Stranger anxiety in infants tends to be influenced by which of the following?

In the first year of life, human development proceeds at a furious pace. Infants progress from squirming and mewing to toddling and shouting in mere months, passing through a flurry of motor, cognitive, social, and emotional milestones along the way. Two of these milestones tend to occur around the same age (7–8 months): independent locomotion (prototypically in the form of hands-and-knees crawling) and a wariness of strangers (often called stranger anxiety)1 . Given the sheer number of milestones infants achieve in this short time, the co-occurrence of two could be coincidental (see Bertenthal, Campos, & Barrett, 1984). However, it seems adaptive for locomotion and stranger anxiety to go hand-in-hand in individual children (Bowlby, 1969; Bretherton, 1992). For instance, perhaps stranger anxiety appears first and then hastens locomotor development in order to help infants regulate the distance from their parents. Equally plausible, locomotion might emerge and then trigger in infants a new awareness of the social environment and a coinciding wariness of strangers (Campos et al., 2000; Mahler, Pine, & Bergman, 1975; Thelen, 2000). The current paper examines whether these milestones systematically co-occur in individual infants, and whether one milestone reliably precedes the other in development.

The beginning of crawling is a much-celebrated milestone for parents. A majority of infants in the United States crawl on hands-and-knees prior to walking, with average age of onset around 8 months (Adolph et al., 2015; Adolph, Vereijken, & Denny, 1998; Capute, Shapiro, Palmer, Ross, & Wachtel, 1985; Gesell, 1928). In addition, other, less-efficient forms of locomotion often emerge before hands-and-knees crawling. For instance, Adolph et al. (1998) found that about half of infants in their sample used some form of “belly” crawl before developing the typical hands-and-knees gait. Other infants “may log roll from place to place, or pivot in circles” (Adolph & Franchak, 2017, p. 4). In other words, according to these authors, “individual infants find different ways to solve the problem of moving” (4).

A clear change in infants’ response to strangers is also typically apparent around the same time. Young infants generally accept proximity and contact by unknown adults. Around eight months, however, infants show a marked increase in fussy and avoidant behavior toward unfamiliar adults, referred to as stranger anxiety, stranger fear, or stranger wariness (Boyer & Bergstrom, 2011; Bronson, 1972; Freedman, 1961; Gaensbauer, Emde, & Campos, 1976; Spitz, 1965; Sroufe, 1977). Although “wariness” may be the most apt term, encompassing everything from staring to averted gaze to frank distress and crying (LoBue and Adolph, 2019; Sroufe, 1977; Waters, Matas, & Sroufe, 1975), we chose to use the classic term “stranger anxiety” as we felt it might be most familiar to parent participants.

Stranger anxiety may serve as an adaptive countermeasure to infants’ locomotion, helping infants maintain a balance between the urge to explore the environment and the need to feel safe and secure (Campos et al., 2000; Freedman, 1974/2016Freedman, /, 2016Freedman, 1974/2016; Mahler et al., 1975). As Bretherton (1985), p.8) put it, “a system that ensures that a child’s explorations do not take it too far from a protective figure can … plausibly be viewed as having survival value.” According to this thinking, the appearance of stranger anxiety and locomotion together seems likely to be functional, rather than coincidental. Perhaps stranger anxiety develops first as part of the unfolding attachment system and subsequently galvanizes infants to locomote in order to regulate their social environment. In support of this possibility, infants in the strange situation paradigm show attenuated fear reactions when they are able to approach the familiar caregiver (e.g. Ainsworth & Bell, 1970; Kotelchuck, Zelazo, Kagan, & Spelke, 1975).

On the other hand, perhaps infants’ desires to explore the environment encourage them to locomote, and subsequently this spurs the onset of stranger anxiety. This possibility seems especially plausible given the evidence that the onset of crawling predicts a huge variety of perceptual, cognitive, emotional, and social developments (e.g., Adolph, Kretch, & LoBue, 2014; Brand, Escobar, Baranès, & Albu, 2015; Campos, Kermoian, & Zumbahlen, 1992; Cicchino & Rakison, 2008; Higgins, Campos, & Kermoian, 1996; Kermoian & Campos, 1988; Uchiyama et al., 2008; see Campos et al., 2000, for a review). Campos et al. (2000) found that locomotor infants were more responsive to maternal separations and paid more attention to distal events than pre-locomotor infants. One explanation for these changes may be that experience locomoting encourages a broader attention to the physical and social environment (Campos et al., 2000; Thelen, 2000; Uchiyama et al., 2008).

If either locomotion or stranger anxiety is systematically prompting the development of the other, we would expect the causal factor to appear reliably first. In addition, if locomotion and stranger anxiety unfold according to either of the above proposals, we would expect the ages of milestone achievement to be correlated across a sample of children. If such a relation is specific to these two milestones, it should persist over and above relations with a third variable also appearing at this age, such as consonant-vowel babbling (Gillis, Schauwers, & Govaerts, 2002). To address these questions, we surveyed parents on the emergence of a number of milestones, including locomotion, stranger anxiety, and babbling2 .

In a large pilot study, we surveyed 304 parents of infants between 6 and 12 months and asked them to approximate the dates of onset of hands-and-knees crawling (specified as traveling two or more body lengths on hands and knees) and stranger anxiety (defined as refusing to go to an unfamiliar adult). Reporting was retrospective, but across a relatively short delay (usually 1–2 months). The pilot data suggested that the ages of the two milestones were correlated across infants. We also found that on average, stranger anxiety appeared before crawling, but a substantial minority of infants developed crawling before stranger anxiety.

To gather milestone data in a more precise and accurate way, in the current study we followed parents prospectively, starting prior to the achievement of the key milestones, and administered a weekly survey designed to identify milestones as they occurred. Given the previously-reported transformative effects of self-produced locomotion, we were somewhat surprised to find the pilot work suggesting a tendency for stranger anxiety to occur before crawling. We considered whether perhaps pre-crawling locomotor attempts might be enough to trigger a broader awareness of the social environment. By the time infants are skilled crawlers, they may have been moving in other ways for some time (Adolph et al., 1998). Thus, in the current study, in addition to hands-and-knees crawling, we also assess other forms of locomotion. To accomplish this, we asked parents about early locomotor forms such as belly or “commando” crawling, as well as rolling or scooting, provided they were seen as attempts to “get across the room” (see below).

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  What causes stranger anxiety in babies?

  About fear of strangers Fear of strangers is very common. It happens as your baby develops a healthy attachment to familiar people – like you. Because babies prefer familiar adults, they might react to strangers by crying or fussing, going very quiet, looking fearful or hiding.

  Which of the following situations is most likely to produce stranger anxiety in an infant?

  Terms in this set (30) Which of the following situations is most likely to produce stranger anxiety in an infant? Meeting a stranger in a research laboratory.

  When does a child develop stranger anxiety?

  Stranger anxiety is a perfectly normal developmental stage that often begins around 6 to 8 months. Stranger anxiety typically peaks between 12 and 15 months and then begins to gradually decrease as your baby continues to grow and develop.

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  Factors related to this variation include the infant's temperament, level of caregiver anxiety, and the developing infant's capacity for the regulation of emotional arousal as measured by physiological markers such as respiratory sinus arrhythmia suppression.