hence it was expected that trends in amount of crying across episodes would be the reverse of trends in exploration. Indeed, as Figure 3 shows, such tends to be the case. Crying was minimal in the preseparation Episodes 2 and 3; it increased during the first separation Episode 4; decreased again when the mother returned in Episode 5; and reached a peak in Episode 6, when the baby was left alone. There was some decline in crying in Episode 7 when the stranger was present, but crying was nevertheless more frequent than in Episode 4, when the baby was previously alone with the stranger. When the mother returned again in Episode 8, crying declined to about the same level as in the first reunion Episode 5. Analysis of variance showed that these episode effects were highly significant [F(6, 612) = 67.38, p <.0001].>
<.0001].>
These findings suggest that separation from the mother in an unfamiliar environment tends to be distressing to 1-year-olds. It is not, however, as distressing as being left alone altogether, as is shown by a comparison between Episodes 6 and 7 (t = 2.77; p < .01). The decline of crying in Episode 7 suggests that the stranger is able to provide some comfort to at least some babies. At the same time this, together with a comparison between Episodes 4 and 6 (t = 451; p < .001), suggests that for most of these infants separation occasioned substantially more distress than did the presence of the stranger per se.
Crying in the reunion episodes is almost entirely interpretable as a continuation of the distress occasioned by the preceding separations. Although the babies tended to be reassured by the mother’s return, not all were calmed immediately.
Search Behavior
This behavior peaked in Episode 6, when the baby was alone, and was relatively weak in the other separation episodes. An episode effect for search behavior was significant [F (2, 204) = 8.06, p < .001]. The stranger’s presence is likely to have affected the baby’s search for his mother in two ways. First, the arrangement of the experimental room was such that if the stranger was sitting in her chair, she was fairly close to the door, so that a baby who was wary of her might be deterred from passing by her to go to the door. Second, if the baby cried or seemed about to cry, the stranger tended to intervene either by distracting him or by picking him up; in either case any tendency to go to the door was thwarted. Furthermore, in Episode 4 babies tended to be less distressed, both because it was the first separation from their mothers and because they were not alone. Despite these complications, a hypothesis that following the mother is more strongly activated by being left alone than by being left in company receives support from findings in the home environment (Stayton, Ainsworth, & Main, 1973).
Seeking Proximity and Contact
An analysis of variance yielded a significant episode effect for seeking proximity and/or contact to the mother [F (3,306) = 26.87, p < .0001]. Figure 4 shows that efforts to gain proximity to the mother were weakest in Episode 2 and increased only slightly after the stranger appeared in Episode 3. Hence, although the stranger’s entrance slowed down exploratory behavior, it did not tip the balance between exploratory and attachment behavior strongly toward the latter. Separation, however, does so, as may be seen in the sharp increase in proximity and contact seeking in the first reunion Episode 5, and in the even more marked increase in the second reunion Episode 8.
FIGURE 4 Mean strength of proximity/contact-seeking behavior in each relevant episode.
An analysis of variance also yielded a significant episode effect for seeking proximity and/or contact with the stranger [F (2,204) = 3.8, p < .025]. Figure 4 shows that such behavior toward the stranger is especially weak in Episode 3, but increases somewhat in the separation Episodes, 4 and 7. Figure 4 shows clearly, however, that infants much more strongly seek proximity and contact with the mother than with a stranger.
Maintaining Contact
FIGURE 5 Mean strength of contact-maintaining behavior in each relevant episode.
An analysis of variance shows a significant episode effect for contact-maintaining behavior directed toward the mother [F (3,306) = 51.02, p < .0001]. As in the case of the proximity seeking, contact maintaining was negligible in Episodes 2 and 3, but was intensified by separation experiences, increasing somewhat in the first reunion Episode 5, and then sharply increasing in the second reunion Episode 8. (See Figure 5.)
It may also be seen from Figure 5 that efforts to maintain contact with the stranger were negligible. Such efforts were virtually absent in Episode 3, and occurred in few cases in the separation Episodes 4 and 7. The mean for contact maintaining to the stranger was 1.01 in Episode 3 and 1.88 in Episode 7. Although absolute interepisode changes are small, an episode effect [F (2,204) = 18.15, p < .0001], is still highly significant.
Distance Interaction
An analysis of variance shows a significant episode effect for distance interaction with mother [F (3,306) = 17.07, p < .0001]. It may be seen from Figure 6 that social interaction with the mother across a distance was highest in Episode 2; proximity-seeking and contact-maintaining behaviors were, in the same episode, lowest. In this episode, most babies are chiefly interested in toys, and they interact with their mothers mainly through signals across a distance, such as smiling and vocalizing, and occasionally by showing or pointing to a toy. Such interaction with the mother sharply decreases in Episode 3, when the stranger also is present. It will be recalled from earlier analyses that the entrance of the stranger increases proximity and contact seeking, but the major reason for the decline of distance interaction is that the baby’s visual attention is shifted from the mother to the stranger. Although distance interaction increases slightly again in the first reunion Episode 5, it is clear that in both reunion episodes most infants seek proximity to or contact with their mothers rather than interact with them across a distance. Thus the behaviors included in the distance-interaction measure—chiefly smiling and vocalizing—seem to be low-intensity attachment behaviors; that is, they are most conspicuous when the attachment behavioral system is activated at relatively low intensity.
FIGURE 6 Mean strength of distance interaction in each relevant episode.
An analysis of variance also shows a significant episode effect for distance interaction with the stranger [F (2,204) = 24.50, p < .0001]. It may be seen in Figure 6 that the baby interacts at a distance with the stranger most in Episode 3, when his mother is also present, and more with the stranger than with his mother in that episode (t = 8.79, p < .0001). Indeed, he tends to interact across a distance more with the stranger in Episode 3 than he did with his mother when he was alone with her in Episode 2 (t = 2.32; p < .02). Nevertheless the baby’s concern about his mother’s accessibility is of greatest importance to him. Distress occasioned by the mother’s departure overrides his interest in the stranger in the separation episodes, especially Episode 7.
In addition to the composite measure of distance interaction just described, a separate examination was made of the incidences of smiling, vocalizing, and looking.
Smiling, Vocalizing, and Looking
Smiling
Figure 7 shows the mean number of smiles emitted by the sample, as well as the number clearly directed toward the mother and the stranger. An analysis of variance shows a significant episode effect for total smiling, F (6,612) = 13.22, p < .0001. First it is evident that smiling was not a conspicuous behavior among these 1-year-olds in an unfamiliar situation. Even in Episode 2, the average baby smiled at his mother but once. Smiling was most frequent in Episode 3, when the babies smiled at both mother and stranger, but they smiled more at the stranger (X̄s = 1.29; X̄m = .65; t = 3.96; p < .0001). Separation from the mother in Episode 4 did not significantly lower the total number of smiles, nor those directed to the stranger. Smiling dropped quite precipitously, however, when the baby was alone in Episode 6.
Overall, babies smiled more at the stranger than at the mother in this unfamiliar situation, and they clearly smiled more when the stranger was present than when left alone. Bretherton and Ainsworth (1974) suggested that by 12 months of age, smiling, though still an attachment behavior, has also become a sociable behavior, directed toward “non-attachment” figures, including strangers. They also found that an infant who smiles at a stranger is also very likely to manifest signs of wariness. Therefore it seems likely that a smile to the stranger is not intended to invite her to come into proximity, but rather expresses friendliness as long as she stays at a distance. Even smiling at the mother may be a mode of interaction across a distance when a baby does not especially seek to be closer to her. Thus, at age 1, smiling seems to be an ambiguous signal. It by no means always signifies that the baby desires to draw the recipient into closer proximity, and therefore the frequency of smiling is an undependable criterion of attachment.
FIGURE 7 Mean frequency of smiling in each episode.
Vocalizing
One of the four significant intersample differences occurred in the case of total vocalizations, including both those judged to be directed toward persons and those that were not [F (3,102) = 2.72; p < .05]. Sample 4 differed from the other three in that more vocalizations were reported for the others. We had noted that Mary Main, who was one of two observers for all Sample-4 babies, was more adept than any other observer in receiving auditory input from the experimental room, simultaneously maintaining a continuous dictated record of what she observed. (For most observers the activity of dictation blocked auditory perception to a much greater extent than it did visual input.)
Nevertheless there was a significant episode effect for the sample as a whole for total vocalization [F (6,612) = 12.71; p < .0001]. As may be seen in Figure 8, vocalizing is most frequent in Episode 2, but decreases sharply in Episode 3, when the stranger is present (X̄2 = 4.14; X̄3 = 2.51; t = 3.39; p < .001). There is a further slight decrease in the first separation Episode 4, but it is in the second separation—including Episode 7, when the stranger is present, as well as Episode 6, when no adult is present—that vocalizing is least frequent. In contrast, vocalization increases in each of the reunion episodes, 5 and 8, although in neither does it regain its initial frequency of Episode 2. It was when these babies were alone with their mothers that they vocalized most.
A baby was considered to have vocalized to a person if he simultaneously looked at her and vocalized. Figure 8 shows that more vocalizations were directed toward the mother than toward the stranger, except in Episode 3. The mean number of vocalizations directed to the mother in all relevant episodes was .69, while the mean number directed to the stranger was .46 (t = 3.14; p < .002).
FIGURE 8 Mean frequency of vocalizing in each episode.
These findings are in striking contrast to those for smiling. It appears that infants in this unfamiliar situation tend to vocalize more frequently than to smile. For example, in Episode 2 the average baby vocalized 4.14 times, but smiled only 1.95 times (t = 3.1; p < .002). Whereas nearly all smiles were directed toward a person, relatively few vocalizations were so directed. Whereas the baby smiled somewhat more frequently at the stranger than at the mother, he vocalized significantly more frequently to the mother than to the stranger. Whereas smiles were most frequent in Episode 3, when both stranger and mother were present, vocalizations were most frequent in those three episodes in which the baby was alone with his mother. Indeed, because vocalizing to the mother was least frequent in Episode 3, and because total vocalizations were less frequent than they had been in Episode 2, it appears that the stranger’s presence tends to inhibit vocalization. This inhibiting effect of the stranger is further reflected in a comparison of latency to vocalize between Episodes 2 and 3. For Episode 2, the latency is 78 seconds, but for Episode 3 it rises to 103 seconds (t = 3.61; p < .001).
Looking
Figure 9 Mean frequency of looking in each episode.
Figure 9 shows the frequency of looking, per 15-second interval, at the mother and at the stranger. The figure clearly shows that babies tend to look more at the stranger than at the mother. Overall the mean frequency per episode of looking at the mother is 5.13, but the comparable mean for looking at the stranger is 6.76 (t = 8.31; p < .0001). That the baby’s attention is captured by the stranger is shown further by an increase in latency to look at the mother, from 20.33 seconds at the beginning of Episode 2 to 43.64 seconds at the beginning of Episode 3. In short, looking serves many purposes—exploration, wariness, sociability, attachment, and so on. Because there is no evidence that looking is differential to an attachment figure at this age, it appears to be an unsatisfactory criterion of attachment. Bowlby (1969) identified looking as an orientation behavior. Ainsworth (1964, 1967) suggested that visual orientation may be differential to an attachment figure under one specific set of conditions: When held by an unfamiliar figure, Ganda babies tended to gaze persistently at the mother across a distance, while their muscular tension also suggested an intense kind of orientation. On other occasions, the baby tended only to glance at the mother occasionally, as though this were all that was necessary to keep track of her whereabouts.
In the strange situation no infant was held by the stranger, except momentarily, after the mother entered the room; consequently there was no opportunity to observe the tense visual-motor orientation noted in Ganda infants when held by strangers. On the other hand, in the strange situation, as in the Ganda homes, it appeared that infants were quite capable of keeping visual tabs on their mothers’ whereabouts through the occasional glance. During Episodes 2 and 3, when the mother was stationary, the average infant seemed to take his mother’s accessibility for granted, glancing at her infrequently. On the other hand (as reported earlier), when the mother got up to leave, most babies looked immediately, even at the end of Episode 3 when the mother attempted to leave unobtrusively and the stranger was doing her best to distract the baby into play with the toys. Furthermore, most babies looked immediately at their mothers when they returned in the reunion Episodes 5 and 8; and under these circumstances, failure to look—or a marked delay in doing so—has been characterized as gaze aversion, a form of avoidant behavior.
On the other hand, a 1-year-old has a strong tendency to monitor the location and behavior of an unfamiliar person. Bretherton and Ainsworth (1974) presented evidence that linked prolonged gazing at the stranger with wariness. In any event, it is likely that a baby feels less confident in his expectations about a stranger’s movements and intents than he does of his mother’s. All in all, frequency of looking at the mother in the strange situation is so greatly influenced by a number of factors that it is very difficult to interpret without a meticulous situational analysis, such as Bretherton and Ainsworth conducted for Episode 3.
There was significant intersample variability in the amount of looking at the mother [F (3,102) = 3.30, p < .05]. Samples 3 and 4, more than Samples 1 and 2, looked at the mother in two or three 15-second intervals. This finding has no clear explanation. Intersample differences in looking at the stranger—and in visual exploration—were not significant, and therefore the intersample difference in looking at the mother does not seem attributable to greater alertness to looking behavior by one set of observers than by the other set.
Resistant Behavior
Analyses of variance indicate significant episode effects for resistant behavior, as directed to both the mother and to the stranger [to mother, F (3,306) = 24.16, p < .0001; to stranger, F (2,204) = 17.09, p < .0001].
Table 5 shows the frequency of various degrees of strength of resistant behavior directed toward the mother and toward the stranger. Unlike other behaviors discussed so far, resistant behavior is shown by a minority of subjects. Therefore, mean scores provide an inadequate impression of changes that take place across episodes.
Because the mothers were instructed not to intervene in Episodes 2 and 3, it is not surprising that extremely few infants were resistant in these episodes. The incidence of resistant behavior increases substantially in the first reunion Episode 5, and still more in the second reunion Episode 8. Separation activates angry resistance to the mother in some cases, but in only a few (6%) is this behavior strong. Nevertheless, 12% in Episode 5 and 22% in Episode 8 showed moderate to strong resistance. As reported earlier, there is also an increase in the strength of seeking to gain and to maintain contact in the reunion episodes, especially in Episode 8. Those children who resist contact and interaction also show moderate to strong proximity-seeking and contact-maintaining behavior. This combination of resistance and of seeking to gain and maintain contact cannot be interpreted from the scores alone. In some cases the baby’s behavior suggests classic ambivalence: The baby seeks to be picked up, yet resists being held, and furthermore may resist being put down. In other cases the baby seems angry because his mother does not pick him up, and he manifests resistance to her efforts to interest him in play by batting away the toys she offers or perhaps by having a full-blown temper tantrum.
TABLE 5 Percentage of Infants Who Exhibited Resistant Behavior to the Mother or to the Stranger
Slightly more infants show resistance to the stranger in Episode 3 than showed resistance to the mother in the preseparation episodes. This is scarcely surprising, for the stranger was instructed to approach the baby and to attempt to engage him in play; in 12% of the cases her overtures met with mild to moderate resistance. The percentage who showed resistant behaviors to the stranger increased somewhat in Episode 4, and more sharply in Episode 7. Even in Episode 7, however, only 42% of the sample showed any degree of resistance to the stranger. In most cases those who resisted the stranger were distressed by the mother’s absence, and when the stranger attempted to distract them with toys they pushed or threw them away; or if she attempted to pick them up, they tended to push away from her or squirm to get down. Some of those who resisted also sought to gain or maintain some degree of contact with the stranger, and thus this behavior, like similar behavior directed to the mother, suggests an ambivalent reaction. In such cases it seems likely that resistance to the stranger can be interpreted as a redirection of the anger occasioned by the mother’s departure. On the other hand, there are undoubtedly some infants, especially those who had taken no initiative in seeking contact with the stranger, whose resistance to the latter may be linked to fear or wary behavior.
Although there was no intersample difference in amount of resistance to the mother, there was a significant difference [F (3,102) = 3.45, p < .05] in resistance to the stranger. This difference was attributable largely to the fact that Sample 1 showed more resistance. It seems likely that this was due to the behavior of Sample 1’s chief stranger, but it is unclear what aspect of her behavior might have evoked more resistance.
Avoidant Behavior
TABLE 6 Percentage of Infants Who Exhibited Avoidant Behavior to the Mother or to the Stranger
Like resistant behavior, avoidant behavior is entirely absent in a substantial number of infants. Table 6 shows the percentages of infants who showed various intensities of avoidance to the mother or of the stranger in the relevant episodes. (Avoidance of the mother was not scored in Episode 2 or 3 because her noninterventive role obviated any instigation to avoidant behavior.) Although 51% show some degree of avoidance in the first reunion Episode 5, only 11% show very strong avoidance. Reunion Episode 8 yielded much the same percentages. Earlier we (Ainsworth & Bell, 1970) interpreted avoidance of the mother in reunion episodes as a defensive behavior. We are now inclined to believe that it is more useful to consider it as a conflict behavior when exhibited in a context in which most children seek proximity to or contact with their mothers. This is an approach—avoidance conflict, in which attachment behavior has been activated, presumably by the separation episodes, but in which avoidance behavior is also activated. In some cases approach and avoidance behavior alternate, but in those who score highest in avoidance we have reason to believe that the baby’s previous experience in close bodily contact with his mother has been disappointing or aversive. This issue is discussed more fully in later chapters.
Avoidance of the stranger seems likely to stem from fear or wariness, without necessarily implying conflict. This is most frequent in Episode 3, when it was shown by 45% of the sample, although by only 5% to a strong degree. Relatively weak avoidance was scored for infants who merely looked away from the stranger, whereas strong avoidance was scored for those who moved away from her, usually to the mother. In the light of Bretherton and Ainsworth’s (1974) analysis, it is by no means evident that all of those who retreated from the stranger to the mother in Episode 3 could be identified as showing strong fear, for nearly all of those few who approached the stranger went directly to the mother.
Oral Behavior
Because infants can comfort themselves by sucking their thumbs or fingers, it seemed possible that they might do so in lieu of crying, perhaps especially in the separation episodes. This did not prove to be the case, however. There was a very high degree of variability in oral behavior, and in each episode in each of the four samples, the standard deviation exceeded the mean. It may also be seen from Appendix VI, Table 30, that there is little change across episodes in the frequency of infants showing oral behavior.