Background
Children exposed to multiple risk factors in the sociodemographic and parental health domains are much more likely to exhibit problems in health and development than those who are exposed to a single or no risk factor [
1‐
5]. For example, although teen-age motherhood is considered an important risk factor for poor child development [
6,
7], by itself it only identifies a small portion of children with early difficulties [
8]. In comparison, when teenage motherhood is added to a combination of other risk factors (e.g., low maternal education, maternal depressive symptoms), identification of children at risk of poorer development improves substantially [
8].
This argues for the utility of the cumulative risk (CR) model, which accounts solely for the number of risk factors to which a person is exposed rather than the intensity of or unique set of risk exposures [
9]. Risk factors are defined dichotomously (e.g., motherhood at ≤ age 19 vs. > age 19) and then summed, ignoring the combination of risk factors [
10,
11]. Indeed, the particular set of risk factors appear less important for developmental impact than the number of factors to which a child is exposed [
9‐
11].
In addition to the consistent finding that cumulative, relative to single or no, risk exposures have worse consequences for children’s health and development [
2,
3,
9,
10,
12], there are substantive reasons for the widespread use of the CR model. Children are typically faced with constellations of risks rather than an isolated instance because risk exposures often co-occur (e.g., single-parent family, low-income household, crowded residence, high crime neighborhood, low quality schools) [
9,
13]. Furthermore, some of the developmental correlates of major sociodemographic factors, such as poverty, are explained, in part, by exposure to multiple risk factors [
14]. Finally, CR exposure research and theory is important because the number of children confronting multiple risk factors is large and expanding around the world [
15].
Yet, despite this understanding being identified at least 20 years ago [
2], the CR approach has rarely been applied to illuminate problems in
early childhood development, because most CR research has focused on school age [
9]. We are aware of only one study [
12] that has examined how different developmental trajectories of CR exposure occurring in the critical early childhood period, from conception to about 2 years of age [
16‐
19], are associated with subsequent child outcomes. Examining the same sample using the same methods as in the present study, that prior study was focused on the single outcome of total behavior problems [
12]. Children exposed to any more than a consistent level of zero risk factors in the first 1000 days of life had a higher likelihood of being reported with a clinical level of total behavior problems at 4.5 years [
12]. Consistent exposure to four or more risk factors across this early period had the highest prevalence at 44% [
12]. Stimulated by these alarming findings, the question remains whether developmental trajectories of CR exposure are associated more broadly beyond behavior problems, potentially affecting children’s health and development more generally. Moreover, little is known about whether the persistence in and timing of CR exposure during this early life period matter for development. Such findings can inform when and how early screening and interventions should occur.
The allostatic load model [
20] highlights the cumulative impact on the body caused by repeated mobilizations of multiple physiological systems in response to risk exposure, and can illuminate how CR can cause developmental disturbances [
21,
22]. Indeed, CR in school age children has been shown to predict allostatic load both concurrently [
11] and prospectively [
23]. More frequent and persistent risk exposure elevates stress and accelerates this impact [
20‐
23]. Moreover, when allostatic load occurs repeatedly, the physiological response systems become recalibrated, remaining on alert and altering their sensitivity to stresses. As well, the malleability of these response systems is compromised, so that they become less proficient in returning to a resting state when the stress desists. Response capabilities are therefore diminished by exposure to CR [
20‐
23]. Drawing from the allostatic model, therefore, we would expect that any CR, but especially persistent CR exposure as well as CR exposure occurring closer to the developmental outcomes of interest, would predict problems in those outcomes.
We examine here the prospective longitudinal associations between exposure across three times in early development to CR, accounting for a range of sociodemographic and maternal health risk factors, and problems in development right before the start of formal education at age 4.5 across health, behavior, and education-related domains. Based on the allostatic load model [
20‐
22], we hypothesize that there will be a higher likelihood of problems across domains when: (1) there is exposure to high CR at any point in early development compared with not at all; (2) exposure to high CR occurs persistently in early development compared with less persistently, and (3) when the timing of first exposure to high CR occurs later in this period and closer to the developmental outcomes compared to earlier.
Discussion
Using longitudinal data that prospectively followed over 5800 children from the late antenatal period until 4.5 years of age, we examined how different patterns of timing and persistence of exposure to a high level (four or more) of sociodemographic and maternal health risk factors in early childhood were associated with problems across health, behavior, and education-related developmental domains. Results indicate that exposure to more than consistently zero risk factors, compared to those who consistently experienced zero risk factors over the early childhood period, is associated with a significantly higher likelihood of experiencing problems shortly before the start of elementary/primary school across nine of 10 outcomes, These outcomes included overall health, obesity, internalizing and externalizing behavior problems, letter naming, counting forward and backward, and mother having concerns about the child starting school and expecting the child not to continuing past secondary education. Consistent exposure to a high level of risk factors in early development was generally associated with the highest prevalence of problems. The exception was injuries, for which there was no association with risk exposures. These findings were therefore generally consistent with Hypothesis 1.
Consistent with Hypothesis 2, the likelihood of experiencing problems in these domains was generally reduced if exposure occurred less persistently, that is on one or two rather than all three assessment periods (antenatal, 9 months, 2 years). Nonetheless, even exposure to high level of risk factors on only one occasion was associated with a significantly elevated likelihood of problem outcomes compared to consistent zero risk exposure. The timing of one dose of high level of risk exposure in early development, whether at antenatal, 9 months, or 2 years, did not generally matter. The exception was that children exposed to a high level of risk factors on one occasion after birth, compared to only in the antenatal period, experienced a reduced likelihood of obesity. Therefore, findings regarding timing of exposure were generally inconsistent with Hypothesis 3. Taking the results overall, it appears that the effect from any exposure to CR is more important than the timing of it. It may also be that timing effects required examining relatively small subsamples, resulting in reduced power to detect consistent differences.
These findings are consistent with those from the few previous studies that have examined CR exposure as early in development as here. We are only aware of three other comparable studies. Sameroff and colleagues reported over 20 years ago that as CR increased, social adjustment and intelligence test scores decreased for children at 4 years of age [
2,
44]. Results more recently from the Avon Longitudinal Study showed that a markedly larger portion (49% vs. 6%) of children with poor development by age five could be identified based on a CR model considering six factors rather than based only on the single exposure to teenage motherhood [
8]. Likewise, a prior study of CR with this same NZ sample using the same methods as used here showed a significantly elevated risk for the single outcome of total behavior problems at 4.5 years of age associated with CR that was not consistently at zero in early childhood [
12]. Internalizing and externalizing behavior problems were not differentiated in that study.
The present study is the first providing evidence that CR exposure is associated with elevations in problem prevalence broadly across multiple developmental outcomes, comprising health, behavior, and education-related domains. The present study also expands on previous findings to show that, although problems are most prevalent for children consistently exposed to high CR in early development, they are elevated even when exposure to a moderate level of CR (1–3 risk factors; trajectory #2) occurs in early childhood rather than remaining consistently at zero. These findings are disquieting because, whereas only about 10% are exposed to a high level of risk (CR ≥ 4) at any time, over 50% of NZ children are exposed to at least one of these risk factors at some point during their early development. Consequently, about one-half of all children may have a significantly elevated likelihood of experiencing problems in health, behavior, and/or education-related domains already prior to their starting formal schooling. It will be important to examine whether this risk continues to manifest itself as the children develop. This will be possible as GUiNZ continues to assess the cohort [
24].
The CR approach is distinguished from examining adverse childhood events (ACEs), which is a distinct separate area of research. ACEs focuses on dysfunctional family experiences in childhood, which are self-reported retrospectively usually in adulthood, and typically without consideration for when in development they occurred [
45,
46] The risk factors considered in the CR index examined here cover a considerably broader range of maternal sociodemographic and health-related conditions, most of which do not indicate dysfunction per se. Moreover, they are measured concurrently in our methodology.
Although the focus in this study has been on problem outcomes associated with exposure to high risk, it should be noted that the majority of children exposed even to a consistent high level of risk do not evidence problems at age 4.5. Future research must identify promotive and protective factors that support resilience in those children despite their high risk exposure. For example, although consistent exposure to high CR in early childhood is associated with a six-fold increased likelihood of being reported with an abnormal level of externalizing problem behaviors, over 70% of the children with this identical high level of risk exposure are not. What is it about them and/or their context that may contribute to their resilience?
The cumulative risk index used here includes various types of conditions. Some would be difficult to improve directly, such as neighborhood deprivation, but other risk factors appear more readily modifiable. Routine screening is feasible during pregnancy for several of the risk factors considered here [
8]. For example, smoking during pregnancy can be reduced through intervention. Moreover, establishing pathways for mothers to continue or return to completing their education could directly reduce risk exposure as well as cause secondary, but important positive effects. For example, increasing education completion likely leads to better job prospects, which can lead to higher income, which can lead to affording less crowded housing possibly in less deprived neighborhoods.
Limitations
The CR approach has some shortcomings. The designation of each of the risk factors here is arbitrary. Likewise, although with precedent from previous research [
12,
29], so is designating as “high” risk the presence of four or more risk factors. Furthermore, information on risk intensity is lost with the CR approach and the CR index is additive precluding the possibility of statistical interactions between risk factors [
9]. The determination of risk exposure here relied on maternal self-report, except for neighborhood deprivation. However, all but two of those risk factors (maternal depression and health) required maternal report of objective conditions. It should be considered that reporting on subjective conditions can vary due to educational attainment and emotional state. Some problem outcomes were present in a small number of children in some of the least prevalent CR trajectories, resulting in wide confidence intervals and low power to detect significant associations.
The OR should not be interpreted to represent the risk of a problem outcome associated with a certain trajectory of CR as it is known to provide an overestimate of the risk when outcomes are more than rare (generally > 10%) [
47]. This is the case especially for the education-related outcomes Even though GUiNZ has one of the highest retention rates among longitudinal birth cohort studies [
48], the analysis sample lost to follow-up at age 4.5 reflects retention bias, as is typical. Because this group was over-represented by children with higher vulnerability (e.g., single parenting, deprived neighborhood), the associations reported here are likely underestimates of the true association between CR exposure and problem outcomes. Finally, these data were collected in NZ, which manifests a distinct social context such that generalizing to other contexts should be done cautiously.
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