Salivary α-Amylase and Intended Harsh Caregiving in Response to Infant Crying

Abstract

This is the first study on adults’ physiological reactivity to infant cry sounds and the association with intended harsh parenting using salivary α-amylase (sAA) as a novel and noninvasive marker of autonomic nervous system activity. The sample consisted of 184 adult twin pairs. In an experimental design, cry sounds were presented and adults’ perception and their intended caregiving responses were measured. Saliva samples were collected after each cry sound. For the majority of the sample, a decrease in sAA across the cry paradigm was observed. However, adults who indicated that they would respond in a harsh way to the crying infant were significantly less likely to show a decrease in sAA. Consistent with previous studies on physiological hyperreactivity in abusive parents, these findings suggest that failure to habituate to repeated infant crying may be one of the mediating mechanisms through which excessive, inconsolable, and high-pitched infant crying triggers less optimal caregiving.

Keywords

infant crying salivary α-amylase child abuse perception harsh caregiving

Excessive, inconsolable, and high-pitched infant crying can be a major trigger of child abuse and neglect, and even infanticide (Frodi, 1985; Soltis, 2004). Especially shaken baby syndrome (SBS), the violent shaking of an infant, which may result in severe head trauma and fractures of the long bone or ribs, has been associated with the prolonged, inconsolable, and unpredictable cry episodes that often occur in the first months after birth (e.g., Barr, Trent, & Cross, 2006). Experimental studies suggest that abusive parents are unable to regulate the physiological stress elicited by infant crying, as they respond with excessive and prolonged autonomic arousal (McCanne & Hagstrom, 1996). However, several significant gaps remain in our knowledge on the nature of this hyperreactivity. In the current study, we examine the association between adults’ physiological reactivity to infant cry sounds and their intended caregiving responses, using salivary α-amylase (sAA) as a novel biomarker to assess activity of the autonomic nervous system (ANS).

Much of the evidence for infant crying as a trigger for child abuse comes from anecdotal reports, surveys, and reviews on SBS (e.g., Dykes, 1986; Levitzky & Cooper, 2000; Ludwig & Warman, 1984). For example, in an epidemiological study, 6% of Dutch parents of 6-month-old infants reported that they had smothered, slapped, or shaken their baby at least once in order to reduce its crying (Reijneveld, Van der Wal, Brugman, Hira Sing, & Verloove-Vanhorick, 2004). Another study reported that 89% of parents of infants had contacted their physicians because of excessive crying prior to the admission to the hospital with SBS (Talvik, Alexander, & Talvik, 2008). Three studies obtained more systematic support for the potential role of crying as a proximal stimulus to SBS by comparing the age-specific incidence of SBS to the normative crying curve. Normally, the average duration of crying increases at 2–3 weeks of age, peaks around 6 weeks, after which it declines to more stable levels by the 4th and 5th month (e.g., Hubbard & Van IJzendoorn, 1991; Hunziker & Barr, 1986; St. James-Roberts & Halil, 1991). The incidence of SBS follows a similar curve: it starts at 2–3 weeks of age and peaks around 8–12 weeks, followed by a decline to more stable levels (Barr et al., 2006; Lee, Barr, Catherine, & Wicks, 2007; Talvik et al., 2008).

Experimental studies in which parents listen and respond to audio- or videotaped samples of infant crying have provided more in-depth knowledge on the perceptual and emotional responses of abusive parents to infant distress signals. These studies have also pointed to acoustic features of cry sounds that are particularly aversive (such as a high pitch) and more likely to elicit less optimal caregiving behavior in vulnerable parents (Frodi, 1985; Out, Pieper, Bakermans-Kranenburg, Zeskind, & Van IJzendoorn, 2010; Zeskind, 2007; Zeskind & Lester, 1978). Compared to nonabusive parents, abusive parents reported feeling less empathic and attentive and more annoyed, distressed, hostile, and sad after listening to cry sounds (e.g., Crouch, Skowronski, Milner, & Harris, 2008; Frodi & Lamb, 1980; Milner, Halsey, & Fultz, 1995). These hostile and negative attributions may increase the parent’s readiness to respond in a harsh, irritable, and aggressive way to the infant, especially in the context of other sources of stress (Sturge-Apple, Skibo, Rogosch, Ignjatovic, & Heinzelman, 2011). Indeed, Crouch and colleagues (2008) showed that parents at risk for abusing their children were especially likely to use excessive force in a handgrip task after they were exposed to hostile cues (i.e., negative priming) and watched a video of a crying infant.

These experimental studies confirmed Knutson’s (1978) theory that abusive parents have a “hyperreactive trait”, which makes them overly responsive to infant distress and more likely to respond with “irritable” aggression. Interestingly, this hyperreactive response pattern has also been shown at a physiological level, suggesting that parents who engage in abusive or neglectful caregiving are unable to successfully regulate the stress elicited by infant crying (for a review, see McCanne & Hagstrom, 1996). Frodi and Lamb (1980) were the first to demonstrate that abusive parents respond with significantly higher peak heart rate (HR) and skin conductance (SC) to infant crying than nonabusive parents. These results have been replicated using a range of stressful child-related stimuli (Crowe & Zeskind, 1992; Friedrich, Tyler, & Clark, 1985; Pruitt & Erickson, 1985; Wolfe, Fairbank, Kelly, & Bradlyn, 1983). In addition, several studies have suggested that abusive parents and adults at risk for abuse display physiological hyperreactivity to pleasant child-related stimuli (e.g., Casanova, Domanic, McCanne, & Milner, 1994), to aversive non-child–related stimuli including a cold pressor task (Casanova, Domanic, McCanne, & Milner, 1992), and during resting conditions (Casanova et al., 1994).

However, there are striking inconsistencies both across and within studies (see also McCanne & Hagstrom, 1996). For example, Casanova and colleagues (1994) reported that adults with a history of abuse displayed a lack of habituation (SC) during a resting period and an increase in SC in response to a video of a smiling infant but not in response to a video of a crying infant, whereas adults without a history of child abuse displayed the opposite pattern of SC reactivity. Frodi and Lamb (1980) have provided strong evidence for the hyperreactivity hypothesis, but they also found lower diastolic blood pressure for abusive versus nonabusive parents during the crying infant videotape. Crowe and Zeskind (1992) found higher HR levels in adults at risk for abusing their children as compared to low-risk individuals, but only after the presentation of the cry stimuli (which included normal and high-pitched cry sounds).

Thus, although each study provided evidence for the physiological hyperreactivity hypothesis, they did so in varying ways: for some but not all physiological measures, for different physiological measures, in response to different infant stimuli, and during different parts of the experimental paradigm. Part of the inconsistency in findings can be explained by the methodological and procedural differences between studies, with infant stimuli varying in nature (crying vs. smiling; cry sounds varying in acoustic structure), duration (10 s to several min), and complexity (standardized cry sounds to videos of parent–infant interactions).

In addition, it is unclear what constitutes “excessive physiological reactivity,” as some level of arousal is necessary for prompt and sensitive caregiving in response to infant distress (Del Vecchio, Walter, & O'Leary, 2009; Joosen, Mesman, Bakermans-Kranenburg, Pieper, et al., 2012; Murray, 1985). In this context, Joosen, Mesman, Bakermans-Kranenburg, Pieper and colleagues (2012) argued that it is vital to distinguish between the parasympathetic and sympathetic components of the ANS. Increases in HR due to a reduction of vagal tone (an index of parasympathetic control of the heart) are considered to support adequate regulation of arousal as well as behavioral and emotional control (Porges, 1996, 2001, 2007; Porges, Doussard-Roosevelt, & Maiti, 1994). HR reactivity may however also originate from activation of the sympathetic nervous system (SNS), which occurs especially when the environment is perceived as hostile or threatening. Given previous associations with a negative attribution bias in abusive parents, a recent study by Joosen, Mesman, Bakermans-Kranenburg, Pieper and colleagues (2012) suggested that especially activity of the sympathetic branch of the ANS may be involved in abusive parents’ hyperreactivity to infant distress.

In the current study, we examine physiological reactivity in adults when they listen to repeated infant cry sounds, using sAA as a novel biomarker to assess activity of the SNS. Two physiological systems comprise the biological stress response (Chrousos & Gold, 1992): activation of the hypothalamic–pituitary–adrenal axis with subsequent secretion of glucocorticoids (e.g., cortisol) and activation of the ANS. The sympathetic branch of the ANS is responsible for the release of catecholamines into the blood stream and prepares the body for a “fight or flight” response. In the past years, the measurement of α-amylase in saliva was discovered as a valuable and minimally invasive biomarker of SNS activity in response to stress (for reviews see Granger, Kivlighan, El-Sheikh, & Stroud, 2007; Nater & Rohleder, 2009). The secretion of α-amylase into saliva is largely regulated by the sympathetic branch of the ANS, although concurrent activity of the parasympathetic branch may augment the effects of the SNS (e.g., Bosch, Veerman, De Geus, & Proctor, 2011). Similar to other measures of the ANS, levels of sAA rise in response to physical and psychological stress, peak within 5–10 min after the onset of the stressor and quickly return to baseline levels (e.g., Gordis, Granger, Susman, & Trickett, 2006; Granger et al., 2007; Nater et al., 2005; Nater & Rohleder, 2009). Levels of sAA are also modestly associated with other indicators of ANS activity, including HR (Stroud et al., 2009) and SC (El-Sheikh, Erath, Buckhalt, Granger, & Mize, 2008).

The current study provides a unique opportunity to examine the association between adults’ physiological and intended caregiving responses to infant crying in more detail. It is part of a larger behavior genetic study of adults’ responses to cry sounds (see Out, Bakermans-Kranenburg, Granger, Cobbaert, & Van IJzendoorn, 2011; Out, Pieper, Bakermans-Kranenburg, & Van IJzendoorn, 2010; Out, Pieper, Bakermans-Kranenburg, Zeskind, et al., 2010). An experimental paradigm was designed with cry stimuli varying in fundamental frequency (pitch), which is one of the most salient acoustic characteristics of cry sounds and indicative of the level of infant distress (e.g., Porter, Porges, & Marshall, 1988; Zeskind, Sale, Maio, Huntington, & Weiseman, 1985). Participants rated the perceived urgency of the cries and reported how likely they would respond with sensitive versus harsh caregiving behavior. The paradigm included continuous HR monitoring (see Out, Pieper, Bakermans-Kranenburg, & Van IJzendoorn, 2010) and repeated saliva sampling (see Out et al., 2011).

Previous analyses revealed that adults were more likely to perceive high-pitched cry sounds as urgent and to respond with more sensitive and prompt caregiving behavior to these cries as compared to the other cry sounds (Out, Pieper, Bakermans-Kranenburg, Zeskind et al., 2010). An increase in cry pitch was however also associated with more intended harsh caregiving, consistent with previous studies on the role of high-pitched crying in eliciting less optimal caregiving behavior in vulnerable parents (Soltis, 2004; Zeskind, 2007). There were no significant differences in HR reactivity depending on the pitch of the cry, but there was a clear increase in HR over the course of the paradigm (Out, Pieper, Bakermans-Kranenburg, & Van IJzendoorn, 2010). Each cry episode elicited a higher HR compared to the previous one, indicating that adults become increasingly sensitized to repeated infant distress signals. In contrast, levels of sAA decreased over the course of the paradigm, although there were significant individual differences in the trajectory of sAA (Out et al., 2011).

Here, our aim is to take the latter finding one step further by relating these individual differences in sAA trajectory to adults’ perception of the cry stimuli and their intended caregiving behaviors. Most studies on physiological hyperreactivity to infant crying focused on ANS activity and there are indications that especially the sympathetic branch of the ANS is involved in the physiological dysregulation observed in abusive parents (Joosen, Mesman, Bakermans-Kranenburg, Pieper, et al., 2012). Since the secretion of sAA is mainly regulated by sympathetic influences, we expect that especially intended harsh caregiving is associated with heightened sAA reactivity and a lack of habituation in response to repeated cry sounds.

Method

Participants

Participants were recruited using the municipal registers with the names and addresses of the residents of five cities in the western region of the Netherlands, through advertisements and a website asking for participants and by word of mouth. The sample consisted of 50 male and 134 female twin pairs, mean age 33.0 years (SD = 10.8). The majority of the twins were born in the Netherlands (93%); the remaining individuals came from China (n = 2), Germany (n = 2), England (n = 2), Cape Verde (n = 2), Oman (n = 2), Sri Lanka (n = 2), and Suriname (n = 6). The sample was from a predominantly middle-class population, and 29% of the participants were parents. Permission for the study was obtained from the ethics committee of the Faculty of Social and Behavioral Sciences, and informed consent was obtained for all participants.

Procedure

Twin pairs were tested individually in two quiet rooms. The visit started with some cognitive assessments; after a short break and an hour long interview, the cry paradigm was administered (lasting about 30 min). Participants were asked to refrain from smoking and drinking alcohol on the day of the session and from intense physical exercise in the 24 hr preceding the session. To control for factors that affect the ANS and the secretion of sAA (Granger et al., 2007), participants completed a questionnaire regarding their health, use of medications, food and drink, smoking, and physical exercise.

Measures

Cry stimuli

Cry sounds were derived from the spontaneous crying of a healthy 2-day-old, full birth-weight and full-term female infant, midway between scheduled feedings. A 10-s portion of the sustained period of crying was selected for presentation, containing seven expiratory sounds. The peak fundamental frequency (F ₀) or pitch of the cry was 515 ± 15 Hz. Two new cry sounds were created by digitally increasing the F ₀ of the original cry by approximately 200 and 400 Hz, respectively, resulting in two new 10-s cry sounds with an overall Peak F ₀ of 714.5 Hz (700 Hz cry) and 895.8 Hz (900 Hz cry). The acoustic and temporal structure of the original cry sound is characteristic of the cries of normal, healthy infants (e.g., LaGasse, Neal, & Lester, 2005; Wasz-Höckert, Michelsson, & Lind, 1985). Fundamental frequencies of 700 and 900 Hz (and even higher) are observed in transient pain cries of healthy infants and in infants being separated from their parents (Porter et al., 1988; Zeskind & Collins, 1987), as well as in cries of infants with medical and neurological conditions (Soltis, 2004). Previous analyses revealed that the original cry was not perceived as urgent or aversive but with every increase in fundamental frequency, ratings for perceived urgency and aversiveness also increased significantly (Out, Pieper, Bakermans-Kranenburg, Zeskind, et al., 2010).

Cry paradigm

Figure 1 illustrates the different phases of the cry paradigm and the measures that were taken during each phase, including the baseline. The cry paradigm was administered using a laptop and Sennheiser HD202 headphones. During the baseline, participants were instructed to relax and look at landscape photographs. In the first weeks of data collection (n = 78), one photograph was presented for 4 min after which a baseline saliva sample was collected. Since visual inspection of the data of these individuals revealed that there was substantial variation in absolute levels of sAA between and within individuals, we decided to obtain two additional baseline samples for the remainder of the sample. For these individuals, three landscape photographs were presented for 6 min in total. After each photograph, a saliva sample was obtained before the next photograph was displayed. The correlations between the three baseline measures were very high (ranging between .79 and .84, p < .01), which allowed us to combine these data with the data from the participants for whom only one baseline sample was collected.

Figure 1.

Procedure of the cry paradigm. Note. S = saliva sample; P = ratings for perceived urgency; C = ratings for intended caregiving behaviors.

During the first part of the task, the cry stimuli were presented in three cycles or episodes. Each episode consisted of the 500 Hz, 700 Hz, and 900 Hz cry sounds, which were presented in a random order. Each cry was followed by the collection of a saliva sample, which took 1 min, during which participants also rated their perception of the cry on four rating scales. Thus, nine cry sounds were presented and after each cry stimulus, a saliva sample was collected (Figure 1). During the second part of the task, participants rated the same cry sounds on seven rating scales to indicate their intended caregiving response. As in the first part, the cry stimuli were presented in three cycles, each cycle consisted of the 500 Hz, 700 Hz and 900 Hz cry sounds, and the order of presentation within each cycle was random. No saliva samples were collected during this part of the task.

sAA

All saliva samples were collected using a hydrocellulose microsponge, which has a plastic applicator shaft (0.4 × 5.2 mm) attached to an arrowhead-shaped sponge (0.7 × 1.8 mm). Participants placed the sponge under their tongue for 60 s to absorb oral fluid produced in the sublingual salivary gland area. After timing 60 s, the experimenter sealed the microsponge in a small tube. Samples were initially stored frozen at −20°C and subsequently transported to the university hospital Leiden where they were stored at −80°C until assayed. After overnight thawing of the saliva at 4°C, samples were centrifuged during 5 min at 2773 g to remove debris, particles, and buccal cells. Saliva supernatants were then diluted 50-fold in physiological saline solution (Versylene^® Fresenius Kabi, Zeist, the Netherlands; cat no. B230551) and mixed. Levels of sAA were measured in singlet, using a commercially available enzymatic colorimetric assay (cat no. 03183742; Roche Diagnostics, Mannheim, Germany). The amylase test kit consists of 4,6-ethylidene(G7)-1[4-nitrophenyl(G1)]-1,4-[alpha]-d-maltoheptaoside (EPS-G7) substrate and bacterial alpha-glucosidase ([alpha]-d-glucoside glucohydrolase; EC 3.2.1.20). α-Amylase degrades the EPS-G7 substrate to reaction products. Alpha-glucosidase degrades uniformly all reaction products to 4-nitrophenol and glucose in the indicator reaction. The color intensity of the 4-nitrophenol formed in the indicator reaction is directly proportional to α-amylase activity.

The test was applied on Integra 800 analyzer from Roche Diagnostics (Mannheim, Germany), according to the instructions of the manufacturer. CFAS (Calibrator For Automated Systems, cat no. 10759350) was used for calibration. The Roche routine amylase assay is standardized to the IFCC reference measurement procedure (Lorentz, 1999), guaranteeing worldwide comparability of the data. α-Amylase activities were measured in IU/L at 37°C and expressed in U/ml in the current report. Within-run imprecision for the control pool ranged from 0.7 to 2.7% for the combined predilution step and the analysis across the entire study period. Between-run analytical imprecision was lower than 5% throughout this study. Accuracy of the amylase activity procedure was guaranteed by regular participation in the Dutch External Quality Assessment Scheme (SKML, the Netherlands).

Perception

During the first part of the task, participants rated the 500 Hz, 700 Hz and 900 Hz cry sounds three times on four 5-point rating scales reflecting their perceptions of the cry sounds: aroused—not aroused, aversive—not aversive, sick—healthy, urgent—not urgent (Zeskind & Lester, 1978). Principal component analyses (PCA) pointed to one underlying component (Out, Pieper, Bakermans-Kranenburg, Zeskind, et al., 2010). Therefore, for each of the nine cry stimuli, the four ratings for perception were aggregated to obtain scores for the overall perceived urgency of the sound. Cronbach’s α (calculated across the three presentations of each pitch) ranged from .88 to .89.

Intended caregiving responses

During the second part of the task, participants indicated their intended caregiving responses to each cry sound on seven 5-point rating scales. They were asked to indicate how likely they would respond with the following behaviors (Zeskind, 1980): pick up, cuddle, wait and see, give pacifier, feed, and focus on something else other than on the crying baby. We additionally included “firm handling” as a harsh response, to cover more extreme forms of insensitive behavior. The Dutch meaning of “firm handling” explicitly refers to a harsh response, being strict, and physically unpleasant for the baby. Based on PCA (Out, Pieper, Bakermans-Kranenburg, Zeskind, et al., 2010), we focused on two components that clearly represent positive and negative styles of parenting, namely, sensitive (e.g., pick up and cuddle) and harsh caregiving responses (firm handling). Thus, for each cry stimulus, the average sensitive response and the rating for harsh response were included as predictors in the multilevel analyses. Cronbach’s αs for the sensitive component (calculated across the three presentations of each pitch) ranged between .87 and .90. Although harsh responses were assessed with a single item, this item was completed three times for each cry pitch, and the ratings were highly reliable (Cronbach’s αs ranged between .79 and .88).

Statistical Analyses

Data processing

For 18 individuals (5%), no saliva measures were available due to insufficient specimen volumes. We excluded 11 individuals with extreme outlying sAA levels (>1000 U/ml) and applied a square root transformation to correct for the positively skewed distribution. Baseline levels were averaged across resting conditions for participants with three baseline measures (n = 261) and correlations between untransformed sAA levels ranged between .79 and .84 (p < .01); for the remaining samples (n = 78), there was one baseline measure available. Because perception and caregiving ratings were available only in relation to the cry sounds (i.e., no ratings were given after the baseline), we imputed the lowest value (1) of the rating scales for perceived urgency (not urgent) and caregiving (no sensitive response and no harsh response) as a baseline value for these ratings. Intended harsh caregiving responses were log-transformed because of the skewed distribution of scores.

Multilevel analyses

Multilevel regression models were employed to account for the hierarchical structure of the data (i.e., the separate measurements of sAA and the ratings for perception and caregiving were nested within individuals and individuals were nested within twin pairs). Three levels were specified: the stimulus level, the person level, and the twin level. The (average) baseline level of sAA and the separate measurements of sAA after the cry stimuli (nine in total) were included as the dependent variable. The statistical model takes into account the dependency of the data within twin pairs and within individuals. In this model, it is not necessary to include the twin status as a separate predictor: specifying a model with twin pair as a separate level is sufficient to correct for the dependency (regardless of whether the dependency arises from the fact that the twins are genetically more similar or that they share a similar environment).

We started with an intercept-only model, which decomposed the variance in sAA into three independent components (pertaining to the three levels). Gender, age, and parental status were then added to the intercept-only model (Model 2). Episode (baseline, cry episode 1, cry episode 2, and cry episode 3) was subsequently added as a continuous variable (Model 3). This model was used as a starting point for two separate analyses; the first analysis included the perception of the cry sounds as a predictor of sAA, whereas the second analysis focused on adults' intended caregiving responses. In the first analysis, we included the ratings for the perceived urgency of the cry sounds and subsequently the interaction with episode to examine whether the pattern of sAA reactivity over the course of the paradigm differed depending on adults’ perception of the cry stimuli. In the second analysis, we included the ratings for sensitive and harsh caregiving responses, as well as the interactions between intended caregiving behaviors and episode in the next step. All independent variables were centered around their mean. Multilevel regression models were fitted using MLwiN, version 2.02 (Rasbash, Charlton, Browne, Healy, & Cameron, 2005). Fixed regression coefficients were estimated by maximum likelihood and tested using two-tailed z-tests. Likelihood ratio tests were used to evaluate the variance of the random intercepts and overall model improvement.

Results

Results of the intercept-only model are presented in Table 1 (Model 1). None of the following factors were significantly associated with sAA levels across the task (ps > .07), and they were omitted from the model: caffeine, heavy physical work, physical exercise on the day of the session or in the preceding week, alcohol consumption, smoking, chewing gum, use of medications/corticosteroids/oral contraceptives, diagnosed hearing problems, oral diseases, hormonal diseases, and time of day. Gender, age, and parental status were added as predictors to the intercept-only model (Model 2), which resulted in an improved fit of the model, χ²(3) = 11.91, p = .01. There were no differences between parents and nonparents (z = 0.27, p = .79) and there was no effect of age on sAA levels (z = 1.50, p = .13), but males had significantly higher levels of sAA across the cry paradigm than females (z = 2.73, p = .01). In Model 3, episode was added. The fit of this model was good in comparison with the previous model, χ²(1) = 23.17, p < .01, with a significant decrease in sAA levels over the course of the task (z = −4.82, p < .01).

Table 1.

Salivary α-Amylase Levels (U/ml) and Intended Caregiving Responses to Infant Crying

	Model 1	Model 2	Model 3	Model 4	Model 5	Model 6	Model 7
Fixed effects
Intercept	10.71 ± 0.30 (<.01)	10.18 ± 0.40 (<.01)	10.17 ± 0.40 (<.01)	10.17 ± 0.40 (<.01)	10.16 ± 0.40 (<.01)	10.19 ± 0.40 (<.01)	10.19 ± 0.40 (<.01)
Gender		1.80 ± 0.66 (.01)	1.80 ± 0.66 (.01)	1.79 ± 0.66 (.01)	1.79 ± 0.66 (.01)	1.90 ± 0.67 (<.01)	1.90 ± 0.67 (<.01)
Age		0.05 ± 0.03 (.13)	0.05 ± 0.03 (.13)	0.05 ± 0.03 (.15)	0.05 ± 0.03 (.14)	0.05 ± 0.03 (.16)	0.05 ± 0.03 (.16)
Parental status		0.18 ± 0.66 (.79)	0.19 ± 0.67 (.82)	0.20 ± 0.67 (.76)	0.20 ± 0.67 (.76)	0.17 ± 0.66 (.79)	0.17 ± 0.66 (.79)
Episode			−0.18 ± 0.04 (<.01)	−0.21 ± 0.04 (<.01)	−0.20 ± 0.04 (<.01)	−0.23 ± 0.04 (<.01)	−0.23 ± 0.04 (<.01)
Perceived urgency				0.07 ± 0.05 (.15)	0.07 ± 0.05 (.13)
Episode × perceived urgency					0.02 ± 0.04 (.65)
Sensitive caregiving						0.05 ± 0.04 (.19)	0.05 ± 0.04 (.27)
Harsh caregiving						0.71 ± 0.32 (.03)	0.59 ± 0.32 (.06)
Episode × sensitive caregiving							−0.03 ± 0.03 (.27)
Episode × harsh caregiving							0.56 ± 0.27 (.04)
Variance components
Twin level intercept	11.08 ± 1.84	10.05 ± 1.74	10.09 ± 1.74	10.11 ± 1.74	10.11 ± 1.74	10.98 ± 1.78	10.98 ± 1.78
Person level intercept	9.92 ± 1.15	9.90 ± 1.14	9.91 ± 1.14	9.89 ± 1.14	9.90 ± 1.14	9.16 ± 1.07	9.18 ± 1.07
Stimulus level intercept	4.49 ± 0.12	4.49 ± 0.12	4.45 ± 0.12	4.45 ± 0.12	4.45 ± 0.12	4.34 ± 0.12	4.34 ± 0.12
Deviance	15235.79	15223.88	15200.71	15198.58	15198.37	14967.97	14963.01

Note. Values refer to the estimates ± SE (two-sided p value z-test).

Based on Model 3, two separate analyses were run: one with perceived urgency and the interaction with episode (Models 4 and 5) and one with the intended caregiving responses and the interactions with episode (Models 6 and 7). The addition of perceived urgency (Model 4) and in the next step the interaction between perceived urgency and episode (Model 5) did not increase model fit, χ²(1) = 2.13, p = .14 and χ²(1) = 0.21, p = .65, respectively.

However, the inclusion of sensitive and harsh caregiving responses to Model 3 was associated with a significantly improved model fit, χ²(2) = 232.74, p < .01; see Model 6. Intended sensitive responses were not associated with sAA (z = 1.32, p = .19), but more intended harsh caregiving responses were associated with higher levels of sAA across the task (z = 2.24, p = .03). In the final model, the interactions between caregiving responses and episode were added. There was a marginally significant improved fit of the model, χ²(2) = 4.96, p = .08 (Model 7), and a significant interaction effect between harsh caregiving and episode (z = 2.07, p = .04). As shown in Figure 2, adults who indicated that they would respond in a harsh way to the infant were significantly less likely to display a decrease in sAA levels over the course of the cry episodes.

Figure 2.

Levels of salivary α-amylase (U/ml, square root transformed) at baseline and for each of the cry episodes (using the estimates for Model 7 of the multilevel analyses, Table 1), separately for different ratings for harsh caregiving responses.

Discussion

To our knowledge, this is the first study on the association between caregiving and physiological responses to infant crying using sAA as a sensitive and minimally invasive biomarker for activity of the ANS. An experimental paradigm was designed with standardized cry stimuli and concurrent assessments of adults’ intended caregiving responses and stress-related changes in sAA. Consistent with previous studies on physiological hyperreactivity in abusive parents (for a review, see McCanne & Hagstrom, 1996), the current study shows that adults at risk for harsh parenting were significantly less likely to habituate in response to repeated cry sounds compared to other adults. Whereas a decrease in levels of sAA was observed for the majority of the sample, this decrease was less pronounced for adults who selected harsh parenting behaviors. As can be seen in Figure 2, the multilevel regression model indicated that at the extremes (i.e., maximum rating for intended harsh response), the physiological response of these adults even took the form of a stress-related increase in sAA.

These results are remarkably consistent with the findings reported by Zeskind (1987) on HR responses in (nonabusive) adults listening to hyperphonated infant cries. Whereas some parents displayed HR decelerations to these extremely high-pitched sounds, other parents displayed strong accelerations in their HR, indicative of a defensive response to aversive sounds. Zeskind (1987) argued that these qualitatively different HR responses may provide the basis for differential caregiving responses to infant crying. The results of the current study confirm this hypothesis and suggest that failure to habituate or sensitization to repeated and aversive infant distress signals may be one of the mediating mechanisms through which infant crying may trigger less optimal caregiving. Casanova and colleagues (1992) argued that physiological hyperreactivity to stressful child stimuli may act as a disinhibitor of normal impulse controls and may increase parents’ readiness to engage in aggressive and abusive behavior toward their children. Especially in the context of multiple sources of stress (e.g., Sturge-Apple et al., 2011), the physiological systems of these parents may fail to regulate the additional stress elicited by a crying infant, thereby diminishing their capacity to respond in a sensitive and supportive way.

Interestingly, the results of the current study are also in line with the findings reported by Joosen, Mesman, Bakermans-Kranenburg, Pieper, and colleagues (2012). They reported that sensitive parenting was associated with an increased HR in parents listening to infant cry sounds, which seems to contradict results from studies on physiological hyperreactivity in abusive parents. However, the HR increase in sensitive parents was paralleled by vagal suppression. During an environmental challenge or demand, vagal tone (parasympathetic control of the heart) is inhibited, which is associated with increased behavioral and emotional control, self-regulatory behaviors, and adequate coping responses (Porges, 1996, 2001, 2007; Porges, Doussard-Roosevelt, & Maiti, 1994). Studies have indeed shown that vagal suppression in parents is associated with sensitive and soothing caregiving behavior in response to infant distress (e.g., Mills-Koonce et al., 2007).

Previous analyses of our study revealed that for the sample as a whole, there was a continuous increase in HR during the cry paradigm (Out, Pieper, Bakermans-Kranenburg, & Van IJzendoorn, 2010). This sensitization to infant distress signals may be largely driven by the parasympathetic branch of the ANS and facilitate prompt and supportive caregiving to the crying infant. In contrast, for the majority of the sample, there was no increase in sAA levels during the cry episodes, except for adults at risk for harsh parenting, which indicates activation of the SNS. Thus, this finding supports the hypothesis of Joosen, Mesman, Bakermans-Kranenburg, Pieper and colleagues (2012), who argued that the physiological hyperreactivity in abusive parents may reflect a chronically overactivated SNS resulting from a poorly functioning vagal brake.

The acoustic and temporal features of the cry change as a function of the infant’s level of distress (Green, Irwin, & Gustafson, 2000; Murray, 1979; Zeskind et al., 1985). In fact, cry pitch is directly related to parasympathetic activity in the infant. Porter and colleagues (1988) demonstrated that decreases in infants’ vagal tone during a stressful procedure were paralleled by increases in cry pitch. Adults are sensitive to these acoustic variations and use these cues as well as contextual information in order to decide when and how to respond (e.g., LaGasse et al., 2005; Wood & Gustafson, 2001; Zeifman, 2004; Zeskind, 1980; Zeskind et al., 1985). Indeed, previous analyses of the current study revealed that high-pitched cry sounds were perceived as more urgent, which paved the way for more immediate and affectionate caregiving (Out, Pieper, Bakermans-Kranenburg, Zeskind, et al., 2010). This process has been described as a synchrony of arousal between parent and infant (Zeskind et al., 1985) where the acoustic structure of the cry elicits parental arousal and caregiving behavior that matches the infant’s level of distress. Although high-pitched cry sounds generally elicit more sensitive parenting, in vulnerable parents they may also elicit less optimal caregiving (for reviews, see Frodi, 1985; Soltis, 2004). Indeed, we found a direct association between an increase in cry pitch and intended harsh responses (Out, Pieper, Bakermans-Kranenburg, Zeskind, et al., 2010). Especially when combined with hostile and negative attributions and a lack of empathy (e.g., Crouch et al., 2008; Milner et al., 1995), high-pitched crying may elicit excessive and prolonged physiological reactivity in parents, increasing the risk of harsh and abusive caregiving. In our study, the perceptual ratings only referred to aspects of urgency. Future studies may include rating scales that specifically refer to hostile and negative attributions and feelings.

A limitation of the current study concerns the short duration of the cry sounds compared to what parents face in everyday life. They may experience considerable stress when their infant is crying excessively and is difficult to console or when the crying is characterized by an unusually high pitch. The physiological dysregulation in parents at risk for abusing their children may thus be more pronounced in these circumstances, increasing the likelihood that they respond with harsh, irritable, and aggressive behavior. Future studies should therefore include cry sounds of longer duration and with more aversive acoustics. A second limitation is the correlational design of the study, which does not allow us to disentangle whether physiological dysregulation leads to harsh caregiving responses or whether a hostile attribution bias results in increased sympathetic activation as well as less optimal caregiving. Intervention studies that focus on reducing parents’ negative and hostile beliefs about their infant and on promoting an accurate perception of their infants’ signals (e.g., Juffer, Bakermans-Kranenburg, & Van IJzendoorn, 2007) may shed more light on this issue, by examining the effects on parents’ stress regulation and caregiving behavior.

A third limitation is the assessment of caregiving responses, with participants reporting their intended behavior and with only one item for the assessment of harsh parenting (reliably completed across the three presentations of each cry pitch). The study was designed to disentangle the influence of adult characteristics and cry acoustics from contextual and other influences, which made it necessary to present the cries in a controlled environment with adults reporting their intended caregiving behavior. We previously showed meaningful and theoretically consistent associations for intended harsh responses to crying (Out, Pieper, Bakermans-Kranenburg, Zeskind, et al., 2010): harsh caregiving was associated with higher ratings for cry aversiveness and lower ratings for sensitive responses, and cry sounds with a higher pitch elicited more intended harsh caregiving, as convergent with the literature. In future studies, it is nevertheless imperative to further validate the use of reported caregiving responses against observed parent–infant interactions. More dynamic measures of perception and observed caregiving are also needed to confirm the association between physiological hyperreactivity to infant crying and risk for harsh parenting. Indeed, a recent study by Joosen, Mesman, Bakermans-Kranenburg, and Van IJzendoorn (2012) has already shown that observed harsh discipline was associated with increased SC reactivity (an indicator of sympathetic activity) in mothers listening to repeated infant cry sounds. Similarly, a study by Sturge-Apple and colleagues (2011), which included observations of maternal behavior, also demonstrated that hyperarousal during a stressful child context was the physiological profile uniquely associated with observed harsh parenting.

In sum, using sAA as a novel and noninvasive marker for activity of the ANS, our study provides further evidence that adults at risk for harsh parenting display a hyperreactive response pattern, reflected in a lack of habituation in response to repeated infant cry sounds. Replication of our findings with additional measures of observed caregiving as well as in ecologically valid contexts is imperative. Most of the studies on physiological reactivity to infant crying in abusive parents were conducted between 1977 (Disbrow, Doerr, & Caulfield, 1977) and 1994 (Casanova et al., 1994). In fact, to the best of our knowledge, no study has addressed physiological hyperreactivity to stressful infant stimuli in abusive parents after the publication of McCanne and Hagstrom’s review (1996). We hope that our findings will stimulate new research using salivary biomarkers which are particularly well suited for these studies, as saliva sampling is minimally invasive, allows for self- and home-based collection, and does not require special laboratory equipment (Granger et al., 2012). If replicated, sAA may assist in the identification of the most vulnerable parents and in the evaluation of intervention programs to prevent child abuse.

Footnotes

Acknowledgments

We thank Philip Sanford Zeskind for providing the cry stimuli and for his advice on their use. We are grateful to Fieke Pannebakker, Jolanda den Heijer, Femke Pijlman, and several student research assistants for their contribution to the various parts of the study. Last but not least, we thank the twins who participated in the study.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: DOU, MJBK, and MHvIJ were supported by awards from the Netherlands Organization for Scientific Research (DOU: Rubicon grant; MJBK: VIDI and VICI grants; and MHvIJ: SPINOZA prize).

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