Introduction

In a 2007 report, the World Health Organization reported there is strong empirical evidence that shows “… many of the challenges in adult society—mental health problems, obesity, stunting, heart disease, criminality, competence in literacy and numeracy—have their roots in early childhood” (Irwin et al. 2007, p. 5). Extensive research has revealed that adverse childhood experiences (ACEs) have been associated with a variety of risky behaviors and negative health and wellbeing outcomes, including: early unintended pregnancies (Hillis et al. 2001), learning and behavior disorders (Burke, Hellman, Scott, Weems, & Carrion, 2011), adult homelessness (Herman, Susser, Struening, & Link, 1997), suicidality (Dube, Anda, Felitti, Chapman, Williamson & Giles, 2001), illicit drug use (Dube et al. 2003), fetal death (Hillis et al. 2001), eating disorders and chronic health conditions(Felitti et al. 1998; Middlebrooks and Audage 2008), and increased morbidity (Brown et al. 2009). Exposures to ACEs are interrelated and typically do not occur independently of other adversities (Dong et al. 2004; Finkelhor et al. 2007). The impact of exposure to ACEs intersects with multiple developmental domains, including social, emotional, behavioral, and cognitive development (D’Andrea et al. 2012; Yoshikawa 2010). There is an increasing need to establish the prevalence of exposure to cumulative ACEs and the association of multiple ACEs to negative health and wellbeing implications across several developmental domains.

Although, there is a wealth of research documenting the negative outcomes of various exposure to childhood adversities, the field remains fragmented with islands of researchers conducting studies on single, specific adversities (D’Andrea et al. 2012), often relying on adult retrospective reports of ACE exposure before the age 18 (Grasso et al. 2016; Shonkoff 2010). Studies examining the co-occurrence of ACEs are limited, especially among varying child developmental stages (Dong et al. 2004; Edwards et al. 2003; Grasso et al. 2016). Researchers across disciplines have highlighted the need to develop and implement interventions to meet the unique needs of young children who have been exposed to early childhood trauma, which cumulatively place them at greater risk for negative outcomes across the lifespan (Shonkoff 2010; Yoshikawa 2010). To aid scholars in the development and adaptation of prevention and intervention efforts targeted toward this vulnerable population, this systematic review sought to examine literature on compounding ACEs experienced during early childhood and the developmental implications.

Literature Review

Adverse Childhood Experiences Background

The first adverse childhood experiences study (ACE Study) was published in 1998 by a group of medical doctors (Felitti et al. 1998), who coined the term “adverse childhood experiences.” Pioneering the field of ACEs, the original ACE Study examined seven different adversities (three categories of childhood abuse and four categories of household dysfunction) and their association to adult risk behaviors, diseases, and health status (Felitti et al. 1998). The findings revealed a strong and graded correlation between all seven ACEs and an increased risk for: depression, alcoholism, drug abuse, suicidality, smoking, risky sexual behaviors, physical inactivity, severe obesity, and poor self-reported overall health (Felitti et al. 1998). Not only did the likelihood of negative behaviors and health outcomes increase with each additional ACE exposure, Felitti et al. (1998) also found that the measured ACEs were strongly interrelated. During Wave II of the ACE Study, the original researchers added three additional adversities to the ACE framework. Today, the ten most common adverse exposures measured include: child physical, sexual, and emotional abuse; emotional and physical neglect; parental violence; household mental illness and substance use; parental separation or divorce; and incarcerated household member (Dube et al. 2001). Since the original ACE Study, researchers across disciplines have examined the association between ACEs and both short and long-term health and wellbeing implications. Despite these findings, a scarce amount of literature focuses on adversities experienced during certain developmental domains.

Although literature includes a plethora of childhood events that may be categorized as an adverse experience, ranging from child maltreatment to lacking love and comfort (Bloom 2000), existing literature lacks clarity as to what constitutes an ACE (Kalmakis and Chandler 2014). The operational definition provided by Kalmakis and Chandler (2014) guided this systematic review:

Adverse childhood experiences are childhood events, varying in severity and often chronic, occurring within a child’s family or social environment that cause harm or distress, thereby disrupting the child’s physical or psychological health and development (p. 1495).

In an effort to include a variety of ACEs, this review did not limit the type, frequency, or number of exposures measured, but rather centered on a critical developmental stage: early childhood. The need for this systematic review is critical to highlight the differences in developmental and wellbeing outcomes experienced by youth across developmental stages.

Early Childhood

The beginning years of life are a time of vulnerability and significant growth, which lay the foundation for future healthy development (Cooper et al. 2009). Research has revealed that exposure to adversities in early childhood, perhaps more than any other childhood developmental stage, disrupts brain development, structure, and functioning (Enlow et al. 2012; Kerker et al. 2015). Such neurological disruptions significantly impair a young child’s mental health, affecting their behavioral, cognitive, and social-emotional development (Cooper et al. 2009; Phillips & Shonkoff, 2000), having implications across the lifespan (Center on the Developing Child at Harvard University 2007, 2010; Keiley et al. 2001; Middlebrooks and Audage 2008). The potential neurological implications from early childhood exposure to ACEs and prolonged toxic stress (i.e., sustained adverse experiences over a long period), decreases the child’s stress threshold, making them more susceptible to adverse reactions from traumatic experiences (Middlebrooks and Audage 2008). Sustained levels of early childhood stress overwhelm the child’s developing immune system leaving them vulnerable to chronic health conditions and infections (Middlebrooks and Audage 2008). Further, early adverse exposures damage the formation of the hippocampus, an area responsible for memory formation and learning, with repercussions persisting into adulthood (Middlebrooks and Audage 2008).

Keiley, Howe, Dodge, Bates, and Pettit (2001) found that the developmental timing of maltreatment exposure was a salient factor in examining negative implications of ACE exposure. Carlson, Furby, Armstrong, and Shlaes (1997) along with Keiley, Howe, Dodge, Bates, and Pettit (2001) suggest that very young children exposed to harmful events or situations are at greater risk for negative outcomes, compared to older children, because they cannot physically escape the situation, resulting in severe developmental implications because the child seemingly does not have the cognitive, emotional, and physical resources to cope or escape. Another study found that exposure to interpersonal trauma, including child maltreatment and interparental violence, in early childhood, specifically during the first two years of life, had significant and enduring implications on cognitive development (Enlow et al. 2012). Exploration of the initial age of ACE exposure and developmental timing may provide more insight into how negative implications of such exposures manifest (Enlow et al. 2012; Lupien et al. 2009). With the rapid neurological development during the first years of life, early childhood is a critical developmental period that lays the foundation for future health and wellbeing outcomes (Cooper et al. 2009; Shonkoff 2010). Given the vulnerabilities and the intersecting and overlapping negative consequences associated with early childhood and exposure to multiple ACEs, this review attempts to bridge the gap between these two siloed, but complexly related, bodies of literature.

This review focused specifically on children in the developmental stage of early childhood, including children between 0 and 6 years old. This review was guided by the Center for Disease and Control and Prevention’s (CDC) age categories and defined early childhood as any child between birth and 5 years of age. The authors recognized that, for some states, the review’s target population includes kindergarten-aged children. In an effort to be as inclusive as possible, the early childhood developmental age range was extended to include all kindergarten-aged children, which in some cases included 6 year-olds, this modification allowed for all studies with a kindergarten-aged sample population, up to 83 months, to be considered. For this review, studies that included infants (0–12 months), toddlers (12–36 months), preschoolers (36–60 months), and kindergarten-aged children (60–83 months) were considered to meet the early childhood inclusion criteria.

Multiple ACE Exposures and Dose–Response Associations

An expansive body of literature examines wellbeing in response to a child’s exposure to single, specific adversities, such as child physical abuse or physical neglect (Edwards et al. 2003; Finkelhor et al. 2007; Sabri et al. 2013). Although exposure to single adversities can have enduring negative consequences, studies examining only single adversities may presume the absence of other adversities, and therefore may overestimate the influence of single exposures on wellbeing outcomes (Edwards et al. 2003). McGee, Wolfe, Yuen, Wilson, and Carnochan (1995) found that over 90% of their adolescent sample had experienced multiple types of maltreatment. Ney, Fung, and Wickett (1994) had similar results, finding that less than five-percent of their seven to eighteen-year-old sample had experienced independent adversities. A 2007 study examining a nationally representative sample of youth between the ages of two and seventeen-years old, found that seven-percent of the sample had experienced seven or more adversities over the course of one year, and 20% of the sample had reported five or more victimizations (Finkelhor et al. 2007). Further, an additional methodological limitation of single ACE studies is the potential for heterogeneous abuse patterns among the sampled population, ranging from chronic, severe exposure to isolated, single incidents (Edwards et al. 2003). Research differentiating between the frequency, chronicity, and number of ACE exposures is needed to better inform knowledge and practice.

Despite limited research exploring the effects of co-occurring, or cumulative, ACEs and health and wellbeing outcomes, available research indicates a dose–response association between the number of ACEs reported and a range of harmful health outcomes and behaviors (Edwards et al. 2003; Felitti et al. 1998). In other words, as the number of ACEs reported increases, so does the likelihood of having a broad range of negative health outcomes, indicating poorer overall health and wellbeing outcomes (Edwards et al. 2003; Felitti et al. 1998). Considering the interrelatedness of ACEs and the cumulative nature of negative health outcomes, early childhood is a critical developmental period. Given the young target population of this review, cumulative, or multiple, adverse childhood experiences, is defined as having two or more adverse exposures. This is supported by Finkelhor, Ormrod, and Turner (2009) who advocated for a lower measurement threshold when looking at samples of young children who are vulnerable and are at a heightened risk of poly-victimization, or exposure to multiple adversities.

Purpose of the Review

Given the extensive empirical research supporting the immense biological, social, neurological, and cognitive development during early childhood and the cumulative risks associated with exposure to multiple ACEs, there is limited research exploring the intersection of these two vulnerable populations: early childhood children and youth exposed to multiple ACEs. As the field of early childhood exposure to ACEs continues to emerge, there is a need for continued research and more rigorous studies, across multiple professions, to help build the knowledge base regarding short- and long-term outcomes for young children exposed to adversities. To the authors’ knowledge, there are no current systematic reviews or randomized control trials specifically targeting children who have experienced multiple ACEs. This systematic review examined and synthesized empirical research on the association between early childhood (0–83 months) exposure to multiple (two or more) ACEs and wellbeing and developmental outcomes, specifically social, behavioral, emotional (or psychological), and/or overall physical health or wellbeing. The goal of this study is to enhance the current knowledge base concerning early childhood ACE exposure in an effort to reduce the gap between empirical evidence and practice. This systematic review will provide a foundation of synthesized evidence for scholars and researchers to develop and tailor early prevention and interventions efforts to meet the unique needs of children exposed to ACEs.

Methodology

Empirical research included in this review focused on cumulative ACEs and the social, behavioral, emotional, and/or physical wellbeing outcomes among children between birth through 6 years of age (83 months). Studies included in this review must have examined early childhood exposure to multiple ACEs and the relation of such exposures to the child’s social, behavioral, and/or physical developmental wellbeing. Given the population age range of this review, and the limited research available, the authors decided to be as inclusive as possible and utilized two or more ACE exposures to define multiple exposures. This review did not include studies examining children in middle childhood, young teens, or teenagers. In an effort to be as inclusive as possible, all kindergarten-aged children, which in some circumstances incudes 6 year olds, were included in this review’s definition of early childhood. This modification allowed for all studies with kindergarten-aged samples to be considered. If a study included a range of child developmental stages (i.e., early childhood, middle childhood, young teens, and teenagers), the outcomes pertaining to children who had been exposed to ACEs in early childhood were extracted, if possible, and utilized if all other eligibility criteria were met. Studies were excluded if the outcomes were not extractable or were unclear.

Collectively the authors decided to include studies that explicitly referred to adverse events as “adverse childhood experiences.” This inclusion criterion excluded all articles that examined specific exposures to pre-specified adversities (i.e., co-occurrence of specific types of child maltreatment, co-occurring parental substance use and mental health issues) but did not explicitly refer to such adversities as “adverse childhood experiences”, which eliminated five studies. This exclusion was made to support taxonomy of the “adverse childhood experiences” terminology; although there is a breadth of literature on multiple and co-occurring adversities, research has remained fragmented, with islands of scholars conducting research on varying, specific combinations of ACEs. For example, extensive bodies of literature explore the co-occurrence of domestic violence and child physical abuse, but do not refer to either traumatic event as an ACE, therefore, further isolating this area of literature from other adversities, which are often interrelated. Resulting in a paradoxical effect: due to the breadth of research on co-occurring and compounding adversities, researchers have subsequently struggled to comprehensively detail the implications of a variety of ACEs (Kalmakis and Chandler 2014). In an attempt to create parsimony, articles examining multiple adversities, without referring to such adversities as “adverse childhood experiences” were eliminated. Moreover, articles were excluded if the study focused on single ACE exposures, protective factors, parental outcomes, or child mental health problems as risk factors for future adversities. Such research is beneficial but outside the scope of this systematic review.

To date there are no intervention programs designed specifically for early childhood children who have been exposed to multiple ACEs. Thus, this systematic review will synthesize current empirical research on early childhood exposure to multiple ACEs and health and wellbeing outcomes. The outcome measures of included studies must have assessed one, or a combination of, the following: overall child wellbeing, socialization, behavioral patterns, or physical developmental outcomes of the exposed child. Excluded studies included qualitative designs, studies that focused on outcomes other than social, behavioral, developmental, or overall child wellbeing (e.g., resilience and protective factors and neurological or biological outcomes), studies that did not specify the age of exposure or developmental stage of the child, and studies that did not specify which adverse exposures the study was assessing. Qualitative studies were excluded from this review due to the disadvantages associated with combining results across varying study designs (Littell et al. 2008).

Search Strategy

This systematic review was guided by Littell, Corcoran & Pillai (2008). Authors of this review did not publish the protocol used for this review, nor received any financial support to complete this review. Additionally, the authors have no relationship to any of the included secondary data analyses or original studies referenced in this review. The authors conducted this review to help advance the current literature base and further pursue their scholarly research interests.

The literature search included English language studies examining early childhood exposure to multiple ACEs and the social, behavioral, emotional and/or physical wellbeing of the child. A preliminary search of articles addressing “adverse childhood experiences” prior to 1997 was conducted to determine if a publication date range was needed for this review. The initial informal search yielded zero articles and therefore this review did not utilize a date restriction for the formal systematic search, however, the formal search only generated articles published between 1998 and 2017. It is important to note that the pioneering ACE Study conducted by Feletti et al. was initiated in 1995 and published in 1998. The ACE Study coined the phrase “adverse childhood experiences,” further validating the publication date range yielded in the formal search.

Guided by the conceptual definitions previously mentioned, four different sources were utilized to identify articles: electronic bibliographic databases; reference mining of included articles; hand searching of specified journals; and a gray literature search. With consultation from an expert in the field, five bibliographic databases were searched including: (1) Academic Search Complete; (2) PubMed; (3) ProQuest; (4) Social Work Abstracts; and (5) Web of Science. Articles meeting eligibility criteria were identified using search terms such as “adverse childhood experience”, “ACE”, “advers*”, “early childhood”, “young child”, and “outcome.” A total of 106 articles were identified through electronic database searching.

Four additional journals were hand-searched, including the Children’s Bureau Express, Child Maltreatment Journal, Child Development Journal, and Contemporary Issues in Early Childhood Journal. Publications by the Administration for Children and Families and Mathematica were also searched. Hand searching of the four journals and two websites yielded 132 articles with referencing mining generating an additional five articles. Table 1 details the number of records identified from each respective source and at each step of the systematic search. A copy of the authors’ search strategy is available upon request, which provides a detailed list of search terms applied for each respective literature source.

Table 1 Chart of Systematic Searches (Electronic Database, Hand Searching & Reference Mining)
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Study Selection

Studies were identified using the search terms and eligibility criteria mentioned above. A total of 243 articles were identified through the search process, then subsequently uploaded into an electronic bibliographic software program (Zotero Version 4.0.29.15). All duplications were removed, yielding a total of 218 articles. The two corresponding authors simultaneously conducted a title and abstract review on the 218 unduplicated articles and identified articles to be included in the full-text review. After the title and abstract review, discussion between the corresponding authors included clarification around neurological outcomes (i.e. brain size and function), studies examining specific child maltreatment types, and retrospective adult and child samples. The majority of articles were excluded due to focusing on: maternal/caregiver outcomes, resilience/protective factors, age range of youth, adult samples, and neurological outcomes. After clarification and resolution of the aforementioned discrepancies between the authors, and implementation of the study’s inclusion and exclusion criteria, 13 articles were identified for full-text review. Five additional articles were identified through reference mining were also included in the full-text review. In total, full-text reviews were conducted on 18 articles.

Data Extraction

Applying the study criteria mentioned previously, the corresponding authors simultaneously and independently assessed each study utilizing the data extraction form developed specifically for this review, discussed discrepancies, and documented final decisions. Data extraction elements included: (a) sample demographics (i.e., age, gender, race, ethnicity); (b) ACEs assessed; (c) outcome measures examined; (d) reporter information; and (e) study setting and sampling methods. To increase interrater reliability, each author’s final decisions were compared and discussed. Five articles were excluded because they focused on single exposures, protective factors, parental outcomes, and child mental health issues as a risk factor for future victimization. Another five articles were eliminated because the adversities were never referred to “adverse childhood experiences.” Three additional articles were eliminated due to the inability to identify children between the ages of zero and 6 years old within the larger sample. Ultimately, five studies were included in the formal review (Flaherty et al. 2006; Freeman 2014; Grasso et al. 2016; Jimenez et al. 2016; Kerker et al. 2015).

Results

Study Characteristics

All five studies examined early childhood exposure to multiple ACEs. Flaherty et al. (2006) assessed for the lowest number of ACE exposures, examining seven, while two studies measured for eight ACEs, one study assessed for ten, and Grasso, Dierkhising, Branson, Ford, & Lee (2016) tested for 17 ACEs. Only five adversities were common between all the studies: (1) child physical abuse, (2) child sexual abuse, (3) psychological or emotional abuse, (4) caregiver substance use or impaired caregiver, and (5) exposure to domestic violence or caregiver/mother treated violently. Caregiver depression or mental health and caregiver criminality or incarceration were included in four of the five studies (excluding Grasso et al. 2016), and physical neglect was assessed for in all but Flaherty et al. (2006). Grasso et al. (2016) assessed for seventeen ACEs, nearly double the ACEs measured in the other four studies, and included adversities such as personal illness or medical trauma, natural disaster, traumatic loss, community violence, kidnapping, school violence, and war/terrorism/political violence. All five studies provided frequencies on cumulative exposures. See Table 2 for detailed information regarding cumulative ACE exposures.

Table 2 Sample characteristics
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Study Design

All five studies were secondary data analyses, descriptive, and involved subsamples from larger, national data sets. Freeman (2014) used Wave 1 of the National Survey of Child and Adolescent Wellbeing (NSCAW), relying on longitudinal data collected between 1999 and 2007 (Cross and Smith 2010; U.S. Department of Health and Human Services Administration for Children and Families, n.d.). Kerker et al. (2015) analyzed Wave II of the NSCAW, which began data collection in 2008 from a different cohort (Cross and Smith 2010; U.S. Department of Health and Human Services Administration for Children and Families, n.d.). Grasso et al. (2016) used the National Child Traumatic Stress Network- Core Data Set (NCTSN-CDS) data set, Flaherty et al. (2006) used the Longitudinal Studies of Child Abuse and Neglect (LONGSCAN) data set, and Jimenez et al. (2016) used the Fragile Families and Child Wellbeing Study. Three of the included studies used longitudinal data (Flaherty et al. 2006; Freeman 2014; and; Jimenez et al. 2016). Kerker et al. (2015) used a cross-sectional design, Grasso et al. (2016) relied on retrospective adolescent reports of early childhood ACE exposure, and Flaherty et al. (2006) was prospective in nature. Further details of methodological quality of the included studies is presented in Table 4.

Sample

Four of the five study samples included children between 0 and 6 years old. Grasso et al. (2016) included an adolescent sample between 13 and 18 years of age, but was included because it relied on retrospective adolescent self-report of ACE exposures in early childhood. Grasso et al. (2016) was the only study that included multiple child development stages, and therefore, results were only extracted for participants who reported ACE exposure during developmental epoch 1, birth through 5 years of age [exact age of exposure was not recorded, rather the authors relied on retrospective reports of age range of exposure]. Four of the five studies included kindergarten-aged children (excluding Grasso et al. 2016), with two of the studies including 6 year-old children (Flaherty et al. 2006; Freeman 2014). Specifically, three of the studies had samples consisting of toddlers and young children (Flaherty et al. 2006; Jimenez et al. 2016; Kerker et al. 2015), while Freeman (2014) was the only study to include infants. Differing from the other four studies, Grasso et al. (2016) was the only included study that was not nationally representative.

Sample age, sex, race, and ethnicity distributions were only provided for three of the five included studies. Flaherty et al. (2006) and Freeman (2014) were the exclusions and did not provide descriptive statistics for the entire sample, but rather provided distributions based on categorical ACE exposures (see Table 2 for detailed descriptives). Efforts were made to contact these authors for additional information regarding sample characteristics.

Sample sizes ranged from 912 to 3485 participants. Three of the five studies included children and families either involved with the child welfare system or at risk of child welfare involvement (Flaherty et al. 2006; Freeman 2014; Kerker et al. 2015). Grasso et al. (2016) sample was comprised of children who had a trauma history and standardized assessment (Steinberg et al. 2014). The Fragile Families data set, used by Jimenez et al. (2016), consisted of children with unmarried mothers at the time of their birth (Reichman et al. 2001). Moreover, all five studies collected data from varying regions across the United States. Table 2 provides further information on sample characteristics.

The majority of the study samples had been exposed to at least one ACE. Kerker et al. (2015) found that 98% of the sampled 18- to 71 month-old children had at least one adverse exposure with more than half (50.5%) reporting four or more ACEs. Finding similar exposure rates, Freeman (2014) reported 42% of their 0–6 year-old sample had reported four or more ACE exposures. Grasso et al. (2016) did not report cumulative ACE exposure statistics for the entire sample nor for each of the three developmental epochs evaluated in the study (0–5, 6–12, and 13–18 years). However, for adolescents who reported ACE exposure in Epoch 1 (0–5 years), the mean number of ACEs experienced for each of the latent class analysis subgroups was reported, finding that the high-varied exposure subgroup, which comprised 22.1% of the early childhood epoch, averaged 5.3 ACE exposures (SD = .12). In other words, of the adolescents who self-reported ACE exposure during early childhood, 22.1% of them averaged 5.3 adverse experiences by the time they were 5 years of age. The remaining two studies reported lower cumulative exposures with Flaherty et al. (2006) reporting 18% and Jimenez et al. (2016) reporting 12.3% of their study samples being exposed to three or more adversities during early childhood. The variance in cumulative ACE exposure may be due to the fact that different studies assess for different ACEs, as seen in this review, with the assessed ACEs ranging between seven and seventeen.

Outcome Measures

Given the developmental stages examined, four of the studies relied on caregiver, teacher, or caseworker reports (Flaherty et al. 2006; Freeman 2014; Jimenez et al. 2016; Kerker et al. 2015) with one study relying on adolescent self-report (Grasso et al. 2016) of early childhood ACE exposures. Outcome variables varied across the five studies, including internalizing and externalizing behavioral issues, academic and literacy skills, child physical health, chronic medical conditions, social development, and mental health patterns. There was considerable overlap between the five studies with the standardized instruments used to measure child behavior, evaluate for child abuse and neglect, and assess for caregiver depression. Four studies utilized the Child Behavior Checklist (CBCL) (Achenbach and Edelbrock 1983) and a version of the Conflict Tactics Scale (CTS) (Straus 1979; Straus et al. 1996), and three studies utilized the Composite International Diagnostic Interview -Short Form (CIDI-SF) to measure caregiver depression (Kessler et al. 1998). Table 3 provides more detail on the standardized instruments, psychometric properties, and corresponding reporter.

Table 3 Data extraction summary
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Behavioral Outcomes

Four of the included studies examined the correlation between early childhood ACE exposure and internalizing and externalizing behaviors (excluding Flaherty et al. 2006). All four utilized the CBCL to measure internalizing (negative internal behaviors including depression and anxiety) and externalizing behaviors (including aggressive and delinquent behaviors). Utilizing Wave I of the NSCAW, Freeman (2014), examined the prevalence of early childhood cumulative ACE exposure and socioemotional development by age six. Results revealed that children who had experienced three ACEs were 4.7 times more likely to exhibit internalizing behaviors compared to children with no exposures (OR 4.70, p = .05). Children with four or more adverse exposures were nearly five times (OR 4.87, p = .05) as likely to exhibit internalizing behaviors, and almost four times (OR 3.75, p = .05) more likely to report externalizing behaviors, compared to children with no reported ACE exposures. Further, children with four or more ACEs were significantly more likely to have greater total problem behavior scores (OR 3.63, p = .05). Although Freeman (2014) examined five different ACE exposure categories (e.g. 0 ACE exposures, 1 ACE, 2 ACEs, 3 ACEs, and 4 + ACEs), the four previously mentioned results were the only statistically significant findings.

Jimenez et al. (2016) examined the association between early childhood ACE exposure and teacher-reported behavioral problems and academic skills among 1,007 kindergarten-aged children. Using four ACE exposure categories (e.g. 0 ACE exposures, 1 ACE, 2 ACEs, and 3 + ACEs), Jimenez et al. (2016) results suggested a dose–response association between the number of early childhood ACE exposures and an increased likelihood of behavioral problems and below-average academic abilities, including literacy, social studies, science, and math skills. On average, kindergarten children with poor academic skills and behavioral outcomes had reported higher ACE exposure rates in early childhood (1.2–1.54 ACEs), compared to their kindergarten counterparts with no reported behavioral issues and on-schedule academic abilities (.92–.97 ACEs). Exposure to three or more ACEs was significantly associated with social issues, attention problems, and aggressive behaviors in kindergarten children. After controlling for the child’s age, gender, maternal race and ethnicity, marital status, and household income, Jimenez et al. (2016) found that children with three or more ACEs were 3.5 times (OR 3.5, CI (95) 1.8–6.5) more likely to have attention issues, nearly three times (OR 2.7, CI (95) 1.4–5.0) as likely to have social delays, and 2.3 times more likely (OR 2.3, CI (95) 1.2–4.6) to have aggressive tendencies.

Unlike the other four studies, Grasso et al. (2016) used adolescent retrospective reports of childhood exposure to seventeen adverse experiences to explore the patterns of ACE exposure across three developmental stages in childhood(i.e., 0–5, 6–12, and 13–18 years). Utilizing latent class analysis, Grasso et al. (2016) identified a three-class solution that best fit Epoch 1, which included only participants who reported experiencing adversities between the ages of 0 and 5 years old. Epoch 1 included the following three subgroups: (1) high and varied early childhood ACE exposures (M = 5.3 ACEs); (2) low and varied early childhood ACE exposures (M = 1.8 ACEs); and (3) low exposure, primarily exposed to domestic violence (M = 1.6 ACEs). For Epoch 1, descriptive characteristics comparing the three subgroups revealed that the high-varied exposure subgroup had higher scores on the CBCL, indicating worse behavioral issues, among the sampled adolescents, than the other two subgroups on the three broadband CBCL scales (i.e., internalizing scale, externalizing scale, and total problem scale). However, these results were only significantly different between the high-varied exposure subgroup and the domestic violence exposed subgroup for the CBCL internalizing and total problem scales.

When Grasso et al. (2016) compared the results of Epoch 1 to the two other older epochs, findings indicated adolescents who reported multiple ACEs in early childhood—typically reporting a combination of emotional abuse and physical abuse, neglect, or impaired caregiver—were the most likely to have clinically significant symptoms of internalizing behaviors and post-traumatic stress disorder (PTSD) as an adolescent. Adolescents who reported early childhood exposure to domestic violence and low exposure to other adversities, were the least likely group to have clinically significant PTSD symptoms and internalizing behavioral issues as an adolescent. Of the adolescents who reported ACE exposure between 0 and 5 years old, 11.8% were identified as potential polyvictims in Epoch 1, and considerably increased to 31.3 and 38.8% in Epochs 2 and 3, respectively.

Kerker et al. (2015) used the CBCL, like Freeman (2014) and Jimenez et al. (2016), to examine the relation between ACEs and mental health patterns in children 18-to-71 months old. Among the entire sample of 912 young children, researchers found that children with scores greater than the clinical symptom range on the CBCL had more ACE exposures than their peers with CBCL scores below the cutoff score (4.3 ACEs vs. 3.5 ACEs, p < .001); however, when stratified this correlation only remained significant for older children in the sample, 36–71 months old (4.4 ACEs vs. 3.6 ACEs, p = .009). Kerker et al. (2015) found that the number of caregiver-reported ACE exposures significantly predicted the child’s score on the CBCL; researchers tested this interaction term, but it was insignificant. For each additional reported ACE, the child’s odds of having a problem score on the CBCL increased 32-percent (OR 1.32, CI (95) 1.14–1.53, p < .001).

Physical Health or Medical Conditions

Flaherty et al. (2006) prospectively examined the association between ACE exposure and child health outcomes at 4 and 6 years of age. Flaherty et al. (2006) found children who had been exposed to one ACE (compared to 0 ACEs) at the age of four, were 1.89 times (OR 1.89; CI (95) 1.02–3.48; p < .05) more likely to have “poor general health” at their follow-up screening at age six. With regards to multiple ACE exposures, the authors found that four-year-old children who had four or more ACE exposures were 2.83 times [OR 2.83; CI (95) 1.10–7.31; p < .05] more likely to need medical attention for a reported illness. However, Flaherty et al. (2006) observed non-significant relations when examining zero ACEs verse two reported ACEs in relation to the child’s overall general health.

Kerker et al. (2015) also assessed the association between ACE exposure among young children (18–71 months) and health conditions. Kerker et al. (2015) specifically assessed chronic medical conditions such as: autism, anxiety, asthma, AIDS, arthritis/joint issues, attention-deficit/hyperactivity disorder, blood problems, cystic fibrosis, cerebral palsy, dental issues, Down syndrome, diabetes, depression, ear infections, hypertension, heart issues, hypertension, headaches, eating disorders, and other health issues. Such conditions were measured using a screening tool developed by Stein et al. (2013) specifically for Wave II of the NSCAW. Significant findings from Kerker and colleague’s secondary data analysis revealed that children with a chronic medical condition were exposed to more ACEs than their peers without chronic medical issues (3.9 ACEs vs. 3.4 ACEs, p = .009); however, this significant finding only held for older children in their sample between 36 and 71 months old (4.1 ACEs vs. 3.5 ACEs, p = .011), when stratified.

Mental Health Development

Grasso et al. (2016) was the single study that examined the relation between early childhood exposure to adversities and the adolescent’s mental or emotional health. Using retrospective adolescent self-reports, Grasso et al. (2016) measured posttraumatic stress symptoms consistent with the diagnostic criteria for PTSD, including re-experiencing, avoidance or numbing, arousal symptomologies, and the frequency of such experiences. For Epoch 1 (exposures experienced during birth and 5 years of age), findings varied for the three definite subgroups identified in the analysis. As measured by the University of California—Los Angeles Post Traumatic Stress Disorder—Reaction Index (UCLA PTSD-RI), descriptive characteristics revealed that adolescents who experience low-varied early childhood exposure were more likely to have re-experiencing symptoms in adolescence, with the domestic violence exposed subgroup being the most likely to exhibit avoidance/numbing symptoms in adolescence. The high-varied exposure subgroup was the most likely to exhibit hyper-arousal symptoms during adolescence. Further, adolescents who reported high-varied early childhood ACE exposure had higher total scores on the UCLA PTSD-RI, indicating clinical range PTSD symptoms. The high-varied exposure subgroup was predominately comprised of females (66.7%), this finding was consistence across all three developmental Epochs.

Academic and Literacy Skills

Jimenez et al. (2016) was the only study that examined the relationship between ACEs and academic performance. In their sample of 1007 kindergarten-aged children, Jimenez et al. (2016) found that as the number of reported ACE exposures increased, so did the likelihood of the children having below average language and literacy skills. This dose–response effect was maintained even after controlling for the child’s age, gender, maternal race and ethnicity, caregiver marital status, and household income. After adjusting for the controls, children with three or more ACEs were nearly two times as likely (OR 1.8, CI (95) 1.1–2.9) to have impaired language and literacy skills, compared to their kindergarten counterpart with no reported adversities. Further, when compared to kindergarten children with no adverse exposures, children with three ACEs were 1.5 times (OR 1.5, CI (95) 0.9–2.5) and 1.8 times (OR 1.8, CI (95) 1.1–2.9) more likely to have below average science/social studies and math skills, respectively, after inclusion of the covariates.

Social Development

Of the included studies, Kerker et al. (2015) was the only study to explore the association between early childhood ACE exposures and social development among a child welfare involved nationally representative sample. Kerker et al. (2015) found a significant interaction (p = .019) between the child’s age and reported ACE exposures. Every additional ACE exposure, reported by the caregiver, children, aged 36–71 months old, were 77% more likely to have a low score on the Vineland Adaptive Behavior Scale (VABS) socialization scales, which includes items assessing coping skills, interpersonal relationships, and play or leisure time. Additionally, when testing the full sample, ACE scores did not significantly differ by VABS score. However, a significant correlation emerged among older children (36–71 months) with more ACEs and low socialization scores on the VABS (5.4 ACEs vs. 3.7 ACEs, p = .049). When tested with the younger children (18–35 months), this correlation was not significant.

Discussion

This systematic review sought to bridge the gap between two, large, but interrelated, bodies of research, early childhood children and youth exposed to multiple ACEs, by examining and synthesizing current empirical research on early childhood exposure to ACEs and the social, behavioral, emotional, developmental, and/or overall wellbeing outcomes. There are several key findings from this systematic review. First, this review found, among the high-risk samples, high rates of cumulative ACEs among children between the ages of zero and 6 years old. Of the studies that provided exposure prevalence for the overall sample, between 12.3 and 70% of the early childhood samples were exposed to three or more ACEs. This finding is alarming given the correlation between early childhood stress and its implications for physiological and neurological development. Gerhardt (2006) reported that infants and young children exposed to chronic stressful environments have high levels of the stress hormone, cortisol, subsequently leading to adult aggressive behavior, vulnerability to stressful and sensitive situations as adults, and drug and alcohol seeking behavior for regulation. Further, Gerhardt (2006) stated high levels of cortisol damage important areas of the brain that undergo rapid development in the formative years, literally leaving children who have experienced traumatic events with smaller brains.

Findings of this systematic review also provide insight into early childhood ACE exposure and behavioral issues. Of the four studies that examined the association between ACEs and behavioral outcomes, all four studies provided evidence for a dose–response relation between cumulative ACE exposures and behavioral problems. Though, all four studies utilized the CBCL to measure child behavior, the behavioral scales used varied between studies, making specificity difficult. Among these young child samples, children with three or more ACEs were significantly more likely to exhibit externalizing (e.g., aggression, attention issues), internalizing behaviors (e.g., anxiety, somatic complaints), and overall problem behaviors than their counterpart with no ACEs.

Previous studies have demonstrated that early childhood behavioral, social, emotional, and developmental delays are associated with poor academic performance and decreased future employment opportunities (Bitsko et al. 2016). Given the young age range of this review, birth through 6 years old, only one study examined the association between ACEs and academic and literacy skills. Although, conclusions cannot be drawn about the impact of early childhood adversity on academic skills, academic abilities and competencies stem from the child’s social and behavioral development. It is important to recognize that Jimenez et al. (2016) found that cumulative early childhood ACE exposure was significantly associated with decreased language and literacy competencies, which underpin academic achievement across the lifespan. Therefore, it is likely that young children exposed to multiple ACEs are at increased likelihood to have poor foundational skills, predisposing them to low adult literacy and future educational achievement, both of which have empirical evidence of being associated with poor health outcomes (DeWalt and Hink 2009; Fiscella and Kitzman 2009; Shonkoff 2010). Despite these findings, Bitsko et al. (2016), in a Centers for Disease Control and Prevention: Morbidity and Mortality Weekly Report, highlighted the importance of early detection and intervention of low literacy rates among young children, stating that early identification of speech, language, and certain developmental delays increases the young child’s likelihood of outgrowing such impairments. Moreover, Fantuzzo et al. (2005) identified childhood participation in early learning experiences as a protective factor for vulnerable children, highlighting the need for early detection and intervention for young children exposed to cumulative ACEs.

Studies included in this review varied in the adversities measured, ranging between assessing for seven and seventeen ACEs. This review highlights the need for comprehensive definitions and measures of adversity, since it is difficult to compare studies examining varying numbers and definitions of adversities. Specifically, Grasso et al. (2016) assessed for seventeen ACEs, which in comparison to other studies, results may appear inflated with higher means. The current body of ACE research assesses adversities that range from child physical, emotional, or sexual abuse, household dysfunction, lack of parental love and comfort, to family stress and poverty (Kalmakis and Chandler 2014). This lack of consistent conceptualization and operationalization hinders research development and the growth of knowledge (Kalmakis and Chandler 2014). On the other hand, Finkelhor et al. (2007) argue for professionals to assess for a broader range of ACEs and victimizations, since research supports that adversities are highly comorbid and children who experience one ACE are likely to experience other, interrelated adversities. For this reason, Finkelhor et al. (2007) advocate for early interventions targeted specifically for children at risk of exposure to multiple adversities, stating that programs and therapies “… should be multifaceted, addressing multiple types of victimizations, as many of the risk factors for one type of victimization are shared among multiple types of victimizations.” (p. 9).

Another key finding is that none of the five studies addressed the role of theory nor addressed theoretical implications. Future researchers should examine the theoretical foundations of ACEs and the potential differences in the theoretical underpinnings of varying exposures. It is possible that exposure to physical neglect and physical abuse may be vastly different, theoretically. Kalmakis and Chandler (2014) state that theoretical development may also be hindered by the lack of consistent operationalization of ACEs.

Study Limitations

Studies examining early childhood exposure to multiple ACEs face several challenges, with perhaps the most obvious and predominate being the need to rely on caregiver or caseworker report of adverse exposures. Given this review’s target population age range, 0–6 years old, it is highly likely adverse exposures are underreported, especially in “at risk” populations, due to the inability of extremely young children to communicate and vocalize the exposures they have directly or indirectly witnessed. Also, some children may not realize they have experienced certain ACEs until they comprehend appropriate and inappropriate behaviors, therefore, it is probable that ACE exposures were underreported in all five studies included in this review. However, as the field of cumulative ACE exposure expands, there is an increasing need to examine and understand the compounding effects of ACE exposure at varying developmental stages to develop and tailor prevention and intervention services to young children with multiple ACE exposures.

As presented in Table 4, all five studies were descriptive in nature, and used large, high-risk samples, making generalizability difficult. Grasso et al. (2016) was the only study that attempted to measure age of exposure, which highlights the need for a systematic measure to assess for the age, frequency, severity, and chronicity of exposures, since it is plausible that age of onset, frequency, severity, and chronicity of exposure may have more severe implications than the number of exposures. Social desirability bias is another limitation of all five studies. Given that all the studies were apart of larger multi-region studies that utilized trained mandated reporters as interviewers, caregivers may have provided socially desirable answers and under-reported ACE exposures. Four of the studies relied on retrospective reports and adult recall, which also could lead to underreporting of ACEs. It is possible that children with social, behavioral, chronic health conditions, and poor overall health may be at increased risk of exposure to adversities during early childhood, and therefore, for cross-sectional designs, the temporal nature of the relations between early childhood ACE exposure and outcomes cannot be determined. Further, none of the included studies addressed sample power in their statistical analyses. Some of the included studies may have been statistically over-powered, which could result in falsely-identified statistically significant associations.

Table 4 Methodological quality
Full size table

The objective of this review was to systematically examine and synthesize research on early childhood exposure to ACEs and the social, behavioral, emotional, developmental, and/or overall wellbeing outcomes. Across a range of methodological approaches, all the included studies provided evidence for a dose–response association between ACEs and negative social, behavioral, psychological or emotional, academic, and physical health outcomes. Although four of the five studies relied on data drawn from studies that utilized random sampling methods and were nationally representative, the generalizability of these findings is limited due to the high-risk nature of the original samples. Nonetheless, this review fills a void in the literature and provides the initial steps towards developing a systematic and comprehensive body of literature on the negative implications of early childhood exposure to ACEs.

Social Work Research and Practice Implications

The social work profession is centered on helping and advocating for marginalized and vulnerable individuals, families, and communities to enhance their overall wellbeing, both individually and collectively. The findings from this review advance the current knowledgebase for professionals working with children and families who have been, or are at risk of, exposure to ACEs. Findings from this review reveal that children at the intersection of early childhood and multiple ACE exposures are more likely than their non-ACE-exposed early childhood counterpart to have detrimental and longstanding negative social, emotional, behavioral, emotional, development, and overall wellbeing implications. Revisions to current policies targeting vulnerable populations, such as child welfare and early childhood children, should incorporate the findings from this review, and attempt to decrease exposure to childhood adversity, especially among extremely fragile populations. Future policies and practices should attempt to minimize and eliminate exposure to ACEs, recognizing that ACEs are not just associated with negative social, emotional, behavioral, and wellbeing outcomes, but that cumulative exposure is associated with increased risk of future risky health behaviors, and unaddressed ACEs in one generation can lead to ACEs for their offspring (Larkin et al. 2014).

Given the interrelatedness of adversities, social workers who work with children and families in all capacities should regularly assess for a multitude of ACEs to better identify their most vulnerable clients. Continued training on ACEs for social workers, and similar professions, working with children and families will help professionals identify child behaviors commonly associated with traumatic exposure to adversities and how to effectively and sensitively communicate, advocate, and intervene with children who have compounding ACEs. Further, it is not uncommon for child welfare involved families to have parents with a ACE backgrounds themselves, and coupled with poverty and oppressions, these multitude of challenges created a unique, high-risk environment for their children (Larkin et al. 2014). Multidisciplinary teams consisting of social workers, psychologists, nurses, substance abuse counselors, and the like, should work together to create an integrated service delivery system that effectively empowers and supports such vulnerable families, and advocate within their community to raise awareness.

Future research should include prospective, longitudinal, and qualitative studies that measure the age of exposure, frequency, and chronicity of exposures. Prospective studies will help lead researchers to causal inferences about the sequence of ACE exposures and negative social, behavioral, psychological, physical, and overall wellbeing outcomes. Longitudinal studies will provide more insight into the importance of age of exposure and child developmental stages. A prospective longitudinal study that follows children from birth through childhood, with yearly assessments, would provide extensive insight into the significance of age of first exposure, the importance of frequency verse chronicity of certain exposures, developmental stages where children are most susceptible to both adverse exposures and negative implications, how outcomes vary by developmental stages, and how protective factors mediate outcomes across the developmental stages.

Nevertheless, the current state of literature on the wellbeing and developmental implications of young children, between the ages of zero and 6 years old, who have been exposed to adversities, is limited. There is a strong need for additional research on adversity in early childhood to provide further insights into when social, behavioral, developmental, and wellbeing implications may begin. In a 2007 report, researchers associated with the World Health Organization stated “economists now assert on the basis of the available evidence that investment in early childhood is the most powerful investment a country can make, with returns over the life course many times the amount of the original investment” (Irwin et al. 2007, p. 5). Shonkoff (2010) also stressed the importance of early childhood intervention, stating “neurobiology tells us that the later we wait to invest in children who are at greatest risk, the more difficult the achievement of optimal outcomes is likely to be, particularly for those who experience the early biological disruptions of toxic stress” (p. 365). Continued efforts to prevent early childhood exposure to multiple—and even single exposures—are needed; however, there is also a need to develop prevention and intervention services tailored to early childhood who have experienced adversity. This review, in conjunction with other research, provides evidence of the compounding implications of early childhood exposure to adversities, especially among high-risk, child welfare involved samples. Given the interrelatedness of adversities, the compounding effects of such exposures, and the negative implications across the lifespan in several developmental areas, funding mechanisms are paramount to develop innovative prevention strategies and interventions designed specifically for young children who have been exposed to ACEs, and will have direct and indirect benefits and cost savings for the child, their family, and society.