Current Directions in Psychological Science 2014, Vol. 23(1) 60 –66 © The Author(s) 2014 Reprints and permissions: sagepub.com/journalsPermissions.nav DOI: 10.1177/0963721413498895 cdps.sagepub.com

Are human infants afraid of heights? At first blush, the answer appears to be a resounding “yes.” More than half a century ago, E. J. Gibson and Walk (1960) reported that crawling infants readily cross a visible surface of support but avoid crawling over an apparent, meter-high drop- off. To ensure infants’ safety, researchers tested babies on a glass-covered precipice, dubbed a “visual cliff” because the drop-off was only illusory (see Figure 1a), rather than a real cliff from which foolhardy infants could fall. The visual cliff is a classic paradigm in developmental psy- chology; the image of an infant peering into a checker- board-patterned abyss is among the most famous icons in developmental science. Furthermore, the basic find- ings are well known to the thousands of students who have sat through introductory courses in developmental psychology, experimental psychology, or perception. Perhaps because the paradigm has such common-sense appeal and apparent face validity (everyone can under- stand the importance of avoiding locomotion over a large drop-off, and most of us have experienced some sort of fear of heights), avoidance and fear are commonly conflated.

Subsequent researchers have expanded on E. J. Gibson and Walk’s (1960) original findings and have introduced a few caveats regarding the role of locomotor experience (Adolph & Kretch, 2012). For example, human infants (and altricial animals, such as kittens) require several weeks of self-produced locomotor experience before they avoid the deep side of the visual cliff (Bertenthal, Campos, & Barrett, 1984; Held & Hein, 1963). Likewise,

on a real cliff, a large gap in the surface of support, or an impossibly steep slope (see Figures 1b–1d), infants plunge right over the edge unless they have many weeks of locomotor experience (Adolph, 1997, 2000; Adolph, Berger, & Leo, 2011; Kretch & Adolph, 2013a). These apparatuses have no safety glass; experimenters catch infants if they begin to fall. Moreover, the apparatuses are continuously adjustable so that researchers can precisely assess the correspondence between infants’ attempts and their actual abilities. Over weeks of crawling and walk- ing, infants become increasingly accurate, attempting drop-offs, gaps, and slopes within their abilities and avoiding those beyond their abilities.

What then is the role of locomotor experience in facili- tating adaptive avoidance responses? The best known hypothesis is that self-produced locomotion leads to fear of heights, and fear leads to avoidance (Bertenthal et al., 1984; Campos et al., 2000; Campos, Hiatt, Ramsay, Henderson, & Svejda, 1978). In support of this account, crawling infants show accelerated heart rate—a standard index of fear—when placed on the deep side of the visual cliff, but prelocomotor infants do not (Campos, Bertenthal, & Kermoian, 1992; Campos et al., 1978). Likewise, kittens, goats, and other animals show stereo- typed fear responses—such as freezing and backing up

498895 CDP10.1177/0963721413498895Adolph et al.Fear of Heights in Infants? research-article2014

Corresponding Author: Karen E. Adolph, New York University, 6 Washington Place, Room 415, New York, NY 10003 E-mail: karen.adolph@nyu.edu

Fear of Heights in Infants?

Karen E. Adolph1, Kari S. Kretch1, and Vanessa LoBue2 1New York University and 2Rutgers University

Abstract Based largely on the famous “visual cliff” paradigm, conventional wisdom is that crawling infants avoid crossing the brink of a dangerous drop-off because they are afraid of heights. However, recent research suggests that the conventional wisdom is wrong. Avoidance and fear are conflated, and there is no compelling evidence to support fear of heights in human infants. Infants avoid crawling or walking over an impossibly high drop-off because they perceive affordances for locomotion—the relations between their own bodies and skills and the relevant properties of the environment that make an action such as descent possible or impossible.

Keywords emotion, locomotion, affordances

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Fear of Heights in Infants? 61

Fig. 1. Apparatuses used to test infants’ reactions to heights. (a) Visual cliff. The surface on the deep side is 102 centi- meters below the centerboard, and the surface on the shallow side is 3 centimeters below the centerboard. (b) Actual cliff used in Kretch and Adolph’s (2013a) study. Height adjusts from 0 to 90 centimeters in 1-centimeter increments. (c) Adjustable gap used in Adolph’s (2000) study. Gap width adjusts from 0 to 90 centimeters in 2-centimeter increments. (d) Adjustable slope used in Adolph’s (1997) study. Steepness adjusts from 0 to 90 degrees in 4-degree increments. (e–f) Bridge apparatus from Kretch and Adolph’s (2013b) study shown in the large (71-centimeter) and small (17-centimeter) drop-off conditions. Bridge width adjusts from 2 to 60 centimeters in 2-centimeter increments.

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with stiff forelegs—when they are placed directly onto the glass or pushed over the edge onto the deep side. Although the placing paradigm is more like being thrown off a cliff than exploring the view from the edge, the common interpretation is that fear of heights mediates avoidance.

Evidence That Infants Are Not Afraid of Heights

Despite half a century of undergraduates learning that infants avoid a drop-off because they are afraid of heights, several sources of evidence suggest that this idea is not accurate. First, researchers have no corroborating evi- dence of fear. The evidence for fear of heights is the avoidance response itself. The argument is circular and goes something like this: Infants avoid because they are fearful; we know they are fearful because they avoid. Physiological measures, such as heart rate, do not pro- vide independent corroboration that fear mediates avoid- ance. Two weeks of crawling experience was sufficient to elicit accelerated heart rate when infants were placed on the deep side of the visual cliff but not to elicit avoid- ance when infants were allowed to cross (Campos et al., 1992). The same infants with pounding hearts in the plac- ing paradigm crawled straight over the glass in the cross- ing paradigm (Ueno, Uchiyama, Campos, Dahl, & Anderson, 2011). Therefore, accelerated heart rate may reflect arousal, not fear.

Similarly, although one might imagine that infants dis- play other indices of fear when they avoid a drop-off, they do not. Infants who avoid crawling over the deep side of the visual cliff ‘‘do not show prototypic fear expressions. Indeed, they often smile!’’ (Saarni, Campos, Camras, & Witherington, 2006, p. 231; also see Sorce, Emde, Campos, & Klinnert, 1985). Infants appear to enjoy the problem of deciding how to cope with obstacles. Likewise, on steep slopes, infants’ facial expressions and vocalizations are primarily positive or neutral, not nega- tive, regardless of whether they go over the edge or avoid (Adolph, Tamis-LeMonda, Ishak, Karasik, & Lobo, 2008; Tamis-LeMonda et al., 2008). Furthermore, there is no increase in negative expressions, such as crying or cling- ing, with infants’ age or locomotor experience. Thus, some researchers have argued that the only valid index of fear on the visual cliff is refusal to crawl or walk over the brink because facial expressions and heart rate are not reliable indicators (Saarni et al., 2006).

A second line of evidence against the notion that fear mediates avoidance is infants’ proximity to the edge. Although readers might assume that “avoidance” means that infants stay away from the drop-off, it does not. In every paradigm—the visual cliff, real cliffs, gaps, slopes, and bridges—infants spend most of each trial right at the

edge of the drop-off, exploring possibilities for locomo- tion by stretching an arm toward the bottom of the preci- pice or by rocking back and forth at the brink (Adolph, 1997, 2000; Kretch & Adolph, 2013a, 2013b; Ueno et al., 2011; Walk, 1966; Walk & Gibson, 1961; Witherington, Campos, Anderson, Lejeune, & Seah, 2005). In fact, on the visual cliff, infants do not get scored as crossing until they have placed all four limbs onto the safety glass (Witherington et al., 2005). On a real cliff or gap, they would have fallen. Moreover, in every paradigm, infants who avoid crawling or walking over the drop-off are likely to attempt descent using alternative methods of locomotion, such as scuttling along the side walls of the visual cliff as well as backing or sliding down real cliffs and slopes (Adolph, 1997; Campos et al., 1978; Kretch & Adolph, 2013a; Witherington et al., 2005). Thus, “avoid- ant” infants refuse to attempt crossing in their typical mode of locomotion, but they do not avoid proximity to the drop-off and do not typically avoid descent if alterna- tives are available. Their behaviors at the brink provide evidence of adaptive, flexible responding, not fear of heights.

A third line of evidence against fear as the critical mediator is that infants show no evidence that they understand the different consequences of falling from different drop-off heights (Kretch & Adolph, 2013b). Adults are more leery of falling from a larger, more dan- gerous height than a smaller one, but infants are not. Both crawling and walking infants carefully scale attempts to cross bridges (see Figures 1e–1f) to the width of the bridge, indicating that they accurately perceive the prob- ability of falling. However, attempts to cross, gait modifi- cations, and exploratory behavior are identical on bridges spanning a large 71-centimeter drop-off (infants’ standing height) and a small 17-centimeter drop-off (infants’ knee height), meaning that infants do not consider the severity of a potential fall when deciding whether to cross, and they are not more reticent to cross a large drop-off than a small one.

A fourth source of evidence to argue against fear of heights is that infants’ actions at the edge of a precipice depend on the constraints of the test situation. In several studies, walking infants treated the same degree of slope differently, depending on whether they were wearing a lead- or feather-weighted vest (Adolph & Avolio, 2000) or rubber- versus Teflon-soled shoes (Adolph, Karasik, & Tamis-LeMonda, 2010). They correctly attempted to walk down steep slopes (with a larger drop-off) while wearing the feather-weighted vest or the rubber-soled shoes but refused to walk down shallow slopes (with a smaller drop-off) in the lead-weighted vest or slippery-soled shoes, where their abilities were diminished. Infants accurately reassessed the situation when constraints changed from trial to trial. If fear were mediating infants’

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Fear of Heights in Infants? 63

responses, they should have treated the drop-offs simi- larly across conditions.

A final argument against fear of heights is that adap- tive responding at the edge of a drop-off depends on whether infants face the obstacle in a newly acquired posture or an old, familiar posture. In one study, infants in an experienced sitting posture refused to span deep gaps that were slightly too large relative to their balance abilities; however, in a novice crawling posture, the same infants repeatedly plunged head first into the precipice (Adolph, 2000). Similarly, novice walkers stepped right into impossibly wide gaps (Adolph et al., 2011). In an experienced crawling posture, infants refused to crawl down impossibly steep slopes or cliffs; however, in a novice walking posture, they walked over the edge (Adolph, 1997; Adolph et al., 2008; Kretch & Adolph, 2013a). Specificity of learning between earlier and later developing postures was so robust that infants alternated between avoiding and plunging on consecutive trials when the experimenter started them in an experienced or novice posture (Adolph, 1997). If locomotor experi- ence teaches infants fear of heights, adaptive avoidance responses should not depend on the posture in which infants are tested.

Why Do Babies Avoid a Drop-Off?

So why do infants avoid a drop-off? Furthermore, what do infants learn from locomotor experience that facili- tates adaptive responding at the edge of a precipice? Answering these questions requires an apparatus more modifiable than the visual cliff. The standard visual cliff has only two drop-off heights, the shallow and the deep side, and the safety glass precludes assessment of infants’ actual abilities because both sides are safe for locomo- tion. In contrast, on a real cliff with continuously adjust- able drop-off height, researchers can assess infants’ attempts to cross relative to their actual ability to navigate the drop-off. For experienced crawling and walking infants, a small drop-off—13 centimeters—is simply a step: If infants attempt to crawl or walk over the edge, they will succeed. However, for novice walkers, the same 13-centimeter drop-off is essentially a cliff: If they attempt to walk, they will fall (Kretch & Adolph, 2013a). The dis- tinction between a stepping-off place and a falling-off place depends on the fit between infants’ physical capa- bilities and the relevant environmental properties—what J. J. Gibson (1979) termed an affordance. On this account, infants decide whether to cross or avoid a drop-off by detecting information for affordances (e.g., limb length, muscle strength, and balance control relative to drop-off height). What infants learn from locomotor experience is how to perceive affordances for locomotion (Adolph, 1997; Adolph & Robinson, 2013; Kretch & Adolph, 2013a).

In her later writings, E. J. Gibson (E. J. Gibson, 1991; E. J. Gibson et al., 1987; E. J. Gibson & Schmuckler, 1989) reinterpreted findings from the visual cliff in terms of perception of affordances (Adolph & Kretch, 2012). E. J. Gibson (1982) did not equate avoidance with fear, and she did not believe that fear necessarily accompanied perception of affordances:

[Affordances] are not the attachment to a perception of feelings of pleasantness or unpleasantness. They are information for behavior that is of some potential utility to the animal . . . I doubt that a mountain goat peering over a steep crag is afraid or charged with any kind of emotion; he simply does not step off. (p. 65)

Although the evidence does not support an account based on fear, it is consistent with an affordance account. Infants explore a precipice to generate information for affordances. Exploration increased on more challenging cliffs, gaps, slopes, and bridges as attempts to crawl and walk decreased (Adolph, 1997, 2000; Kretch & Adolph, 2013a, 2013b). Indeed, on the visual cliff, infants patted the glass with their hands, leaned their weight on the glass, and laid their face on it (Ueno et al., 2011; Walk, 1966; Walk & Gibson, 1961; Witherington et al., 2005). After one trial, they figured out the illusion, and crossed the deep side (Campos et al., 1978; Walk, 1966). When experienced crawlers and walkers were tested on modifi- able apparatuses with no safety glass, they rarely erred. Attempts to crawl or walk were matched to their abili- ties—they only attempted risky obstacles that were within 1 to 3 centimeters of their actual ability on real cliffs, gaps, and bridges and that were within 2 to 6 degrees of slant on slopes (Adolph, 1997, 2000; Adolph et al., 2008; Kretch & Adolph, 2013a, 2013b). Moreover, experienced infants retained impressive accuracy when their abilities were altered with weighted shoulder packs or slippery shoes (Adolph & Avolio, 2000; Adolph et al., 2010). Such finely attuned perception of affordances requires many weeks of locomotor experience because infants must identify the relevant parameters for their new action sys- tems and learn to calibrate the settings of those parame- ters under varying conditions. Infants fail to show transfer from earlier to later developing postures because affor- dances and the information to specify the relevant rela- tions are different for sitting, crawling, cruising, and walking (Adolph & Robinson, 2013).

Unanswered Questions

What would constitute convincing evidence that infants are afraid of heights? Although infant fear is a controver- sial topic because of considerable individual variation

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(Saarni et al., 2006), infant distress is not. Unfortunately for parents, infants are quite adept at displaying negative affect. In the laboratory, infants in the same age range as those who avoid drop-offs are also capable of negative affect. When approached by a stranger, for example, some 8- to 22-month-old infants produce negative affect in facial expressions and vocalizations; they may also withdraw from the stimulus, cling to their mothers, show accelerated heart rate, and stiffen their bodies (e.g., Sroufe, 1977; Waters, Matas, & Sroufe, 1975). Thus, in addition to accelerated heart rate, evidence for fear of heights should include converging evidence, such as negative affect and withdrawal from the edge of the drop-off. However, infants do the opposite: Their facial expressions and vocalizations are positive or neutral, and they spend most of their time exploring at the edge of the drop-off and finding alternative means of crossing.

So when does fear of heights develop, and how is it acquired? To our knowledge, there are no longitudinal data to address this question. Previous work suggests that acquiring fear of heights does not necessitate direct condi- tioning experiences, such as a traumatic fall (Menzies & Clarke, 1993; Poulton, Davies, Menzies, Langley, & Silva, 1998). Indirect pathways may include observational learning or transmission of negative verbal information (Rachman, 1977). Individual differences in temperament or trait anxiety might also play a role by making some children more likely to acquire fear of heights than others (Mineka & Zinbarg, 2006).

A final question for future research is whether infants may show heightened sensitivity to other stimuli, without displaying evidence of fear. Fear of snakes and spiders, for example, is common in adults, but recent research indicates that infants and young children do not show fear of snakes or spiders (LoBue, 2013). Instead, they display positive or neutral affect, and they approach, rather than withdraw, from photographs, videos, and live snakes and spiders. However, infants and young children do show heightened visual sensitivity to snakes and spi- ders, akin to their heightened attention to drop-offs. Possibly, early perceptual sensitivity to stimuli, such as heights and snakes, facilitates fear learning later in devel- opment (LoBue, 2013).

Conclusions: Who Is Afraid of Heights?

Although infants, children, and adults can perceive affor- dances for locomotion, fear of heights is not universal at any age. Some sensation seekers enjoy the thrill of heights and seek out activities such as mountain climbing, para- sailing, or skydiving (Salassa & Zapala, 2009; Zuckerman, 1983). Others have more trepidations: About 30% of adults report nonclinical height fear, and another 5%

have full-blown height phobia (Agras, Sylvester, & Oliveau, 1969; Depla, ten Have, van Balkom, & de Graaf, 2008; Huppert, Grill, & Brandt, 2013). Despite substantial variability in adults, fear of heights in infants is described as a universal development, akin to language acquisition or learning to walk. Indeed, some researchers claim that fear of heights is innate (Menzies & Clarke, 1993; Poulton et al., 1998). From an evolutionary standpoint, avoidance of falling-off places is undoubtedly adaptive, making accounts of innate or early developing height fear popu- lar and satisfying. However, the processes mediating avoidance of a cliff in infancy appear unrelated to the development of fear of heights. Young infants respond flexibly to novel challenges by detecting the fit between the environment and their own abilities. They actively explore, assess, and generate creative alternatives that suit the constraints of the current situation. Far more adaptive than an automatic fear response is the ability to perceive affordances. Although the construct of fear is attractive, it is not necessary to describe the flexible and adaptive behavior of infants at the edge of a drop-off.

Recommended Reading

Adolph, K. E., & Kretch, K. S. (2012). (See References). Describes research on the visual cliff and research inspired by the visual cliff and places the original paradigm and subsequent studies into historical context.

Adolph, K. E., & Robinson, S. R. (2013). (See References). A description of the development of locomotion, from prena- tal movements to mature walking and beyond, relating the phenomena to general conceptual issues of learning and development.

Campos, J. J., Hiatt, D., Ramsay, D., Henderson, C., & Svejda, M. (1978). (See References). A series of studies that explores the relations between locomotor experience and responses to the visual cliff, including the placing and crossing para- digms.

Gibson, E. J., & Walk, R. D. (1960). (See References). The clas- sic article reporting the first comparative experiments on the visual cliff, including several animal species and human infants.

Kretch, K. S., & Adolph, K. E. (2013a). (See References). A recent study using an actual, adjustable cliff that provides evidence against fear of heights in infancy.

Declaration of Conflicting Interests

The authors declared that they had no conflicts of interest with respect to their authorship or the publication of this article.

Funding

The project was supported by Eunice Kennedy Shriver National Institute of Child Health and Human Development Grant R37HD033486 to Karen E. Adolph. The content is solely the responsibility of the authors and does not necessarily represent the official views of the Eunice Kennedy Shriver National

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Fear of Heights in Infants? 65

Institute of Child Health and Human Development or the National Institutes of Health.

References

Adolph, K. E. (1997). Learning in the development of infant locomotion. Monographs of the Society for Research in Child Development, 62(3, Serial No. 251).

Adolph, K. E. (2000). Specificity of learning: Why infants fall over a veritable cliff. Psychological Science, 11, 290–295.

Adolph, K. E., & Avolio, A. M. (2000). Walking infants adapt locomotion to changing body dimensions. Journal of Experimental Psychology: Human Perception and Performance, 26, 1148–1166.

Adolph, K. E., Berger, S. E., & Leo, A. J. (2011). Developmental continuity? Crawling, cruising, and walking. Developmental Science, 14, 306–318.

Adolph, K. E., Karasik, L. B., & Tamis-LeMonda, C. S. (2010). Using social information to guide action: Infants’ locomo- tion over slippery slopes. Neural Networks, 23, 1033–1042.

Adolph, K. E., & Kretch, K. S. (2012). Infants on the edge: Beyond the visual cliff. In A. Slater & P. Quinn (Eds.), Developmental psychology: Revisiting the classic studies (pp. 36–55). London, England: SAGE.

Adolph, K. E., & Robinson, S. R. (2013). The road to walking: What learning to walk tells us about development. In P. Zelazo (Ed.), Oxford handbook of developmental psychol- ogy (pp. 403–443). New York, NY: Oxford University Press.

Adolph, K. E., Tamis-LeMonda, C. S., Ishak, S., Karasik, L. B., & Lobo, S. A. (2008). Locomotor experience and use of social information are posture specific. Developmental Psychology, 44, 1705–1714.

Agras, S., Sylvester, D., & Oliveau, D. (1969). The epidemiology of common fears and phobia. Comprehensive Psychiatry, 10, 151–156.

Bertenthal, B. I., Campos, J. J., & Barrett, K. C. (1984). Self- produced locomotion: An organizer of emotional, cogni- tive, and social development in infancy. In R. N. Emde & R. J. Harmon (Eds.), Continuities and discontinuities in development (pp. 175–210). New York, NY: Plenum Press.

Campos, J. J., Anderson, D. I., Barbu-Roth, M. A., Hubbard, E. M., Hertenstein, M. J., & Witherington, D. C. (2000). Travel broadens the mind. Infancy, 1, 149–219.

Campos, J. J., Bertenthal, B. I., & Kermoian, R. (1992). Early experience and emotional development: The emergence of wariness of heights. Psychological Science, 3, 61–64.

Campos, J. J., Hiatt, S., Ramsay, D., Henderson, C., & Svejda, M. (1978). The emergence of fear on the visual cliff. In M. Lewis & L. Rosenblum (Eds.), The development of affect (pp. 149–182). New York, NY: Plenum Press.

Depla, M. F. I. A., ten Have, M. L., van Balkom, A. J. L. M., & de Graaf, R. (2008). Specific fears and phobias in the general population: Results from the Netherlands Mental Health Survey and Incidence Study (NEMESIS). Social Psychiatry & Psychiatric Epidemiology, 43, 200–208.

Gibson, E. J. (1982). The concept of affordances in develop- ment: The renascence of functionalism. In W. A. Collins (Ed.), The concept of development: The Minnesota symposia

on child psychology (Vol. 15, pp. 55–81). Mahwah, NJ: Erlbaum.

Gibson, E. J. (1991). An odyssey in learning and perception. Cambridge, MA: MIT Press.

Gibson, E. J., Riccio, G., Schmuckler, M. A., Stoffregen, T. A., Rosenberg, D., & Taormina, J. (1987). Detection of the traversability of surfaces by crawling and walking infants. Journal of Experimental Psychology: Human Perception and Performance, 13, 533–544.

Gibson, E. J., & Schmuckler, M. A. (1989). Going somewhere: An ecological and experimental approach to development of mobility. Ecological Psychology, 1, 3–25.

Gibson, E. J., & Walk, R. D. (1960). The “visual cliff.” Scientific American, 202, 64–71.

Gibson, J. J. (1979). The ecological approach to visual percep- tion. Boston, MA: Houghton Mifflin.

Held, R., & Hein, A. (1963). Movement-produced stimulation in the development of visually guided behavior. Journal of Comparative and Physiological Psychology, 56, 872–876.

Huppert, D., Grill, E., & Brandt, T. (2013). Down on heights? One in three has visual height intolerance. Journal of Neurology, 260, 597–604.

Kretch, K. S., & Adolph, K. E. (2013a). Cliff or step? Posture- specific learning at the edge of a drop-off. Child Development, 84, 226–240.

Kretch, K. S., & Adolph, K. E. (2013b). No bridge too high: Infants decide whether to cross based on bridge width not drop-off height. Developmental Science, 16, 336–351.

LoBue, V. (2013). What are we so afraid of? How early atten- tion shapes our most common fears. Child Development Perspectives, 7, 38–42.

Menzies, R. G., & Clarke, J. C. (1993). The etiology of fear of heights and its relationship to severity and individual response patterns. Behaviour Research and Therapy, 31, 355–365.

Mineka, S., & Zinbarg, R. (2006). A contemporary learning theory perspective on anxiety disorders: It’s not what you thought it was. American Psychologist, 61, 10–269.

Poulton, R., Davies, S., Menzies, R. G., Langley, J. D., & Silva, P. A. (1998). Evidence for a non-associative model of the acquisition of a fear of heights. Behaviour Research and Therapy, 36, 537–544.

Rachman, S. J. (1977). The conditioning theory of fear acqui- sition: A critical examination. Behaviour Research and Therapy, 15, 375−387.

Saarni, C., Campos, J. J., Camras, L. A., & Witherington, D. (2006). Emotional development: Action, communication, and understanding. In N. Einsenberg (Ed.), Handbook of child psychology. Vol. 3: Social, emotional, and personality development (6th ed., pp. 226–299). New York, NY: John Wiley.

Salassa, J. R., & Zapala, D. A. (2009). Love and fear of heights: The pathophysiology and psychology of height imbalance. Wilderness & Environmental Medicine, 20, 378–382.

Sorce, J. F., Emde, R. N., Campos, J. J., & Klinnert, M. D. (1985). Maternal emotional signaling: Its effects on the visual cliff behavior of 1-year-olds. Developmental Psychology, 21, 195–200.

at ARKANSAS STATE UNIV on January 29, 2015cdp.sagepub.comDownloaded from

66 Adolph et al.

Sroufe, L. A. (1977). Wariness of strangers and the study of infant development. Child Development, 48, 731–746.

Tamis-LeMonda, C. S., Adolph, K. E., Lobo, S. A., Karasik, L. B., Dimitropoulou, K. A., & Ishak, S. (2008). When infants take mothers’ advice: 18-month-olds integrate perceptual and social information to guide motor action. Developmental Psychology, 44, 734–746. doi:10.1037/0012-1649.44.3.734

Ueno, M., Uchiyama, I., Campos, J. J., Dahl, A., & Anderson, D. I. (2011). The organization of wariness of heights in expe- rienced crawlers. Infancy, 17, 376–392.

Walk, R. D. (1966). The development of depth perception in animals and human infants. Monographs of the Society for Research in Child Development, 31(5, Serial No. 107).

Walk, R. D., & Gibson, E. J. (1961). A comparative and ana- lytical study of visual depth perception. Psychological Monographs, 75(15, Whole No. 519).

Waters, E., Matas, L., & Sroufe, L. A. (1975). Infants’ reactions to an approaching stranger: Description, validation, and functional significance of wariness. Child Development, 46, 348–356.

Witherington, D. C., Campos, J. J., Anderson, D. I., Lejeune, L., & Seah, E. (2005). Avoidance of heights on the visual cliff in newly walking infants. Infancy, 7, 285– 298.

Zuckerman, M. (1983). Sensation seeking and sports. Personality and Individual Differences, 4, 285–293.

at ARKANSAS STATE UNIV on January 29, 2015cdp.sagepub.comDownloaded fro