Journal of Family Psychology 2009, Vol. 23, No. 5, 615– 625
© 2009 American Psychological Association 0893-3200/09/$12.00 DOI: 10.1037/a0016323
Intimate Partner Violence Moderates the Association Between Mother–Infant Adrenocortical Activity Across an Emotional Challenge Leah C. Hibel
Douglas A. Granger
Purdue University
Pennsylvania State University
Clancy Blair
Martha J. Cox
New York University
University of North Carolina at Chapel Hill
The Family Life Project Key Investigators This study examined the relationship between mother and infant adrenocortical levels and reactivity to an emotion eliciting task. The impact of intimate partner violence (IPV) on these relationships was assessed as a moderator. The sample (n ⫽ 702 mother–infant dyads) was racially diverse and from predominantly low-income, rural communities. During a home visit, the dyad’s saliva was sampled before, 20 min, and 40 min after standardized tasks designed to elicit the infant’s emotional arousal and later assayed for cortisol. Mothers completed self-report measures of their partner’s violence, and parenting behaviors were assessed via structured interview and mother– child interactions. In response to the task, infants had positive, and mothers had negative, cortisol slopes. Contrary to expectations, there were no IPV-related differences in mean pretask cortisol levels or reactivity in the mothers or infants. Mother–infant dyads from households characterized by either (1) violence or (2) restrictive and punitive parenting behaviors exhibited correlated cortisol reactivity measured in response to the infant challenge task. The findings suggest that social contextual features of the early caregiving environment may influence individual differences in the coordination between maternal and infant adrenocortical reactivity. Keywords: intimate partner violence, cortisol, mothers, infants, stress reactivity
The examination of the social contextual influences on individual differences in stress physiology has been a central theme in child development research (see Gunnar & Vazquez, 2006, for review). In a seminal integration of findings on this topic, Boyce and Ellis (2005) propose a theoretical framework suggesting highly stressful early environments promote hyper-reactivity in stress responsive
systems, such as the hypothalamic-pituitary-adrenal (HPA) axis. Volumes of literature link individual differences in the psychobiology of stress reactivity and recovery to adverse early experiences including maltreatment, neglect, sexual abuse, and severe maternal deprivation (e.g., Cicchetti & Rogosch, 2001; De Bellis, 2005; Gunnar, Morison, Chisholm, & Schuder, 2001; but see Tarullo & Gunnar, 2006, for
Leah C. Hibel, Department of Child Development and Family Studies, Purdue University; Douglas A. Granger, Behavioral Endocrinology Laboratory, Department of Biobehavioral Health, Pennsylvania State University; Clancy Blair, Department of Applied Psychology, New York University; Martha J. Cox, Center for Developmental Science, University of North Carolina at Chapel Hill; and the Family Life Project Key Investigators, Department of Biobehavioral Health, Pennsylvania State University; Department of Human Development and Family Studies, Pennsylvania State University; Center for Developmental Science, University of North Carolina at Chapel Hill; and Frank Porter Graham Child Development Center, University of North Carolina at Chapel Hill. We would like to thank the many families and research assistants that made this study possible and to Peter Molenaar for his generous statistical assistance. We would also like to thank Jacob Hibel and Christine Fortunato for their careful edits of this manuscript. Many thanks to the four anonymous review-
ers and to the editor of this Journal; their comments greatly strengthened this manuscript. The first author was supported by a National Science Foundation Graduate Research Fellowship. Support for this research was provided by the National Institute of Child Health and Human Development grant P01 HD39667, with co-funding from the National Institute on Drug Abuse. The Family Life Project Key Investigators include Lynne Vernon-Feagans, Martha Cox, Clancy Blair, Peg Burchinal, Linda Burton, Keith Crnic, Ann Crouter, Patricia Garrett-Peters, Mark Greenberg, Stephanie Lanza, Roger Mills-Koonce, Debra Skinner, Cynthia Stifter, Emily Werner, and Michael Willoughby. In the interests of full disclosure, Dr. Granger is the founder and president of Salimetrics, LLC. Address correspondence to Leah C. Hibel, Department of Child Development and Family Studies, Purdue University, 101 Gates Road, West Lafayette, IN 47907. E-mail:
[email protected]
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review). To date, little empirical attention has addressed the possible influence of domestic violence exposure on HPA axis activation in mothers and their children. This is particularly surprising given the high prevalence of partner violence in young families’ homes (Heise & Garcia-Moreno, 2002) and the host of behavioral and mental health problems in mothers and children exposed to violence (see Campbell & Lewandowski, 1997, for review). A leading hypothesis is that the association between violence exposure and mental health is at least in part due to individual differences in the regulation of stress-linked physiological processes (e.g., Repetti, Taylor, & Seeman, 2002; Davies, Sturge-Apple, Cicchetti, & Cummings, 2008). In an effort to fill this knowledge gap, this study employs a “biosocial family systems” approach (e.g. Booth, Carver, & Granger, 2000) to explore the impact of intimate partner violence (IPV) on maternal and infant cortisol levels and stressrelated reactivity.
IPV and Children Violence between intimate partners is a leading cause of morbidity and mortality in women of child bearing age, affecting approximately 1.3 million women in the United States each year (Tjaden & Thoennes, 2000). The Centers for Disease Control and Prevention (CDC) considers (IPV) to be threatened, attempted, or completed physical or sexual violence or emotional abuse by a current or former intimate partner (CDC, 2003). Studies show that women living below the poverty line or who have children under the age of 12 are victimized at higher rates (Bureau of Justice Statistics, 2007; Heise & Garcia-Moreno, 2002) and are disproportionately at risk for developing mental health problems (Bell, 1990). The pervasiveness of IPV in young families is underscored by estimates that 1 in 10 children in the United States live in homes in which they are exposed to violence (e.g., Moore, Probst, Tompkins, Cuffe, & Martin, 2007), with the majority of these homes containing children under the age of 5 years (Fantuzzo, 1991). Marital conflict has been directly related to negative child outcomes including internalizing and externalizing behavior problems, attention-seeking, decreased social and cognitive competence, and low grade point averages (see Grych & Fincham, 1990, for review). More recent studies show exposure to IPV during infancy is associated with the expression of trauma symptoms such as disturbances in eating and sleeping and greater distress to conflict (Bogat, DeJonghe, Levendosky, Davidson, & von Eye, 2006; DeJonghe, Bogat, Levendosky, von Eye, & Davidson, 2005; Layzer, Goodson, & deLange, 1986). Davies and Cummings (2006) underscore the importance of children’s emotional processes (e.g., negative emotionality) as a mechanism linking partner conflict with childhood behavior problems. Witnessing violence and aggression between parents is thought to undermine children’s emotional security (Davies & Cummings, 1994). More specifically, when faced with a stressor, young children turn to their immediate caregivers to make sense of the problem and for help in coping with the crisis. Davies and colleagues hypothesize that when parents are the
cause of the crisis, children are left without external resources to regulate their behavioral or physiological arousal (e.g., Davies, Sturge-Apple, Cicchetti, & Cummings, 2008). It is tempting to speculate that circumstances such as these may influence individual differences in the set-point or threshold of HPA reactivity and recovery (Gunnar & Donzella, 2002).
The Role of the Mother Attachment theory champions the role of the mother in the socioemotional development of the child (Bowlby, 1969). External regulation by the mother is thought to facilitate the organization of the infant’s behavioral and physiological activity by providing a scaffold for the infant to develop the capacity to self-regulate (e.g., Feldman, Greenbaum, & Yirmiya, 1999; Hofer, 1994). A key component of mother–infant interactions involves coordinated or coregulation of biological processes as each member of the dyad’s behavior influences the behavior and physiology of the other (Hofer, 1987). Individual differences in maternal characteristics, such as caregiving behaviors and psychopathology, influence the nature of the organization of infant attachment behaviors (Egeland & Farber, 1984; Radke-Yarrow, Cummings, Kuczynski, & Chapman, 1985) and moderate the strength of the relationship between maternal and child physiology (Field, Healy, Goldstein, & Guthertz, 1990; Sethre-Hofstad, Stansbury, & Rice, 2002). Although some studies show mothers in violent relationships to compensate by becoming more effective parents (e.g., Levendosky, Huth-Bocks, Shapiro, & Semel, 2003), hostility and conflict between romantic partners often negatively influences parenting behaviors (e.g., Krishnakumar & Buehler, 2000). Specifically, parents who are engaged in violent and disruptive relationships with their partners are more likely to exhibit harsh and aggressive parenting techniques and less likely to be consistent in their parenting and disciplinary strategies (e.g., Levendosky & GrahamBermann, 2000). They may also decrease their time and emotional investment in their children (e.g., Letourneau, Fedick, & Willms, 2007; Sturge-Apple, Davies & Cummings, 2006). Thus, IPV has the potential to harm child witnesses both directly and indirectly through damages to the parent– child relationship (Levendosky, & GrahamBermann, 2001).
HPA Axis Activity, Reactivity, and Coordination The major component of the psychobiology of the stress response involves activation of the HPA axis (e.g., Chrousos & Gold, 1992). Secretion of corticotrophin releasing hormone (CRH) from the hypothalamus initiates the release of adrenocorticotropic hormone (ACTH) into the blood stream from the pituitary. ACTH stimulates cortisol, a glucocorticoid, to be secreted from the adrenal glands. This response is considered adaptive in the short-run because glucocorticoids facilitate mobilization of resources to accommodate changing or novel environmental demands (Sapolsky, 1996). As repeated experiences become familiar,
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the HPA axis habituates, such that the magnitude of its reactivity to those events is dampened (e.g., McEwen, 1998). However, if the events are extreme in intensity or duration, and they occur on an unpredictable schedule, the result may be frequent and prolonged HPA activation. Chronic activation of the HPA axis has been linked to wear and tear on the body (i.e., allostatic load) via glucocorticoid overexposure (McEwen, 1998). An extensive literature has examined links between physical and psychological stress, atypical behavior, and negative emotionality to individual differences in cortisol levels and reactivity to threat or challenge. The majority of this work is focused on the individual as the unit of analysis; however, family-oriented psychologists have broadened this endeavor to test whether physiological processes are coordinated between individuals (e.g., Powers, Pietromonaco, Gunlick, & Sayer, 2006) in an attempt to better understand reciprocal influences within a dyad. Studies have revealed cortisol levels and stress-related reactivity are sometimes associated between mothers and their children (e.g., SethreHofstad et al., 2002; van Bakel & Riksen-Walraven, 2008), between siblings (Schreiber et al., 2006), and between romantic partners (e.g., Goslin, Booth, & Granger, 2009; Powers et al., 2006). This emerging literature has yet to standardize the language used to describe this phenomenon. Across studies the association between individuals in physiological activity is referred to as attunement, concordance, synchrony, coregulation, similarity, or coordination. In this study, for convenience, we use the terms “coordination” or “concordance” interchangeably to refer to the association between mothers’ and infants’ cortisol activity or reactivity. Whatever the terminology, these observations may be important because they raise the possibility that characteristics of dyadic interactions (e.g., maternal sensitivity, maternal depression) (Field et al., 1990; Sethre-Hofstad et al., 2002) may influence the degree of association between mothers’ and children’s cortisol levels and stress-related reactivity. To the best of our knowledge, no studies have explored the consequences of IPV on the coordination of HPA axis activity or reactivity between mothers and infants.
Present Study In the present study, the moderating role of partner violence on maternal and infant adrenocortical activity, reactivity, and concordance was examined in a large epidemiologic sample of economically disadvantaged rural families. We anticipated that the unpredictable nature and high intensity of conflict in homes characterized by IPV would result in a state of constant and heightened readiness to respond to ever-present threat and challenge. This hypervigilance will set the stage for a coordinated maternal response to infant HPA reactivity. Thus, the degree of maternal reactivity will be related to the degree of infant reactivity in IPV households. Moreover, we expected mothers and infants exposed to IPV would have elevated adrenocortical arousal and exaggerated HPA stress reactivity. Finally, we hypothesized that positive maternal behaviors would buffer the infant, such that infants exposed to IPV
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with positive caregiving would not show elevated HPA levels or reactivity. Similarly, infants exposed to IPV with negative or harsh caregiving would show more extreme elevations in HPA levels and reactivity.
Method Participants The Family Life Project (FLP; National Institute of Child Health and Development [NICHD], P01HD39667) was designed to study families in two areas of high child poverty: the Rural South and Northern Appalachia (see Burchinal, Vernon-Feagans, Cox, & FLP Investigators, 2008, for description). Data for this analysis came from the larger project’s home interview and assessments, when the infant was approximately 7 months old (range ⫽ 5.0 –13.4 months, M ⫽ 7.6 months), with 85% of the sample at age 9 months or below (Blair, Granger, Willoughby, & Kivlighan, & the FLP Investigators, 2006). From the total of 1,292 dyads eligible, 702 were included in the analyses. Specifically, 88 families were not available for a home visit, 224 dyads were excluded because of the mother’s refusal to complete the Conflict Tactics Scale (CTS)1, 123 dyads were excluded because the infant took acetaminophen in the past 48 hours (Hibel, Granger, Kivlighan, Blair, & the FLP Investigators, 2006), 57 dyads were excluded because the mother refused to donate saliva specimens, and 98 dyads were excluded because of procedural difficulties collecting the saliva samples from the infant. Based on the mother’s race and income-to-needs ratio, this subsample was 67% White and 33% African American, with 73% of the dyads 200% below the poverty line. Many (37.1%) of the mothers were not married, and the majority (88.8%) of the single mothers had never been married.
Procedures Overview. As part of the larger project’s 2- to 3-hr home visit, the FLP assessments included self-report measures, semistructured interviews, and mother– child interaction tasks. Selfreport questionnaires include those assessing IPV and demographic information. For the mother–infant interactions, mothers were given a set of toys and were instructed as follows: “We would like for you to play with [infant’s name] as you normally would if you had a little free time during the day” (Cox, Paley, Burchinal, & Payne, 1999). Infants underwent 1
Mothers who refused to complete the CTS were more likely to exhibit negative parenting behaviors 2(1) ⫽ 16.92, p ⬍ .0001 and less likely to exhibit positive parenting behaviors 2(1) ⫽ 52.97, p ⬍ .0001. Overall, dyads excluded for missingness and medication usage did not differ from the analytic sample based on restrictive and punitive 2(1) ⫽ 3.34, p ⫽ ns parenting groups or based on IPV grouping 2(1) ⫽ .24, p ⫽ ns. However, excluded dyads were significantly less likely to have mothers exhibiting positive 2(1) ⫽ 16.40, p ⫽ .0001 and more likely to have negative 2(1) ⫽ 5.10, p ⫽ .05 parenting behaviors. The excluded dyads were also more likely to be from the North Carolina site 2(1) ⫽ 58.79, p ⫽ .0001.
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“challenge tasks” (taken from the Laboratory Temperament Assessment Battery; Goldsmith & Rothbart, 1988) designed to elicit fear and frustration, including a mask presentation challenge, a barrier challenge, and an arm restraint challenge. Three saliva samples were collected surrounding the challenge tasks and were later assayed for cortisol (Hellhammer, Wust, & Kudielka, 2009).
Behavioral Assessments IPV. To assess violence between partners, the Conflict Tactics Scale was administered to all mothers regarding their spouses (or partners), regardless of whether or not the spouse/partner lived in the household with the infant. The Couple form of the Conflict Tactics Scale–Revised (Straus, Hamby, Boney-McCoy, & Sugarman, 1996) consists of 19 items. Following Jouriles and colleagues, only items pertaining to physical or threatened physical aggression were used to determine membership in the IPV versus non-IPV group (see Jouriles, Norwood, McDonald, & Peters, 2001, for review). Two hundred eighty-five mothers indicated their partner used or threatened at least one violent act in the last 12 months; these mother–infant dyads were classified in the IPV group. Of these mothers, 28.8% reported having something thrown at them; 39.3% were pushed, grabbed, or shoved; 13.7% had been slapped; 10.2% were kicked, bit, or hit; 19.3% had been hit with an object; 5.3% had been beaten up; 6.7% were choked; 1.8% were threatened with a gun or knife; and 1.1% had a gun or knife used on them. Four hundred eighteen mothers indicated their partner or spouse did not use any of the above violent tactics, and these dyads were classified as the non-IPV comparison group. Parenting behaviors. Mother– child interactions during a standard free-play activity were digitally recorded and subsequently coded to assess levels of mothers’ sensitivity, detachment, intrusiveness, positive regard, negative regard, and animation in interacting with the child (see Cox et al., 1999; NICHD Early Child Care Research Network, 1999). Mothers were given three standard, age-appropriate toys and were instructed to play with their infants as they normally would. Free-play episodes lasted approximately 10 min. Ratings for each behavior were made on a 1 to 5 scale, with 1 ⫽ not at all characteristic, 2 ⫽ minimally characteristic, 3 ⫽ somewhat characteristic, 4 ⫽ moderately characteristic, and 5 ⫽ highly characteristic. Based on the results of factor analyses conducted with an oblique rotation (i.e., Promax), maternal positive engagement (␣ ⫽ .89) was defined as the mean of mothers’ scores for four characteristics: detachment (reverse-scored; level of emotional uninvolvement or disengagement), positive regard (level of positive feelings expressed toward child), animation (level of energy), and stimulation for development (appropriate level of scaffolding of activities with child). Maternal intrusion (␣ ⫽ .69) was defined as the mean of mothers’ scores for three characteristics: sensitivity (reversed, level of responsiveness to child’s needs, gestures, and expressions), intrusiveness (degree to which mother imposed her own agenda on the interaction ignoring the baby’s signals), and negative regard (level of harsh, negative feelings expressed toward
child). Reliability was determined by calculating the intraclass correlation for ratings made by two coders to approximately 30% of the tapes randomly drawn. Reliability was acceptable for both maternal intrusion (r ⫽ .88) and maternal positive engagement (r ⫽ .80). The HOME Inventory (Caldwell & Bradley, 1984) was employed to assess restrictive and punitive parenting styles. This measure examines the quantity and quality of support given to a child in the home environment. The HOME revealed 93% interrater agreement using raw percent agreement with high internal consistency (␣ ⫽ .89) for the total scale. Items included “caregiver shouts at the infant,” “caregiver is hostile to the infant,” “caregiver used more than 1 physical punishment in the last week,” “caregiver scolds the infant during the visit,” “caregiver physically restricts the infant,” and “caregiver slaps or spanks the infant during the visit.” These items were completed by the home visitors based on conversations with the mother and observations of maternal behavior through out the interview. Laboratory Temperament Assessment Battery (Lab-TAB). Three “challenge tasks” designed to elicit emotional reactivity were administered to the infant. The task procedures have been previously validated (e.g., Buss & Goldsmith, 1998; Kochanska, Tjebkes, & Forman, 1998; Stifter & Braungart, 1995) and are only briefly mentioned here. The tasks are from the Lab-TAB (Goldsmith & Rothbart, 1988). For the first task, the mask presentation, mothers were seated beside their child, and for the last tasks, barrier and then arm restraint, mothers were out of the infants’ sight. For the mask task, infants were presented with 4 unusual masks, one at a time. The experimenter wore each mask for 10 s while calling the child’s name and moving from side to side in front of the child. For the barrier task, infants were presented with an attractive toy and encouraged to play with it for 30 s. The experimenter then placed the toy behind a clear plastic barrier just beyond the child’s reach for 30 s. The toy was then returned and the procedure was repeated twice more. For the arm restraint task, the experimenter crouched behind the infant and gently restrained his or her arms for 2 min or until 20 s of hard crying ensued. Mothers were asked not to intervene but were told they could stop the task at any time. After the completion of the tasks, infants were given 1 min to self-soothe, after which the mother was allowed to soothe the infant.
Biological Assessments To assess changes in cortisol indicative of the infants’ and the mothers’ adrenocortical response to the emotion challenge tasks, 3 saliva samples were collected: a pretask sample collected prior to administration of the challenge tasks, a sample 20 min after the completion of the tasks (or prior, if the infant reached “peak emotional arousal”), and then a final sample 20 min later. The infant was considered to have reached peak arousal if he or she produced 20 s of hard crying at which point the tasks were terminated. The majority (95%) of the infants completed all tasks. For the infants, unstimulated whole saliva was collected using absorbent materials (either cotton swabs or Visispears;
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Harmon, Hibel, Rumyantseva, & Granger, 2006) and expressed into 2 ml cryogenic storage vials. For mothers, whole unstimulated saliva was collected using the passive drool technique (Granger et al., 2007). The collection of mother and infant saliva samples was coordinated to occur simultaneously. After collection, samples were immediately placed on ice, then stored frozen (⫺20 °C) until shipped on dry-ice overnight to the Behavioral Endocrinology Laboratory at the Pennsylvania State University. Samples were then stored frozen at ⫺80 °C until assayed. All samples were assayed for salivary cortisol using a highly sensitive enzyme immunoassay (Salimetrics, State College, PA). The test used 25 l of saliva, had a range of sensitivity from .007 to 3.0 g/dl, and average intra- and interassay coefficients of variation less than 10% and 15%. All samples were assayed in duplicate and the average of the duplicates was used in all analyses. Cortisol levels were natural log transformed to correct for skewed distributions. All analyses used transformed values; for ease of interpretation, raw levels are reported.
Analytical Strategy Latent growth curve modeling (e.g. Davies et al., 2007) was used for the main analyses. First, IPV was assessed as a moderator of the correlation between the maternal and infant cortisol intercepts (levels) and slopes (described below). Group differences based on these variables were determined through the comparison of constrained (correlations between maternal and infant intercepts and slopes designated to be equal across the two groups) and unconstrained models. Correlations between maternal and infant intercepts along with maternal and infant slopes were freed to vary between groups in the unconstrained model. Given the arbitrary nature of intercept designation, any relation between intercepts and slopes will be considered a statistical artifact (see Rovine & Molenaar, 1998, for description). Therefore, these parameters will not be interpreted. Examining slopes provides insight into the overall adrenocortical functioning of the individual across the entire task. The mean slope parameter represents the constant change in cortisol across the three assessments (pretask, posttask1, and posttask2). A linear slope was derived by weighting each manifest natural log transformed cortisol measurement to correspond to the number of minutes elapsed from the collection of the pretask sample, with the pretask designated as 0. This resulted in the posttask1 and posttask2 being weighted .40 and .60, respectively. The weighting allows the correct distance to be represented between each collection point. Centering the slope variable on the pre-task assessment causes the intercept to reflect the mean cortisol level at the pretask, adjusted for sampling time of day and the other cortisol measurements. All cortisol levels were weighted equally onto the latent intercept variable. Second, the effect of IPV on maternal and infant cortisol levels (intercept) and reactivity (slope) was assessed. Mean differences between groups were determined by fixing the
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alpha coefficient (the mean parameter of a latent variable) in one group and assessing differences based on membership to the second group. Third, parenting behavior was assessed as a moderator of maternal and infant levels, reactivity, and concordance. Moderation was determined through the use of multiple groups analysis; therefore, a median split of the parenting behavior scales were used to determine membership in either the high or low maternal engagement and intrusive parenting behavior groups. Previous studies have found harsh parenting, including physical punishment, to be associated with both behavioral and physiological consequences for children (e.g., Bugental, Martorell, & Barazza, 2003; see Gershoff, 2002, for review), thus, we placed dyads in which the mother exhibited any restrictive or punitive behaviors in one group (RP); while mothers not exhibiting these behaviors were placed in the nonrestrictive or punitive (non-RP) group. 2 analyses were used to determine membership differences in maternal caretaking groups (engagement, intrusion, and RP) between the IPV and non-IPV groups. Engagement, intrusion, and RP maternal caregiving behaviors were then examined as potential moderators of maternal and infant adrenocortical levels, reactivity, and concordance. The characteristics of the sample, the repeated interview schedule, the length of each interview protocol, and the age of the infants required that these in-home assessments were scheduled when families were available. On average, the pretask sample was collected from mothers and infants at 1:37 p.m. (SD ⫽ 2 hr 56 min; range ⫽ 8:10 a.m.– 8:08 p.m.), with 62% of the collections completed after 12 noon. Sample collection time did not differ between the IPV and non-IPV dyads t(700) ⫽ .41, p ⫽ ns. All analyses covaried sampling time of day, to control for the diurnal rhythm of cortisol production.
Results Preliminary and Descriptive Analyses Table 1 provides the raw means, SDs, and correlations of both maternal and infant cortisol levels before, and in response to, the challenge tasks. Maternal and infant cortisol levels were highly correlated within, and weakly to moderately correlated between, individuals in the dyad (rs ⫽ .23–.90), with maternal and infant pretask cortisol levels being moderately correlated r(700) ⫽ .27, p ⬍ .01. Using latent growth curve modeling, the relationships between maternal and infant intercepts and slopes across the task were examined. The model fit the data well 2 (7) ⫽ 4.86, p ⫽ .68. Overall, mother’s intercept (␣ ⫽ .21 g/dL, SE ⫽ 0.03) was positively correlated with infant’s intercept (␣ ⫽ .23g/dL, SE ⫽ 0.04; r ⫽ .14, SE ⫽ .01, p ⬍ .0001), however, mother’s slope was not correlated with infant’s slope (r ⫽ .01, SE ⫽ .01, p ⫽ ns). Mothers exhibited a significant decrease in cortisol levels across the task with a slope coefficient of ⫺0.67 (SE ⫽ 0.03; Z ⫽ 5.43, p ⬍ .0001), whereas infants exhibited a significant increase in cortisol levels across the task with a slope coefficient of 0.15 (SE ⫽ 0.07; Z ⫽ 2.34, p ⬍ .05).
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Table 1 Means (Cortisol in g/dL), SDs, and Intercorrelations of the Primary Variables in the Study for Mothers and Infants (N ⫽ 702) Variable 1. 2. 3. 4. 5. 6. 7.
Child pretask cortisol Child posttask cortisol 1 Child posttask cortisol 2 Mother pretask cortisol Mother posttask cortisol 1 Mother posttask cortisol 2 Time of day
ⴱⴱⴱ
M
SD
1
2
3
4
5
6
7
.22 .25 .22 .20 .16 .14 13:37
.31 .27 .26 .15 .13 .11 2:56
— .56ⴱⴱⴱ .52ⴱⴱⴱ .27ⴱⴱⴱ .27ⴱⴱⴱ .27ⴱⴱⴱ ⫺.25ⴱⴱⴱ
— .83ⴱⴱⴱ .23ⴱⴱⴱ .26ⴱⴱⴱ .26ⴱⴱⴱ ⫺.31ⴱⴱⴱ
— .24ⴱⴱⴱ .26ⴱⴱⴱ .25ⴱⴱⴱ ⫺.35ⴱⴱⴱ
— .86ⴱⴱⴱ .82ⴱⴱⴱ ⫺.49ⴱⴱⴱ
— .90ⴱⴱⴱ ⫺.45ⴱⴱⴱ
— ⫺.47ⴱⴱⴱ
—
p ⬍ .001.
Main Analyses Does IPV moderate concordance between mother and infant adrenocortical levels or reactivity? A multigroup approach was used to examine IPV as a possible moderator of mother–infant adrenocortical correlations. 2 differences between the constrained and unconstrained models revealed the unconstrained model (the two-group model) to have a significantly better fit (2 difference ⫽ 29.66, p ⬍ .05). Mother–infant dyads in the non-IPV group had correlated intercepts (r ⫽ .18, SE ⫽ .04, p ⬍ .0001), whereas the IPV dyads did not, (r ⫽ .09, SE ⫽ .05, p ⫽ ns) (see Figure 1). However, comparing the correlation coefficients did not reveal a significant difference between these parameters, t ⫽ 1.25, p ⫽ ns. Mothers in the IPV group had cortisol slopes that were correlated with their infants’ slope (r ⫽ .18, SE ⫽ .09, p ⬍ .01), whereas mothers’ slopes in the non-IPV group were not correlated with their infants’ slopes (r ⫽ ⫺.11, SE ⫽ .07, p ⫽ ns) (see Figure 1). Comparisons of these correlation coefficients revealed a significant difference between the IPV and non-IPV dyads, t ⫽ 2.33, p ⬍ .05.
Sampling time of day Mother’s Pre-task 1
.18*** .09
0
Mother’s Post-task1
Mother’s Intercept
1
40
0 Infant’s Intercept
Mother’s Slope
1
Infant’s Post-task1
40
Infant’s Slope
1
1 60
Mother’s Post-task2
Infant’s Pre-task
1
60 -.11 .18*
Infant’s Post-task2
Figure 1. The structural equation model examined the relationship between mothers’ and infants’ overall cortisol levels and reactivity to the stress task. Values of dyads in violent homes are presented in bold. Correlations between mother–infant intercepts were not significantly different between the two groups. Violent dyads exhibited correlated reactivity to the stress task, whereas nonviolent dyads did not; these parameters significantly differed between the two groups. To increase readability, regression coefficients with time of day were left out. ⴱ p ⬍ .05. ⴱⴱⴱ p ⬍ .001.
Does IPV moderate maternal and infant cortisol levels or reactivity? The next model focused on the moderating effects of IPV on the means of the latent intercepts and slopes across the tasks. Alphas were estimated for the intercept and slope for both the mother and the infant. Alpha coefficients in the non-IPV dyads were fixed; thus, allowing the alpha of the IPV dyads to represent significant differences between the groups. No alpha coefficients reached significance (␣s ⫽ ⫺.04 to .09, ps ⫽ ns), showing the mean intercepts and slopes were not different between the IPV and non-IPV groups. Do maternal caregiving behaviors moderate maternal or infant adrenocortical levels, reactivity, or concordance? Maternal caregiving groups (high/low engagement; high/low intrusion; non-RP/RP) were first examined as possible moderators of mother-infant adrenocortical correlations across the entire sample. 2 differences between the constrained and unconstrained models revealed no significant difference in the overall models based on positive engagement or intrusive caregiving behaviors. However, a two-group approach with RP parenting moderating motherinfant adrenocortical correlations gave a significantly better fit (2 difference ⫽ 28.10, p ⬍ .05). Correlations of mother– infant intercepts between the non-RP (r ⫽ .17, SE ⫽ .04, p ⬍ .0001) and RP (r ⫽ .03, SE ⫽ .06, p ⫽ ns) groups were marginally different, t ⫽ 1.90, p ⫽ .06, whereas the correlation between maternal and infant slopes were significantly different between the two groups. Similar to the IPV dyads, RP mothers (N ⫽ 144) exhibited cortisol slopes that were significantly correlated with their infants’ slope in response to the task (r ⫽ .24, SE ⫽ .11, p ⬍ .01); however, non-RP dyads (N ⫽ 558) did not exhibit correlated slopes (r ⫽ ⫺.02, SE ⫽ .07, p ⫽ ns). The strength of the correlation was significantly different between the RP and non-RP groups, t ⫽ 2.03, p ⬍ .01. Constraining alpha coefficients for the non-RP dyads allowed the alphas of the RP dyads to represent significant differences between the groups; however, no alpha coefficients reached significance (␣s ⫽ ⫺.08 to .09, ps ⫽ ns) showing the mean intercepts and slopes did not differ between the RP and non-RP groups. Do maternal caregiving behaviors moderate maternal and infant adrenocortical levels, reactivity, or concordance, within the IPV group? Cross-tabulation of IPV grouping and parenting behavior groupings revealed mothers in the
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IPV group to exhibit significantly more RP behaviors (2 ⫽ 5.88, p ⬍ .01) but no difference in membership to the engagement or intrusion parenting groups. Parenting behavior was then assessed as a moderator of the relationship between IPV and maternal and infant cortisol levels, reactivity and concordance. Within the IPV group, 2 differences between the constrained and unconstrained models revealed no significant difference in the overall models based on engagement, intrusion, or RP parenting. This analysis shows positive maternal behaviors do not buffer, and negative maternal behaviors do not amplify the impact of IPV on mother-infant adrenocortical levels, reactivity, or concordance.
Discussion IPV is a family level problem, with negative implications for mother victims, exposed children, and for mother– child relationships (e.g. Campbell & Lewandowski, 1997; Levendosky, & Graham-Bermann, 2000). Confirming prior research we found maternal and infant pretask cortisol levels were positively correlated (e.g., Sethre-Hofstad et al., 2002; Stenius, Theorell, Lilja, Scheynius, Alm, & Lindblad, 2008). Also, on average, infants showed an increase in adrenocortical activity in response to the emotionally challenging events (see Blair et al., 2006, 2007). By contrast, mothers’ did not show a cortisol increase in response to witnessing their infant’s participation in the task. Motherinfant dyads from households characterized by either (1) violence or (2) restrictive and punitive parenting behaviors exhibited positively correlated cortisol slopes measured in response to the infant challenge task. The findings have several noteworthy implications and are discussed in relation to the possible mechanisms linking maternal and infant HPA activity and regulation. Multiple frameworks can be used to explain the positive correlation between maternal and infant cortisol levels and reactivity. Most obvious is the possibility that there are genetic determinants of HPA activity and responsiveness, and that the association between mother and infant is a function of heredity (Wust, Federenko, van Rossum, Koper, & Hellhamer, 2004). Indeed, research has revealed a scattered pattern of associations in HPA axis activity between mothers and their children (Granger et al., 1998; SethreHofstad et al., 2002), and among siblings, both twins and singletons (Goslin et al., 2008; Schreiber et al., 2006). Yet, Schrieber et al. (2006) reported the majority of resemblance in twin’s cortisol levels was attributable to shared environmental factors. Moreover, studies report modest positive associations between cortisol levels or responsiveness, among young adult couples, (Powers et al., 2006) and husbands and wives in established marriages (Goslin et al., 2009). In the context of the present study’s design we are unable to test the genetic compared to environmental contributions, we therefore assume our findings are influenced by both. Shared events or experiences have the potential to influence physiological responses between dyads because of Mutual Coordination (Kendon, Harris, & Key, 1975). That
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is, two individuals presented with the same situation (challenging/stressful or positive) may simply exhibit similar physiological responses. The degree of the shared arousing context (i.e., highly arousing versus moderately arousing) could dictate the degree to which the dyad jointly responds. Individuals spending a large proportion of time together have the potential to exhibit concordant diurnal fluctuations resulting from continuous shared experiences (Stenius et al., 2008). Thus, it is possible that mutual coordination could be driving the correlation between mother and infant pretask cortisol levels. Another possibility is that individuals could have correlated adrenocortical activity due to Parallel Coordination. For example, two individuals may experience the same stressor, but due to differences in cognitive and social development, or past experiences, perceive the situation differently. One individual may react based on experience with a previous, but similarly stressful trauma, while the other may show a response due to an unrelated experience (e.g., perceived lack of control, fear). This possibility is unlikely to have driven the association between maternal and infant responses to the task. The challenge task directly engaged the infants, while the mothers, on the other hand, experienced the challenge indirectly, as witnesses. In other words, the stressor for the infant (i.e., the Lab-TAB) was not the same as the mother’s stressor (i.e., infant distress). In this study, a “shared experience” (Parallel or Mutual Coordination) explanation is probably not justified for maternal– infant coordinated responses to the task. A third possibility is that covariation within the dyad is because of reciprocal influences between members of the dyad (Reciprocal Coordination; Schore, 2000). Behavioral and emotional reactivity to threat or challenge by one dyad member may, under certain circumstances, shape the responsiveness of the other member. Again, the subjective experience or perception of the stressor does not have to be shared between dyad members, but the experience of one individual influences the other’s experience. Thus, the infants in the current study could be responding to the challenge tasks with anger or fear, whereas the mother may be anxiously responding to the infant’s distress. Within the violence exposed group, this possibility seems a viable hypothesis and worth further exploration. However, to disentangle who is affecting whom would require comparisons between the dyad using a cross-lagged time series biomarker collection design. Mother– child interactions provide a unique context in which the caregiver provides cues that influence and regulate the state of infant behavior and physiology (e.g., Gunnar & Donzella, 2002; Hofer, 2006). Synchronous behavioral exchanges between the mother and the infant are thought to facilitate secure attachments (Isabella, Belsky, & von Eye, 1989), and create a biobehavioral system that supports the development of the infant’s own regulation systems over time (Field, 1994; Hofer, 2006). As expected, differential patterns of physiological and behavioral stress reactivity have been found to vary by caregiver– infant attachment patterns (Schieche & Spangler, 2005; Spangler & Grossman, 1993). Specifically, infants clas-
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sified as insecure-avoidant exhibit dissociation between their behavioral and physiological reactivity (i.e., little behavioral distress but high physiological reactivity; HillSoderlund et al., 2008). Biobehavioral dissociations at the individual level may have implications for our understanding of associations in adrenocortical reactivity at the dyadic level. For example, if an infant does not exhibit behavioral distress but is physiologically aroused, would we expect mother–infant cortisol levels to be coordinated? This may be particularly relevant to the present analyses, given both IPV and punitive parenting styles have been found to predict lower attachment security (Posada & Pratt, 2008). Thus, it is possible that attachment security may moderate the relationship between maternal–infant adrenocortical coordination within the IPV and punitive parenting groups, by accounting for infants that exhibit dissociated patterns of physiological and behavioral reactivity. The most significant contribution made by the present findings is that conflict and harsh familial interactions appear to moderate the association between maternal and infant HPA responses to challenge. It may be of more than passing interest that in preliminary analyses of parents and children from 400 intact families, Booth and colleagues reveal that the strength of the association between spouses, between siblings, and between parent– child HPA activity is moderated by the degree of conflict reported between the specific dyad members (Goslin et al., 2009). That is, higher levels of conflict were linked to stronger associations in HPA activity with the dyad. Similarly, Powers and colleagues revealed coordinated HPA responsiveness in young romantic couples (Powers et al., 2006) when engaged in conflict-oriented discussions. Why would the adrenocortical association between individuals be moderated by relationship conflict, violence, and/or harsh interpersonal behavior rather than closeness, intimacy, and relationship quality? Both the shared nature and the increased potential for conflict to result in a physiological response from both individuals involved (e.g., Sapolsky, 1994) may facilitate a mutual or parallel response in the conflicting individuals. In relation to our findings, the context of partner violence may have influenced the mother’s perception or interpretation of her infant’s cues during the challenge tasks (Lamb & Easterbrooks, 1981), potentially encouraging a reciprocal adrenocortical response to the infant’s reactivity. Being a victim of IPV is also thought to undermine a women’s sense of control (Umberson, Anderson, Click, & Shapiro, 1998), potentially spilling over into her parental responses to infant distress (e.g., Krishnakumar & Buehler, 2000).
Limitations and Future Directions Studies are clearly needed to follow-up this emerging pattern of findings and test hypotheses about the nature and direction of adrenocortical associations between individuals. One worthwhile approach would be to expose the same dyad to a series of challenges over time. One challenge might be directed at the infant with the mother present as a witness, on another occasion a task might be directed at the mother with the infant as a witness, and on another occasion
the task could be a challenge designed to involve both members interacting together. Testing how the strength of the adrenocortical association varies in mother–infant adrenocortical reactivity across these tasks would extend our understanding. It is possible to imagine that in some circumstances, the direction of the association could even be negative. The sample for this analysis is an epidemiologically valid representation of children and families living in the areas from which it is drawn. As such, it is representative of a substantial segment of the low-income population in the United States in nonurban areas; however, this limits its generalizability to urban populations. Furthermore, the age of the sample and the continuing development of the HPA axis (Gunnar & Vasquez, 2006) may make the findings specific to young infants. Future studies assessing violence and nonviolence exposed dyads should also incorporate measures of verbal aggression and psychological abuse (e.g., Reid & Crisafulli, 1990) to better understand how each of these aspects of conflict impact maternal and infant adrenocortical reactivity. Furthermore, while studies have shown children do not need to be physically present at the time of the altercation to exhibit implications (Osofsky, 1999), examining the degree of exposure or partner residence may be worthwhile next steps. Finally, a task specifically pulling for reactivity to conflict may elucidate physiological changes surrounding conflict in both mothers and children (e.g., Granger, Weisz, & Kauneckis, 1994).
Conclusion The importance of taking a “biosocial family systems” (Booth et al., 2000) approach is clearly highlighted in the current set of analyses. Family systems researchers view family members as a continuous, inter-related, and an interdependent whole (Cox & Paley, 1997). This conceptualization of the family may be mirrored in biological processes as well. The frequent shared and parallel interactions and experiences, along with the high potential for family members to impact each other’s thoughts, feelings, and actions, may facilitate the coordination of physiology. Thus, family members’ physiology should be viewed as continuous and inter-related, with the strength of the association dependent on the quality and the context of the relationship between family members. This may be particularly salient in mother–infant dyads given the intrinsically dependent nature of this relationship and the role of the mother as the infant’s behavioral and physiological regulator. Thus, examining maternal and infant adrenocortical levels and reactivity simultaneously reveals multiple levels of family and individual functioning. It is therefore important for future research to understand how physiology at the individual level interacts at the dyad, triad, or even entire family group level to function as a distinct influence on child and family development.
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Received May 19, 2008 Revision received February 20, 2009 Accepted March 30, 2009
New Editors Appointed, 2011–2016 The Publications and Communications Board of the American Psychological Association announces the appointment of 3 new editors for 6-year terms beginning in 2011. As of January 1, 2010, manuscripts should be directed as follows: ● Developmental Psychology (http://www.apa.org/journals/dev), Jacquelynne S. Eccles, PhD, Department of Psychology, University of Michigan, Ann Arbor, MI 48109 ● Journal of Consulting and Clinical Psychology (http://www.apa.org/journals/ccp), Arthur M. Nezu, PhD, Department of Psychology, Drexel University, Philadelphia, PA 19102 ● Psychological Review (http://www.apa.org/journals/rev), John R. Anderson, PhD, Department of Psychology, Carnegie Mellon University, Pittsburgh, PA 15213 Electronic manuscript submission: As of January 1, 2010, manuscripts should be submitted electronically to the new editors via the journal’s Manuscript Submission Portal (see the website listed above with each journal title). Manuscript submission patterns make the precise date of completion of the 2010 volumes uncertain. Current editors, Cynthia Garcı´a Coll, PhD, Annette M. La Greca, PhD, and Keith Rayner, PhD, will receive and consider new manuscripts through December 31, 2009. Should 2010 volumes be completed before that date, manuscripts will be redirected to the new editors for consideration in 2011 volumes.
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