Early Experience and Emotional Development: The Emergence of Wariness of Heights

Because of its biological adaptive value, wariness of heights is widely believed to be innate or under maturational control. In this report, we present evidence contrary to this hypothesis, and show the importance of locomotor experience for emotional development. Four studies bearing on this conclusion have shown that (1) when age is held constant, locomotor experience accounts for wariness of heights; (2) "artificial" experience locomoting in a walker generates evidence of wariness of heights; (3) an orthopedic ally handicapped infant tested longitudinally did not show wariness of heights so long as he had no locomotor experience; and (4) regardless of the age when infants begin to crawl, it is the duration of locomotor experience and not age that predicts avoidance of heights. These findings suggest that when infants begin to crawl, experiences generated by locomotion make possible the development of wariness of heights. Between 6 and 10 months of age, major changes occur in fearfulness in the human infant. During this period, some fears are shown for the first time, and many others show a step-function increase in prevalence (Bridges, 1932; Scarr & Salapatek, 1970; Sroufe, 1979). These changes in fearfulness occur so abruptly, involve so many different elicitors, and have such biologically adaptive value that many investigators propose maturational explanations for this developmental shift (Emde, Gaensbauer, & Harmon, 1976; Kagan, Kearsley, & Zelazo, 1978). For such theorists, the development of neurophysiological structures (e.g., the frontal lobes) precedes and accounts for changes in affect. In contrast to predominantly maturational explanations of developmental changes, Gottlieb (1983, 1991) proposed a model in which different types of experiences play an important role in developmental shifts. He emphasized that new developmental acquisitions, such as crawling, generate experiences that, in turn, create the conditions for further developmental changes. Gottlieb called such 4 'bootstrapping" processes probabilistic epigenesis. In contrast to most current models of developmental transition, Gottlieb's approach stresses the possibility that, under some circumstances, psychological function may precede and account for development of neurophysiological structures. There is evidence in the animal literature that a probabilistic epigenetic process plays a role in the development of wariness of heights. Held and Hein (1963), for instance, showed that darkreared kittens given experience with active self-produced locomotion in an illuminated environment showed avoidance of heights, whereas dark-reared littermates given passive experience moving in the same environment manifested no such avoidance. In these studies, despite equivalent maturational states in the two groups of kittens, the experiences made possible by correlated visuomotor responses during active locomotion proved necessary to elicit wariness of heights. So long as they are prelocomotor, human infants, despite their visual competence and absence of visual deprivation, may be functionally equivalent to Held and Hein's passively moved kittens. Crawling may generate or refine skills sufficient for the onset of wariness of heights. These skills may include improved calibration of distances, heightened sensitivity to visually specified selfmotion, more consistent coordination of visual and vestibular stimulation, and increased awareness of emotional signals from significant others (Bertenthal & Campos, 1990; Campos, Hiatt, Ramsay, Henderson, & Svejda, 1978). There is anecdotal evidence supporting a link between locomotor experience and development of wariness of heights in human infants. Parents commonly report that there is a phase following the acquisition of locomotion when infants show no avoidance of heights, and will go over the edge of a bed or other precipice if the caretaker is not vigilant. Parents also report that this phase of apparent fearlessness is followed by one in which wariness of heights becomes quite intense (Campos et al., 1978). In sum, both the kitten research and the anecdotal human evidence suggest that wariness of heights is not simply a maturational phenomenon, to be expected even in the absence of experience. From the perspective of probabilistic epigenesis, locomotor experience may operate as an organizer of emotional development, serving either to induce wariness of heights (i.e., to produce a potent emotional state that would never emerge without such experience) or to facilitate its emergence (i.e., to bring it about earlier than it otherwise would appear). The research reported here represents an attempt to determine whether locomotor experience is indeed an organizer of the emergence of wariness of heights. Pinpointing the role of locomotion in the emergence of wariness of heights in human infants requires solution of a number of methodological problems. One is the selection of an ecologically valid paradigm for testing wariness of heights. Another is the determination of an outcome measure that can be used with both prelocomotor and locomotor infants. A third is a means of determining whether locomotion is playing a role as a correlate, an antecedent, an inducer, or a facilitator of the onset of wariness of heights. This research was supported by grants from the National Institutes of Health (HD16195, HD-00695, and HD-25066) and from the John D. and Catherine T. MacArthur