Background
An inverse socioeconomic gradient has been documented for numerous health outcomes, including adult and infant all-cause mortality and a large number of diseases, health conditions, and health status measures [e.g., [
1]]. This may not generalize to adolescent populations, however. West argued, based mainly on the results of studies of social class and adolescent health from the United Kingdom, that there seemed to be a relative equalization of the SES-health gradient during adolescence, followed by a re-emergence in early adulthood [
2]. A more complex picture has emerged from additional research, where inverse relationships have been observed inconsistently across measures of SES, health outcomes, and adolescent characteristics [e.g, [
3‐
6]]. Social determinants of adolescent health, including SES, may be particularly relevant to the study of adolescent injuries; this area of research is the focus of our study.
Injuries are unique among health conditions because of their acute nature, and because they are, by definition, externally caused. This external causation emphasizes the potential relevance of the physical and social environment, including socioeconomic factors, in contributing to injury risk. Research to date has consistently demonstrated that SES is inversely associated with injury mortality in both children and adults, but findings have been less consistent for non-fatal injuries [
7]. The literature addressing SES and injury morbidity among adolescents has been particularly inconclusive. While some studies including adolescents have reported an inverse SES-injury gradient [
8‐
17], others have failed to observe such a gradient or have even found some evidence of a positive association between SES and injuries among youth [
18‐
24].
One possible explanation for these discrepant research findings is that SES may be differentially related to injuries according to cause. For example, there is some evidence that SES may be positively related to sports and recreation injuries among children and youth [
23‐
25], yet inversely associated with some types of traffic injuries [e.g., [
10,
15,
17]] and intentional injuries [e.g., [
10,
26,
27]]. This might be explained in part by differences in the intermediate factors or pathways that could account for an SES-injury relationship. For example, SES may be related to injuries in children and adolescents through such factors as parental care and supervision [
28,
29], the neighbourhood environment [
30], and behaviours [
24]. These characteristics likely relate differently to injury risk from different causes (for example, physical activity behaviours may be related to recreation injuries, while the neighbourhood environment may be important for traffic-related injuries).
A second and related possible explanation for the inconsistent findings in previous studies is the inclusion of different measures of SES. There is no consensus among researchers on how aspects of social position should be conceptualized, or how terms such as social class, SES, and socioeconomic position should be defined and used. For example, some authors distinguish between concepts of status or prestige and class or economic resources [
31‐
33], while others distinguish among dimensions of social stratification based on access to various forms of capital (including material or financial capital, human capital, and social capital) [
34‐
36]. Some also draw a distinction between social class itself, which is defined in terms of societal relationships, and the manifestation of social class in various dimensions of socioeconomic position [
31]. In this paper we use the term SES to refer to all aspects of social standing collectively, by convention. There is also no agreement on how social position is best measured. Some nations (such as the United Kingdom) have a long tradition of viewing social stratification in terms of occupational class [
37], while North American research has tended to place greater importance on income, education, and occupational status in relation to income and education [
36]. However measured, though, it is generally recognized that different indicators of SES (such as household income, education, occupational status or class, and neighbourhood characteristics) tend to reflect distinct underlying aspects of social position, and thus may be expected to be differentially related to health outcomes [
33].
Given the above complexities, it is not surprising that it is challenging to identify which of the various aspects of SES is most relevant for adolescent populations [
5,
38] and for injury outcomes. The possible pathways through which SES may be related to adolescent injury include behaviours (e.g., physical activity and risk behaviours) and environmental characteristics (e.g., potential hazards in the physical environment) that could be influenced, for example, by both status and access to material resources. Another possible contribution of SES measurement to inconsistent research findings in relation to adolescent injury is the notion that SES indicators may hold different meaning for different adolescent populations, depending on social and economic policies [
10]; this may help to explain international differences in research findings.
A greater understanding of the nature of the relationship between SES and injuries among adolescents could help to inform injury prevention priorities and would contribute to a wider body of knowledge addressing the social foundations of adolescent health. Toward this understanding, we empirically examined possible explanations for the inconsistent results among previous studies. Specifically, this study uniquely aimed to identify whether the observed relationship between SES and adolescent injury differed according to: i) different measures of SES; or ii) different causes of injury (recreation injuries versus non-recreation injuries).
Discussion
Summary
The results of this study suggest that differences related to the measures of SES chosen and the causes of injury under study may both contribute to discrepancies in past research on SES and non-fatal injuries among adolescents.
Measures of SES
The most striking result in our study was the variability in the observed relationship between SES and injuries across different measures of SES. This is not entirely surprising, given the discrepant findings in previous studies. No consistent trends in the SES-injury association are apparent in the literature when comparing studies incorporating parental and household SES indicators based on parental occupation [
10,
20,
22,
24], parental education [
18,
23], household income [
23], or adolescent perceptions of family affluence [
22,
24]. There is an intriguing pattern, though, whereby studies that have measured SES at the neighbourhood level have tended to observe an inverse relationship between SES and total (rather than cause-specific) non-fatal childhood or adolescent injuries [
8,
9,
11‐
13,
15‐
17], while studies that have focused on individual SES indicators have been less consistent, with several finding no relationship or a positive association between SES and injuries [
20,
22‐
24]. Interpretation of this pattern is limited by other differences between the two groups of studies, though. For example, studies incorporating area-based indicators of SES have tended to be ecological in nature, comparing population-based rates of health care use for injuries across neighbourhoods or regions [
8,
9,
11‐
13,
15‐
17], while studies relying on individual-level SES measures have tended to be based on injury self-reports among a defined sample of children or adolescents [
20,
22‐
24] (with some exceptions [
10,
18]). These two types of studies (ecological and individual) likely differ in terms of the distribution of included injuries according to severity and cause as well as in their measures of SES, which may help to explain the differing findings (particularly in view of evidence that the observed SES-injury relationship likely depends on the cause of injury [
10,
24,
49]). The two groups of studies may also be subject to different types of potential biases (for example, recall bias in studies based on self-reports, and possible biases related to health care system factors in studies based on medical records) [
50]. Further, our study allowed for a comparison of the observed SES-injury relationship across both neighbourhood and individual-level SES measures within the same sample, and the findings did not follow the literature trend; any evidence of an association between SES and injury was positive.
Potential explanations for the heterogeneity we observed across SES indicators include conceptual differences in the SES measures (i.e., in terms of the underlying constructs captured), differences in measurement error, or chance. While chance may account for the few statistically significant gradients observed, it seems unlikely to account for the variability across measures.
Regarding measurement error, the level of missing data was relatively high for household income and for the Blishen SES index. In previous analyses with this sample, adolescents who were missing household income information were less likely to be living in higher income neighbourhoods; thus, "missingness" was not completely random [
51]. The heterogeneity of the results is still apparent, though, excluding the findings related to household income. The household and parental SES measures were sometimes reported by the adolescent and sometimes by a household adult. Neither SES levels nor correlations among the SES measures differed systematically based on the reporter, however [
51].
It seems likely that conceptual differences among the measures of SES account for some of the heterogeneity in our findings. This is supported by pairwise correlations among these measures in the sample, which were generally below 0.6 [
51]. The results do not allow us to define which aspects of socioeconomic position are captured by the different SES indicators, or which indicators are most salient for adolescents. For example, at the individual level, income may be seen as an indicator of material resources, while occupation and education may reflect both economics and status or prestige [
32]. When parental indicators are incorporated, there is the added complexity that any influence of occupation and education is indirect. It is possible that the positive relationship we observed between the Blishen SES index and injuries among males is a reflection that this indicator is more meaningful for male youth, relative to other SES measures; indeed parental education may have lower relevance for some adolescents because it reflects the more distant past [
52]. If additional research supports the positive association, potential explanatory pathways could be explored. For example, we could explore whether the SES index reflects social status among adolescents, and if so, whether social status in turn is associated with injury risk behaviours among male youth.
Correlations between individual-level and neighbourhood indicators of SES for both genders were typically below 0.30 in our study [
51], suggesting that they were measuring different constructs, or that the neighbourhood indicators were poor proxies for individual SES. Even when used as proxies for individual status, area-based indicators of SES may capture area-level characteristics [
53]; neighbourhood SES indicators may be particularly relevant for injuries due to their potential association with characteristics such as physical hazards [
30]. This does not seem to be a plausible explanation, though, for our observation of a positive relationship between the neighbourhood indicators of SES and injuries among females. Again, potential intermediate factors could be explored in future research. For example, neighbourhood SES measures may be proxies for aspects of material resources among adolescents, and female youth with higher material wealth may have greater access to injury risk activities.
Causes of injury
There was some evidence that the relationship with SES was different for recreation versus non-recreation injuries. For females, the positive SES-injury gradient was generally more consistent for recreation injuries. Similarly, among males, the parental Blishen SES index was positively related to recreation injuries but not non-recreation injuries. This positive association is consistent with previous research [
23‐
25,
49]. Behaviours such as overall levels of physical activity could be explored as possible intermediate factors in the relationship between SES and recreation injuries; such behaviours may be associated with recreation injury risk among adolescents [
54,
55], and adolescents with higher SES may be exposed to higher physical activity or sports participation levels [e.g., [
56,
57]].
Although SES was not consistently related to non-recreation injuries, the grouping together of all such injuries may have limited our ability to detect associations with more specific injury circumstances. Inverse relationships have been observed for SES and both traffic injuries [
10,
12,
15] and intentional injuries [
10,
26,
27] among children and adolescents.
Limitations
An important contribution of this study was the comparison of several different measures of SES in relation to injuries within the same adolescent sample. There were some important limitations, however. First, the sample size was not sufficient to examine injury causes beyond the breakdown into recreation and non-recreation injuries. Non-recreation injuries may be relatively heterogeneous with respect to causes and their relationship to SES (for example, as described, the literature suggests that some types of traffic and intentional injuries may be inversely related to SES). Related to this, while physical assault and suicide attempt were included as possible causes of injury in the data captured for the survey, given the interview format and the self-reported nature of these data, it is possible that intentional injuries may have been under-reported, and thus underrepresented in the analysis (approximately 2% of the most serious injuries in the unweighted sample were reported to be intentional). The sample size was also insufficient to explore age differences within the adolescent sample.
It was not possible to determine the severity of adolescent injuries in this study. Previous research has yielded mixed results in terms of SES and injury severity for children and adolescents [
11,
12,
24], suggesting that severity could play a role in contributing to inconsistent study findings. The more consistent inverse relationship that has been documented between SES and injury mortality also supports the importance of considering severity [
7]. This is also complicated by the notion that injury severity is likely related to injury cause (for example, recreation and non-recreation injuries may differ in severity).
A further potential limitation was that our measures of injury relied on adolescent self-reports. Recall errors have been identified in surveys of childhood injury [
50,
58], and recall bias has been proposed as a potential explanation for positive relationships in studies of SES and injuries [
23,
59]. Although this warrants exploration in terms of the positive relationships observed, it seems unlikely that such bias explains the heterogeneity in our findings.
Additional indicators of SES that may be worth exploring in relation to adolescent injury include measures of adolescents' own social position [
2,
5], subjective evaluations of SES [e.g., [
38]], and measures of deprivation [e.g., [
60]]. Subjective SES measures were not available for this study; deprivation measures were excluded because they may not reflect a range of SES levels.
Finally, a multi-level modeling approach to the analysis of SES and injuries may help to distinguish between individual and neighbourhood-level associations. Our focus on separately exploring individual and neighbourhood SES indicators reflected our aim of identifying possible reasons for heterogeneity among the findings of previous studies. Our results reveal that additional development work is necessary at each level of analysis (individual and contextual), to further elucidate the meaning of indicators of adolescent SES and how they relate to injuries. Multi-level modeling approaches will be an important component of this work, and in particular for area-level SES measures, will be useful for distinguishing between aspects of the SES-injury association that are related to characteristics of neighbourhoods themselves, and those that are related to the aggregate characteristics of individuals living within neighbourhoods. In terms of neighbourhood characteristics, it may be informative to explore which particular aspects of the social and physical environment (for example, traffic hazards or crime) are contributors to injury risk, and how SES variables capture these characteristics. In light of recent attention being given to issues such as neighbourhood income distribution and health [
61], it may also be worthwhile to explore whether the degree of SES homogeneity within neighbourhoods is related to injury, or whether the role of individual SES in contributing to injury risk is modified by neighbourhood characteristics. This may be especially interesting given that in our sample, household income and area-based income quintiles variables were only modestly correlated (Spearman rank correlation of approximately 0.3) [
51].
Conclusion
We found that the relationship between SES and injury varied depending on the choice of both measures of SES exposure and injury outcomes. The findings emphasize the importance of considering how different measures of SES may operate through potential pathways that may include behavioural, social, and environmental factors. Our results also highlight challenges in measuring different dimensions of socioeconomic position. A future priority will be to further develop our understanding of both the meaning and relevance of SES indicators and the theoretical basis for a potential SES-injury relationship among adolescents.
Despite the variability in our findings, any apparent association between SES and non-fatal adolescent injuries was positive. This has implications for injury prevention, since evidence of a positive association between SES and non-fatal adolescent injuries argues against focusing prevention efforts mainly on lower SES groups. A positive or null relationship with SES is consistent with some (but not all) previous studies in this area, and suggests a need for caution in generalizing the findings of inverse SES gradients for injury mortality to non-fatal injuries.
Our results support previous literature documenting the complex nature of the relationship between SES and adolescent health. The findings suggest that to understand the potential relationship between SES and non-fatal injuries among youth, key conceptual and measurement issues related to both SES and injury will need to be addressed. This may help to explain variability in injury risk among adolescents and may aid in identifying priority areas for injury prevention. Understanding the potential contribution of SES will also provide insight into the social context that underlies more proximate exposures to injury risk among adolescents.
Competing interests
The author(s) declare that they have no competing interests.
Authors' contributions
BKP participated in the design and coordination of the study, carried out all statistical analyses, and drafted the manuscript. KNS participated in the design, coordination, and supervision of the study, as well as revisions to the manuscript. JJK participated in the design, coordination, and supervision of the study and revisions to the manuscript, as well as providing statistical guidance. IAG, MKC, and DM participated in the design, coordination, and supervision of the study, as well as revisions to the manuscript. All authors read and approved the final manuscript.