Lower educational level remains associated with lower body height among Austrian conscripts born from 1961 through 2002

Abstract Growth patterns and final body height are influenced by genetic and socio-environmental factors. A major impact of education on growth has been documented. Body height increases with an increasing educational level. The present study focuses on the association patterns between body height and educational level among 1,734,569 Austrian male conscripts aged 17 to <19 born between 1961 and 2002. Four levels of education were classified to examine their association with body height. Over 42 years, the percentage of conscripts at the lowest educational level decreased dramatically from 37.5% to 1.7%. All educational classes showed increasing body heights over time. Despite a marked improvement in the living standard, body heights at different educational levels did not converge. In Austria, educational and social advancement was associated with higher population body heights. Young men at the lowest educational level, however, remain shorter and their body height gap to the highest educational level has widened.


Introduction
Human variation in height is highly influenced by genetic factors (Zhong et al. 2017;Yengo et al. 2022) but also modulated by the socioeconomic and psychosocial environment such as psychosocial stress, malnutrition, poor health, and limited access to health care (Lasker and Mascie-Taylor 1989;Krzyzanowska 2007;Bogin 2021).Given that genetic determinants of growth and height do not fluctuate rapidly over time, the marked increase in mean body heights in Europe during the last 180 years, namely the secular trend, has been interpreted as a visible sign of improved living conditions (Cole 2003;Kalka et al. 2019;Valente et al. 2022).
A well-documented phenomenon is a positive association between educational level and body height (Viviani et al. 2020;Arntsen et al. 2023).The educational level is an important socioeconomic indicator, but it also mirrors social inequality.Socio-political and economic development advances were accompanied by a reduction of social boundaries, an increased probability of social mobility, and a profound modernisation of the educational system.This is also true for Austria, where the living standard has improved markedly since the 1960s.Investigating how education level, as an important socio-environmental factor, may affect body height differently over time is, therefore, of special interest.Military conscripts represent population-based data on young men, hence providing a unique powerful basis for examining the impact of macrosocial changes on growth.Hence, the present study analysed the pattern of associations between educational level and body height among Austrian conscripts and if differences in body height according to educational level decreased during the last 60 years.

Subjects and methods
We used the anonymised data of male conscripts born in Austria with Austrian citizenship between 1961 and 2002.The enlistment of conscripts included an obligatory medical examination to verify military fitness.In the course of this examination, as detailed previously (Yang et al. 2020), body height was measured by trained staff.At the time of medical examination, the conscripts were aged 17 to <19 years.The first examinations included in the present analysis took place in 1979 (birth cohort of 1961), and the last in 2021 (birth cohort of 2002), totalling 1,734,569 conscripts.
Educational level was classified by the military into four categories based on the number of years of school education as well as school type: low (fewer than 9 years of compulsory school); medium (completed compulsory school); high (graduated from professional training or served an apprenticeship); and very high (qualified for university entrance).These four educational levels reflect the Austrian educational system.After 4 years of elementary school, the first selection takes place at the age of 10, when the decision is made whether to attend an 8-or 9-year secondary school (Gymnasium) or only a 5-year compulsory school from the age of 10 (Hauptschule).In addition to the children's academic performance, this decision is significantly associated with the parent's educational level and household income (Lamei et al. 2015).Compulsory education ends after 9 years of schooling at the age of 15.After that, students either remain in higher education until they have passed the Matura and attained university entrance qualifications, or they begin a 3-year vocational training programme in the dual system.At the highest level of education, 18-year-old recruits may have acquired university entrance qualification; at the lowest level, they have not even completed the 9 years of compulsory schooling.
Although data on the number of years of school education and type of school were available to all, military classified educational level categories were missing in 58.5% of conscripts in 1961 and 19.8% in 1964.For those with missing data, their educational levels were imputed by multivariate imputation by fully conditional specification using the option FCS in SAS procedure Proc Mi and variables year of birth, number of years of school education, age, and the province of living address.Therefore, full data for education level are available from 1961 onwards.
Time trends of mean height were analysed by a smoothed trend line by generalised additive models using proc GAM in SAS with a spline with n = 3 degrees of freedom.The model included the year of birth as well as the age of the conscript.The analysis was repeated for each educational level, thereby allowing maximal flexibility regarding the form of the association within education level and interaction effects between educational level and year of birth.The increase in mean height per year was presented by line plots as well as dots for means for age 17 and 18 years to illustrate the dynamics of growth.Time dependence of the effect of education was tested by an interaction term education*year of birth in a linear model.

Results
As presented in Figure 1(a), showing the estimated height by the regression model as well as dots representing the mean for 17 and 18 years olds separately, we found a strong, positive association between educational level and body height.Smoothed time trends in body height between birth cohort 1961 and 2002 increased similarly by the educational level (all p < 0.0001).The significant interaction term education*year of birth (p < 0.001) pointed to time dependence of the effect of year of birth.Although Austrian young men of all educational levels showed increasing body heights over time, body heights of individuals with different educational levels did not converge.For the earliest birth cohorts, 1961-1965, medium to very high educational levels showed higher mean heights by 1.45 cm, 2.86 cm, and 2.91 cm compared to the low educational level (Table 1).For the most recent birth cohorts, 1998-2002, corresponding values of difference in mean heights were 1.04 cm, 2.61 cm, and 3.26 cm.While the difference between the low, medium, and high educational levels decreased over time, the difference in body height between the lowest and highest educational level widened from 2.91 cm to 3.26 cm over time.

Discussion
In Austria, since 1961 all males with Austrian citizenship reaching their 18 th year of age are required to attend conscription with the exemption of severely disabled persons.Therefore, our data set comprises the whole total male population in that age range and participation is independent of socio-demographic determinants which can lead to a potential selection process over time.Overall, body height increased from the birth cohort of 1961 to 2002 in Austria, however, the secular trend has slowed (Kirchengast et al. 2022).In the present study, this was true of all four educational strata.Considering the improvement of the living conditions and the economic situation during this period in Austria, this trend was expected.Additionally, profound changes in the Austrian educational system took place.Since the early 1970s, attendance at grammar schools (including textbooks) became free of charge, and entrance exams have been abolished, removing a social barrier to higher education.These policies were implemented to facilitate social advancement and reduce social inequalities.Despite socioeconomic improvements, educational level remains closely related to height.These findings are in accordance with those of several other studies focussing on the association patterns between educational level and body height.Similar effects of educational level on growth and body height were reported in Swedish men (Magnusson et al. 2006), Greek conscripts (Papadimitriou et al. 2008), Hungarian students and conscripts (Gyenis and Joubert 2004), Spanish conscripts (Ayuda and Puche-Gil 2014), and Dutch conscripts (Huang et al. 2015).All those studies described the same trend, such that there is a positive association between educational level and body height, and a low educational level remained significantly associated with shorter body height.
Based on these results we hypothesise that improved living standards, increasing urbanicity, and the turn of Austria into a Welfare state in the 1970s are positively associated with growth and body height and also with the population's educational levels.
Conscripts with the lowest educational level decreased markedly from 37.5% in birth cohorts of 1961-1965 to only 1.7% in birth cohorts of 1998-2002 (Figure 1(b)).This was also true for the medium educational level, while conscripts corresponding to high educational level increased significantly.The increased educational level was accompanied by increased mean body heights in Austrian young men.From a public health perspective, the association between body height and education level may be important, as an association between body height, education level and the risk of cardiovascular diseases has been shown, including myocardial infarction and stroke (Krieg et al. 2022).
In summary, educational and social advancement is associated with higher population body heights.Austrian young men who remained at the lowest educational level, however, remain shorter and the gap in mean body height between the lowest and highest educational level remained constant.
This research received no specific grant from any funding agency, commercial entity, or not-for-profit organisation.

Figure 1 .
Figure 1.(a) Body height by educational level and year of birth; (b) Educational level of conscripts in percent by years of birth.

Table 1 .
Body height (cm) by years of birth and educational level.