Background
Internalizing problems encompass the inner emotional distress of anxiety and depression, which are frequently comorbid [
1,
2] and often begin in childhood [
3]. To illustrate, Australia’s national Child and Adolescent Survey of Mental Health and Wellbeing found 10% of youth had internalizing problems [
4]. Anxiety disorder impacted 7% and major depressive disorder impacted 3% (over half with depression had comorbid anxiety). When internalizing problems develop in childhood they are associated with academic, social, and family dysfunction [
5]. Moreover, they can persist into adolescence and adulthood to become long term disorders, with substantial negative life impacts in social, educational, employment and health domains [
3,
6,
7]. In pediatric healthcare, understanding the development of internalizing problems in children is important, to inform discussions with families and facilitate early intervention.
Conceptual models include various risk factors and vulnerabilities that can influence the development of internalizing problems in children and youth [
8‐
10]. The premise of these models is that different factors may influence risk at different developmental time points, resulting in the manifestation of internalizing problems from childhood to adulthood. Rapee, Schneiring and Hudson [
10] summarised knowledge on key etiological factors for youth anxiety disorders as including genetics, temperament, parenting and individual experiences including adversity. They highlighted limited knowledge about child temperament in relation to mechanisms of environmental factors that requires population-based longitudinal studies. Hankin and colleagues’ [
8] review of vulnerabilities and risk mechanisms for youth depression and anxiety similarly highlighted the core roles of child affectivity, information processing biases, and stressors in different domains. They noted that conceptual and empirical work to understand the mechanisms that contribute to youth anxiety/depression are relatively novel in the field with considerable research required in this domain. Extensive childhood research has focused on family factors including parent mental health, parenting (overprotective and/or harsh approaches) [
11,
12] as well as genetic inheritance [
13,
14]. In sum, research has pointed to the key role of both genetic and environmental factors in the development of anxiety and depression.
Environmental factors such as early childhood traumatic events have been posed in literature as a risk for internalizing problems in youth [
15‐
17]. Trauma is defined as invoking actual or perceived significant harm to an individual or their loved ones [
18]. Some examples include vehicular accidents, physical injury, sudden death of a loved one, abuse, and natural disasters. The rate of exposure to such ‘high magnitude events’ for youth has been reported as 25–31% of the population (encompassing childhood and adolescence) [
19,
20]. For very young children in highly disadvantaged areas, rates can also be this high with 26% exposed to traumatic events that increase their risk for anxiety [
21]. Experiencing traumatic or adverse events in childhood is linked to the development of adolescent and adult psychopathology, including internalizing disorders [
22‐
25]. The neurodevelopmental mechanisms linking adversity to psychopathology include heightened emotional reactivity, threat-related information processing and disruptions in reward processing [
26,
27].
In another body of literature broader life-stressors place youth at risk for internalizing problems; events with negative consequences that outweigh a child’s ability to cope but are not considered traumatic [
15,
28]. Illustrative examples include parental separation, moving house, or death of a pet. In Costello and colleagues’ [
20] population study (
N = 1420) 29% of youth (age 9–13 years) had experienced a ‘recent stressor’ in the past 3 months. Research suggests that general life-stressors can contribute to youth internalizing problems [
29‐
31]. Children with anxiety disorders tend to experience more stressful life events prior to onset of the disorder and perceive these events as more impactful compared to their non-anxious counterparts [
28,
29,
32‐
35]. Life-stressors are also associated with youth depression [
36‐
41]. Studies suggest that uncontrollable and unpredictable events, especially earlier in childhood, are relevant to the development of anxiety while undesirable major stressful events link to depression onset [
8]. Although childhood adversity is a risk for internalizing, relatively little is known about how adversity exposure including its developmental timing influences mental health outcomes [
42].
One of the strongest child-focused risk factors for internalizing problems is known in empirical literature to be an inhibited temperamental style [
43,
44]. Inhibited temperament is the tendency for fearfulness/withdrawal in unfamiliar situations and is observed in 15–20% of children [
45,
46]. Studies show that inhibited children are at greater risk of developing anxiety disorders than non-inhibited children [
47‐
52]. Jaffee and colleagues [
53] also found that child inhibition was a significant predictor of adolescent depression (odds ratio 3.09,
p < .01). However, while inhibition is an important individual risk factor, not all children with this temperamental style develop internalizing problems.
Theory on ‘stress sensitization’ combines trauma and other life-stressors, posing that experiencing trauma early in life can sensitize individuals to having more negative reactions (such as internalizing symptoms) to other later life-stressors [
54‐
56]. Therefore, individuals who experience childhood trauma seem to require less stress later to produce a reoccurrence of symptoms [
35,
57]. McLaughlin, Conron, Koenen and Gilman’s [
58] population study (
N = 34,653 adults) found that exposure to childhood adversity plus stressful life events in the previous year predicted more internalizing problems for men and women, compared to prior year stressors without child adversity. One recent study of children focused on this theory. Grasso and colleagues [
15] explored stress sensitization amongst preschool age children in disadvantaged United States neighbourhoods. In their sample of 213 clinically referred and non-referred young children, 58% experienced a traumatic event in their lifetime (age 4-years) while 38% had recent life-stressors in the past 3 months. Recent life-stressors were more frequent for those young children who had experienced trauma (46% vs. 33%;
p < .05). Young children with the combination of traumatic events and recent life-stressors were at highest risk for internalizing problems, compared to those with trauma alone, recent stressors alone, or neither type of environmental stress.
In theory, temperamentally inhibited young children may be especially susceptible to environmental impacts due to their constitutional sensitivity to novelty/change [
59]. They may be particularly sensitive to negative events in their environment. Consequently, inhibited preschool children may therefore be especially prone to stress sensitization if faced with a combination of early life trauma followed by later life-stressors. In stress-sensitization theory, a traumatic event early in life can sensitize individuals toward more negative responses to later life stresses. Therefore, it is important to know whether inhibited young children experiencing trauma in early childhood are especially vulnerable to internalizing difficulties when faced with life stressors later in childhood. To our knowledge this hypothesis has not been explored empirically.
A couple of studies with inhibited children have more generally explored risk for internalizing problems when encountering daily stressors, or a traumatic event (but not stress-sensitization). Previously, Brozina and Abela [
60] explored whether inhibition predisposed school-age children (
N = 384, age 8–13 years) to negative impacts of daily hassles. They found symptoms of anxiety increased significantly amongst children initially high in inhibition who then experienced high levels of daily hassles across 6 weeks. Otto and colleagues [
61] explored whether early childhood inhibition was a risk for school-age children (
N = 105,
Mage 9-years) to develop posttraumatic stress symptoms after watching televised coverage of the 9/11 terrorist attacks. They found inhibited children were actually less likely to develop anxiety because they viewed less television after 9/11, thereby avoiding this potential stressor.
In sum, the mental health of young inhibited children may be especially sensitive to environmental impacts. Elucidating risk factors that further contribute to the development of internalizing disorders in this already vulnerable population can help clinicians to identify those who are most in need of early intervention. Therefore, this population study of inhibited preschool children aimed to explore whether those experiencing traumatic events in early childhood (6 months – 4 years) were especially vulnerable to later internalizing difficulties when faced with more general life-stressors in mid childhood (7–10 years). We expected inhibited children with early-life trauma would have more internalizing difficulties in mid childhood compared to those without early-life trauma. We predicted inhibited children with more general life-stressors recently in mid childhood would have higher internalizing difficulties. We then anticipated early-life trauma would sensitize inhibited young children to negative impacts of later general life-stressors, with the combination leading to highest mid-childhood internalizing difficulties.
Results
In this population sample of inhibited preschool children, 29 (6%) had experienced a potentially traumatic event in the first 4 years of life, of whom 83% experienced one traumatic event (n = 24) and 17% experienced two traumatic events (n = 5). The types of potentially traumatic events experienced by inhibited preschoolers in the general population were - child hospitalization (41%, n = 12), death of a loved one (38%, n = 11), parent life threatening illness (14%, n = 4), natural disaster (7%, n = 2), parent abandoned child (3%, n = 1), parent removed from home (3%, n = 1), sibling life threatening illness (3%, n = 1), car accident (3%, n = 1), and animal attack (3%, n = 1).
Later in mid childhood, 256 (56%) of the children in this general population sample of inhibited pre-schoolers experienced broader recent life-stressors (Table
2). Most commonly this was separation from a parent for a week or longer, reported for one in 10 inhibited children. The other more common life-stressors were moving home, death of a pet, parent hospitalization, or a significant person moving away. Report of early-life trauma was not associated with number of recent life-stressors in mid-childhood (ANCOVAs:
wave 1:F (1,405) =0.00,
p = 0.948;
wave 2:F (1,404) =0.28,
p = 0.559;
wave 3:F (1,411) =0.04,
p = 0.836). The group of children with early trauma did not differ significantly from the group with later life stressors on demographic variables in Table
1, apart from the proportion of children with single or widowed parents (21% in early trauma group vs 8% in later life stressors group,
p = 0.037).
Table 2
Recent Life Stressors Experienced by Inhibited Children in Mid Childhood
Separation from parent for a week or longer | 9.9 (44) | 11.5 (49) | 11.3 (48) |
Moving home | 4.9 (22) | 6.8 (29) | 7.5 (32) |
Death of a pet | 6.3 (28) | 4.2 (18) | 6.1 (26) |
Parental hospitalization | 4.7 (21) | 5.4 (23) | 3.3 (14) |
Significant person moving away | 3.8 (17) | 4.5 (19) | 3.5 (15) |
Childcare change | 5.0 (22) | 3.8 (16) | 2.6 (11) |
Parental separation/divorce | 3.1 (14) | 4.2 (18) | 1.6 (7) |
Reduction in standard of living | 2.0 (9) | 2.1 (9) | 3.1 (13) |
New children in home | 2.5 (11) | 2.6 (11) | 1.6 (7) |
Unsafe neighborhood | 1.4 (6) | 2.6 (11) | 1.2 (5) |
New parent figure | 2.0 (9) | 1.2 (5) | 1.4 (6) |
Loss of home | 0.5 (2) | 0.2 (1) | 0.7 (3) |
Relationship between early-life trauma and mid-childhood internalizing symptoms (anxious and depressive) was assessed by MANCOVA for parent and child report at each wave (see Table
3). Inhibited preschoolers with and without early-life trauma differed on later mid-childhood internalizing symptoms in two of the three waves by parent report, with small effect sizes (Table
3, partial η
2 = .02) [
77]. Subsequent ANCOVAs (see Table
4) showed that the inhibited children with early trauma had more anxious but not depressive symptoms, than those without early trauma in mid childhood in these waves. The inhibited children with early trauma also had more anxiety disorder in one wave than those without early trauma (
wave 1: 52% vs. 41%, χ
2(1,
n = 405) =1.35,
p = 0.245;
wave 2: 55% vs. 36%, χ
2(1,
n = 402) =4.39,
p = 0.036;
wave 3: 38% vs. 27%, χ
2(1,
n = 403) =1.52,
p = 0.218). However, the child report MANCOVA showed no relationship between early trauma and mid childhood internalizing difficulties.
Table 3
Early Life Trauma and Mid Childhood Internalizing Symptoms: MANCOVA results
Parent report |
Wave 1 (age 7–8 years) | 2, 403 | 2.34 | 0.99 | 0.097 |
Wave 2 (age 8–9 years) | 2, 400 | 4.12 | 0.98 | 0.017 |
Wave 3 (age 9–10 years) | 2, 409 | 5.07 | 0.98 | 0.007 |
Child report |
Wave 1 (age 7–8 years) | 2, 368 | 1.59 | 0.99 | 0.206 |
Wave 2 (age 8–9 years) | 2, 372 | 0.41 | 1.00 | 0.665 |
Wave 3 (age 9–10 years) | 2, 380 | 0.74 | 1.00 | 0.477 |
Table 4
Subsequent ANCOVA Results for Early Life Trauma and Mid Childhood Internalizing Symptoms
Anxiety (SCAS) |
Wave 2 (age 8–9 years) | 26.01 (2.35) | 19.15 (0.65) | 7.92 | 0.005 |
Wave 3 (age 9–10 years) | 25.82 (2.34) | 18.11 (0.64) | 10.15 | 0.002 |
Depression (SMFQ) |
Wave 2 (age 8–9 years) | 3.65 (0.75) | 2.73 (0.21) | 1.40 | 0.238 |
Wave 3 (age 9–10 years) | 3.55 (0.69) | 2.43 (0.19) | 2.46 | 0.118 |
Life-stressors in mid childhood concurrently related to inhibited children’s internalizing difficulties. Anxiety symptoms showed small positive correlations by parent and child report in two of the three waves (wave 1: parent report, r = 0.07, p = 0.151; child report, r = 0.10, p = 0.064) (wave 2: parent report, r = 0.18, p > 0.001; child report, r = 0.11, p = 0.035) (wave 3: parent report, r = 0.13, p = 0.007; child report, r = 0.13, p = 0.011). Depression symptoms showed small positive correlations by parent report in the second wave and child report in the third wave (wave 1: parent report, r = 0.06, p = 0.176; child report, r = 0.06, p = 0.243) (wave 2: parent report, r = 0.15, p = 0.002; child report, r = 0.10, p = 0.060) (wave 3: parent report, r = 0.09, p = 0.077; child report, r = 0.12, p = 0.018). ANCOVA compared recent life-stressors for children with and without anxiety disorder. Inhibited children with anxiety disorder experienced more recent life-stressors than those without anxiety disorder at one wave (wave 1:M (SD) 0.45 (0.76) vs, 0.46 (0.90), F (1,426) = 0.01, p = 0.962) (wave 2:M (SD) 0.68 (1.14) vs. 0.39 (0.77), F (1,413) = 10.31, p = 0.001) (wave 3:M (SD) 0.47 (0.84) vs. 0.41 (0.79), F (1,408) = 0.91, p = 0.341).
MANCOVA then tested for stress sensitization, comparing internalizing symptoms for the four groups - early trauma plus recent stressors, early trauma only, recent stressors only, or neither type of environmental stress. There was a difference between the four groups on child internalizing symptoms by parent report in the second and third wave (see Table
5 for details). Subsequent ANCOVAs (see Table
6) indicated a difference between the four groups on anxiety and depression symptoms at those waves. However, inhibited children with both early trauma and recent life-stressors did not have the highest internalizing symptoms. Rather, children with early trauma only, or recent life-stressors only, had more internalizing symptoms than the group without trauma or stressors (see Table
6). Likewise, there were no differences in anxiety disorder between the four groups to indicate stress sensitization (trauma plus stress, stress only, trauma only, neither) (
wave 1: 58% vs. 42% vs. 47% vs. 40%, Fisher’s exact test =2.04,
p = 0.554) (
wave 2: 50% vs. 43% vs. 59% vs. 33%, Fisher’s exact test =7.96,
p = 0.045) (
wave 3: 33% vs. 29% vs. 40% vs. 27%, Fisher’s exact test =1.82,
p = 0.611).
Table 5
Four Groups with Different Combinations of Early Life Trauma and Recent Life Stressors: MANCOVA results
Parent report |
Wave 1 (age 7–8 years) | 6, 802 | 1.49 | 0.98 | 0.176 |
Wave 2 (age 8–9 years) | 6, 798 | 3.58 | 0.05a | 0.002 |
Wave 3 (age 9–10 years) | 6, 814 | 3.58 | 0.51a | 0.002 |
Child report |
Wave 1 (age 7–8 years) | 6, 732 | 1.64 | 0.97 | 0.133 |
Wave 2 (age 8–9 years) | 6, 740 | 1.38 | 0.98 | 0.219 |
Wave 3 (age 9–10 years) | 6, 756 | 1.30 | 0.02a | 0.254 |
Table 6
Subsequent ANCOVA Results for Four Groups with Different Combinations of Early Life Trauma and Recent Life Stressors
Anxiety (SCAS) |
Wave 2 (age 8–9 years) | 17.80 (0.76), 270 | 22.60 (1.21), 17 * | 28.25 (3.03), 106 * | 22.81 (3.60), 12 | 6.88 | < 0.001 |
Wave 3 (age 9–10 years) | 17.05 (0.75), 276 | 20.90 (1.21), 20 * | 26.00 (2.79), 108 * | 25.41 (4.17) 9 | 5.82 | 0.001 |
Depression (SMFQ) |
Wave 2 (age 8–9 years) | 2.37 (0.25), 270 | 3.64 (0.39), 17 | 4.11 (0.98), 106 * | 3.01 (1.16), 12 | 3.16 | 0.025 |
Wave 3 (age 9–10 years) | 2.21 (0.23), 276 | 2.97 (0.36), 20 | 2.80 (0.83), 108 | 5.24 (1.24), 9 | 2.77 | 0.041 |
Discussion
In stress sensitization theory, a traumatic event in early life may sensitize individuals toward more negative responses to later life-stressors [
54‐
56]. In temperament theory, inhibited children can be particularly sensitive to the effects of their environment [
59]. Therefore, this population-level study of inhibited preschool children explored whether those experiencing traumatic events in early childhood were especially vulnerable to later internalizing difficulties when faced with more general life-stressors in mid childhood. Six percent of inhibited preschoolers in the Australian context (one in every 14) experienced a potentially traumatic event in their first 4 years of life. Later in mid childhood, 60% of the inhibited population sample experienced broader life-stressors. Those inhibited children with early-life trauma developed slightly more mid-childhood anxiety symptoms and clinical disorder (parent source). Those facing higher general life-stressors in mid-childhood also had slightly more anxious and depressive symptoms (parent and child sources). However, in regard to stress sensitization, those inhibited children with the
combination of early-life trauma and broader recent life-stressors did not show the most mid-childhood internalizing problems.
Although stress-sensitization effects were not detected for inhibited children in the Australian general population, our findings (as in the United States) [
21] indicated higher anxiety symptoms in children exposed to traumatic events. However, our sample had comparatively low levels of exposure to traumatic events, and even lower levels of exposure to multiple traumas. It could be that multiple severe traumas are the level to induce stress sensitization [
78,
79].
A reason for the lower level of traumatic events in our sample may have to do with economic factors. Compared to the general Australian population, traumatic events experienced by young children in the United States (USA) might be higher due to economic disparity [
80,
81]. In the USA, 25–31% youth (spanning childhood and adolescence) in the general population experience potentially traumatic events [
19,
20]. In disadvantaged locations trauma rates for very young American children are also at this rate (26%) [
21]. It is possible that our sample had lower rates of traumatic events than the samples included in US studies of sensitization because of economic disparities between those samples and ours. Briggs-Gowan and Grasso’s [
15,
21] sample included 45% of families in poverty and severe trauma (family violence, burns, serious falls, poisonings). Their stress sensitization finding for young children was with clinically referred and non-referred children from poor USA neighborhoods (45% poverty, 48% in treatment). Two thirds experienced early childhood trauma, of which a third were multiple traumatic events (up to four). Our population sample of inhibited preschoolers in Australia, by comparison, had experienced fewer traumatic events in early childhood (6%, usually one and occasionally two events). Stress sensitization may take place only with early severe and prolonged adversity. Perhaps in line with this view, a study of adults by McLaughlin et al. [
58] found 3+ ‘adverse childhood experiences’ forecast adult internalizing problems whereas 0–2 did not (although this was not restricted to trauma).
It is also possible that inhibited young children have lower trauma exposure because of their cautious, novelty- and risk-avoidant behavioral style, together with that of their parents (biological relatives, due to genetic factors). Along this line, children with attention deficit hyperactivity disorder (ADHD, extreme behavioral disinhibition) tend to experience more traumatic events than those without this disorder [
82]. For clarification, rates of trauma would need to be compared between inhibited and non-inhibited children in epidemiological samples.
Interestingly, the number of broader life-stressors experienced by inhibited children in mid-childhood seems higher than population rates for children in general. Costello and colleagues [
20] reported that 29% of youth aged 9–13 years experienced recent stressful events in the past 3 months. In our inhibited sample 56% of children aged 7–10 years experienced recent life-stressors in the past 3 months. Temperamentally inhibited children do tend to experience a variety of daily-life circumstances to be stressful in comparison to their uninhibited peers. Also consistent with other international studies, we found that more mid-childhood life-stressors was associated with more child internalizing problems [
28‐
35,
37]. It is clear that temperamentally inhibited children facing general life-stressors are at some risk for developing internalizing problems.
The present study had the following limitations. Families facing high adversity may have chosen not to participate in the initial population trial. However, the sample’s demographics show that many families facing risks were recruited. While tertiary educated parents and families with more income were over-represented, still 12% of parents had not completed high school and one in five families had a welfare card that indicates financial hardship. The sample included a wide range of parent education and family income (including lower incomes
1), along with diversity in cultural backgrounds. Longitudinal retention was high and those retained in mid childhood were sociodemographically comparable. The study findings can therefore be generalised to a wide socioeconomic spectrum, although perhaps not the lowest end [
65]. Another limitation is that retrospective report by parents of traumatic events in their child’s life relies on memory. Conceptualisation of early trauma and recent life-stressors in our study aimed for consistency with definitions in Grasso et al.’s early childhood study [
15]. While both included retrospective report of potentially traumatic events, recall in Grasso et al.’s study [
15] was over a shorter period of time. It may be that some traumatic events that took place in inhibited young children’s lives were forgotten by their parents over time in the present study.
With these limitations in mind, findings of this study could have some positive clinical implications. Many parents with inhibited young children express concerns about whether some negative event ‘caused’ their child to be highly anxious. Clinicians may therefore be able to offer reassurance to many parents with inhibited youngsters that it is relatively uncommon for children of this temperamental style to have experienced traumatic events. Further, for those that have, effects on their mental health in the longer term appear to be small. Guidance could be offered to foster resilience. Children can model distress behaviors by observing parents’ own responses to life stressors [
10,
12,
83‐
87]. For parents to maintain calm and positive parenting could therefore be protective when their inhibited child is faced with stressors [
10]. Early prevention programs have been developed to facilitate inhibited children’s resilience to life stressors as they grow. Cool Little Kids, for example, guides parents in responding to their young child’s emotional distress in ways that develop positive coping skills [
69]. Buthmann and colleagues note a need for deeper understanding of interactions between parental mood and temperamental reactivity of the developing child that could be explored in future early intervention research for internalizing [
88].
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