All cause and cause specific mortality in 15-24-year-olds in Denmark 2010 to 2022: nationwide study of socioeconomic predictors

Study population

We conducted a nationwide study including all individuals in Denmark between 15 and 24 years from 2010 to 2022. Any individual within that age range was included for as long as they met the age restriction criteria. All data were updated annually. As such, the study population constituted an open cohort study, in which we included individuals as they either turned 15 years or excluded them once they turned 25. The study period of 13 years (1 January 2010 through 31 December 2022) included an average of 716 523 individuals per year, corresponding to a total of 9 314 807 person years.

Information about any person who is a permanent resident in Denmark is included in the Danish Civil Registration System,19 which contains basic information such as legal gender, date of birth, immigration, nationality, and parents' identification number. Data from this register served as the backbone for the current study. We used the personal identifying number to link unique information from different national registers, to obtain information about deaths, parents' socioeconomic status, and ethnic group.

Mortality data

Detailed information about deaths was acquired from the register of causes of death.20 In Denmark, when an individual dies, a doctor performs a post-mortem examination and adds cause of death to the death certificate. The certificate has information about the manner and cause of death, that is whether the death was from natural causes such as a medical condition or from unnatural such as external factors comprising accidents, suicides, and homicides. In cases where the death is deemed unnatural or unexpected, an in-depth post-mortem examination is done by a physician from the Danish agency for patient safety in collaboration with the police. Based on this examination, a decision can be made to proceed with an autopsy, aiming to determine the precise manner and underlying cause of death. The underlying cause of death is defined as the disease or injury that initiated the series of events that led to the person's death. The unknown category for the manner of death is only used when an autopsy has been conducted and proved inconclusive. In all cases, death certificates are filled in and the information is transferred to the register of causes of death.20 The Danish health data authority manually reviews details regarding all unnatural deaths to ensure the accuracy of the recorded information. Data for all cause mortality were available for 2010-22 and obtained from the Danish civil registration system.19 Information about cause specific deaths as obtained from the register of causes of death was available from 2010 to 2021 and not 2022 because the registration process is longer causing some delay in data access.20

Socioeconomic measures

Socioeconomic position was operationalised by three variables: parents' employment status, educational level, and family income. This information was obtained from national registers on employment status (labour force statistics),21 highest attained education (population's education register),22 and family disposable income (income statistics register).23 Employment status was categorised into three groups: both parents working; one parent working, one receiving benefits; and both parents receiving benefits. Educational level was categorised into four groups, based on the parent who attained the highest educational level: elementary school (10 years mandatory education); low education, covering upper secondary school (high school (three years) and vocational education (four years including works experience)); medium education covering higher education following upper secondary school (Academy Profession degree (two years), Bachelor's degree (three years), and Professional Bachelor's degree (three to four years)); and high education (Master's degree following bachelor's degree (two years) and PhD following Master's degree (three years)). Family income, as derived from the Income Statistical Register,23 was operationalised into quintiles and also used continuously.

Other covariates

Gender was defined as man or woman based on information from the Danish civil registration system.19 Information about ethnic group was obtained from statistics Denmark24 and was defined as Danish origin or first or second generation immigrant.

Statistical analysis

We used Stata version 17 for all analysis. The socioeconomic variables had missing information for parents' employment status (4.8%), parents' education (6.1%), parents' disposable income (4.8%), and immigration status (0.1%). Information about age and gender was complete. Given the relatively low number of deaths from a statistical standpoint, we opted to use multiple imputation by chained equations25 (Stata procedure mi impute chained, 10 imputations) to address missing values and retain the full sample. The model included age, gender, immigration status, and socioeconomic measure (parents' employment status, educational level, or income). Socioeconomic data were updated yearly, at the beginning at the year. The procedure was run in strata of calendar year. Poisson regression was used to calculate incidence rates, incidence rate ratios, and 95% confidence intervals (95% CIs) for all cause death and cause specific death. In 34 cases, the manner of death was recorded as unknown therefore these people could not be included in analysis of cause specific mortality. We estimated for each imputation separately and then combined using Rubin's rule.26 Models were adjusted for gender, age, immigration status, and calendar year. Furthermore, we adjusted standard errors allowing for clustering of data by year to consider the precondition of the Poisson analysis that observations were independent. Restricted cubic splines with income as a continuous variable were modelled to illustrate the shape of the risk curve over the full spectrum of income allowing for non-linearity.27 28 We used four knots set at the fifth, 35th, 65th, and 95th percentile of the income distribution. We tested for a statistical interaction between socioeconomic position and both gender and age group. This test was done by including main effects as well as interaction terms and subsequently testing the combined effect of interaction terms using an F-test.

Patient and public involvement

No patients were directly involved in the study because it is register based.

Key results

In this nationwide study of all deaths in 15-24-year-olds during 2010 to 2022 in Denmark, we reported that mortality in young people was consistently socially patterned. This result was observed for three measures of socioeconomic position (parent's employment status, educational level, and income). All cause mortality as well as cause specific mortality were markedly higher in socioeconomic disadvantaged groups compared with more affluent groups. Dose dependent associations generally showed higher mortality from natural deaths (ie, medical conditions and diseases), accidents, suicides, and homicides with lower levels of socioeconomic position. Modelling income showed socioeconomic inequality across incomes and the gradient was especially steep for the 60% less affluent individuals. Results were similar in young men and women and between age groups (15-17-years-old v 18-24-year-olds).

Our results support previous indications of socioeconomic inequality in mortality in young people.10 12 14 29 30 We observed a consistent social gradient in natural deaths which is as novel a finding as no socioeconomic inequality in medical mortality reported in previous studies in young men and women.11 12 30 These studies used various measures of socioeconomic position such as social class, housing tenure, car access, and manual versus non-manual work. Our findings add a more distinct and finely divided social gradient that also includes the large middle class and is not limited to marginalised groups.

Mechanisms

Mechanisms explaining the association between social inequality and mortality in young people are complex and not fully understood. As described in 1995, socioeconomic factors are likely fundamental causes of disease because money, knowledge, power, prestige, and social network are resources that influence the extent to which people avoid risks for morbidity and mortality.31 Socioeconomic factors are fundamental because they persistently affect multiple disease outcomes via multiple risk factors, and they are passed on through generations. Although the extent of socioeconomic mobility is continuously under debate, socioeconomic position is hereditary.32

Some types of cancers that lead to natural death are associated with the accumulation of risk factors over time and socioeconomic position is a determinant of exposure, acting through living conditions, work environment (eg, toxins and psychosocial stress), and health behaviour (eg, smoking and alcohol use). Social inequality that affects mortality in young people may also result from differences in discovery, diagnosis, and treatment of diseases. Denmark is a welfare society with free access to high quality welfare benefits including medical services at all levels. Yet, tackling a course of a disease also requires health literacy, compliance to treatment, and the ability to navigate and understand the healthcare system.33 These issues are where parents' educational level is likely to have a large effect because longer education reduces the effect of these potential barriers and therefore a favourable disease outcome, as outlined by the theory of fundamental causes.31

The mechanisms of inequality may differ in nature depending on the manner of death, most importantly if the death is caused by natural or unnatural causes. Unnatural as opposed to natural deaths are the ultimate result of an immediate event, such as a traffic accident or a suicide. Adolescence constitutes the important transitional period from child to adulthood entailing more dependence, orientation towards peers, and social mobilisation—all factors that are associated with increased autonomy.34 During this period, the young brain is further developed including the maturing of the frontal lobe that is responsible of executive functions, planning, reasoning, and impulse control, alongside remodelling of the dopaminergic system that is essential in reward and sensation-seeking behaviour. These are adjustments that help young people to assess external risks such as situations in the traffic,35 36 Socioeconomic position correlates with differential susceptibility to exposures and willingness to engage in risk behaviours,37 even more pronounced in children and young people who are experiencing distress in the form of adverse childhood experiences,38 which are also more frequent in those who have parents with low socioeconomic position.

Accidents were a common cause of death accounting for 35% of all deaths and 58% of unnatural deaths in 15 -to-24-year-olds in Denmark from 2010 to 2022. A high alcohol intake in young people may also be a factor because accidents are sometime related to alcohol intake,18 and studies have shown that alcohol causes disproportionally more harm in disadvantaged groups compared with more affluent groups.39 40 Differences in drinking patterns between socioeconomic groups have been proposed to explain a maximum of 30% of the variability in alcohol related harm41 However, in a recent prospective cohort study, alcohol-related harm in young people was more common in socioeconomically disadvantaged adolescents despite similar levels of alcohol consumption, regardless of differences in drinking pattern or substance use.42

Employment status somewhat mirrors parental level of functioning and receiving public benefits is associated with chronic illness and other functional disabilities,43 which in turn, can cause reduced family functioning and social-emotional and behavioural problems in young people.44 Furthermore, economic hardship may cause disruption of supportive parenting behaviours as described in the family stress model,45 compromising parental monitoring and support, which might lead to more risky and adverse health behaviours in young men and women. A mix of accumulating environmental exposures, health risk behaviour, and differences in vulnerability may constitute some of the mechanisms explaining a social pattern in mortality in young people.

Strengths and limitations

A considerable strength is that we used complete and annually updated data from nationwide registers including all 15-to-24-year-olds during the study period. We considered the risk of bias due to selection and misclassification to be negligible. We operationalised socioeconomic position with three measures, each one contributing to cover attributes and resources affecting life circumstances and health. Parents' employment mirrored functional level and health, education represented an indicator of parent knowledge, and family income represented an indicator of financial and material resources. Including all three measures provides a more detailed definition of socioeconomic position and allowed us to detect possible differences between the measures. The measures are connected and are of different importance during a life course.46 Socioeconomic position, however, was a proxy for other determinants and resources of diverse nature in turn affecting major determinants of health such as environmental exposure, health behaviour, and healthcare.47

Fortunately, premature death in young men and women is relatively rare. Consequently, even though all deaths occurring during the study period were included, statistical power was low especially for suicide and homicide. Nevertheless, a pattern of social imbalance was systematically observed across all causes of death. Of note, we did not differentiate between one parent and two parent households, which may have affected results. Also, our model for associations between socioeconomic position and mortality from natural deaths was not adjusted for the affect on income that may come from having a sick child.