Morphometric traits predict educational attainment independently of socioeconomic background

Background Aim of this study is to describe the relationship between anthropometric traits and educational attainment among Estonian schoolchildren born between 1937 and 1962. We asked whether height, cranial volume and face width (a testosterone-dependent trait), measured in childhood predict later educational attainment independently of each other, family socioeconomic position (SEP) and sex. Associations between morphometric traits and education and their interactions with biosocial variables are of scholarly importance because higher education is nearly universally associated with low fertility in women, and often with high fertility in men. Hence, morphometric traits associated with educational attainment are targeted by natural selection and describing the exact nature of these associations is relevant for understanding the current patterns of evolution of human body size. Methods Data on morphometric measurements and family background of 11,032 Estonian schoolchildren measured between seven and 19 years of age were obtained from the study performed by Juhan Aul between 1956 and 1969. Ordinal logistic regression was used for testing the effects of morphometric traits, biosocial variables and their interaction on the cumulative probability of obtaining education beyond primary level. Results Of biosocial variables, family SEP was the most important determinant of educational attainment, followed by the sex, rural vs urban origin and the number of siblings. No significant interactions with morphometric traits were detected, i.e., within each category of SEP, rural vs urban origin and sex, taller children and those with larger heads and relatively narrower faces were more likely to proceed to secondary and/or tertiary education. The effect of height on education was independent of cranial volume, indicating that taller children did not obtain more educations because their brains were larger than those of shorter children; height per se was important. Conclusions Our main finding – that adjusting for other morphometric traits and biosocial variables, morphometric traits still robustly predicted educational attainment, is relevant for understanding the current patterns of evolution of human body size. Our findings suggest that fecundity selection acting on educational attainment could be partly responsible for the concurrent selection for smaller stature and cranial volume in women and opposite trends in men.


Background
Educational attainment is a strong and robust predictor of essential life outcomes including occupational status, happiness, health, and life expectancy [1]. Since the beginning of the twentieth century, educational attainment is also known as the most important predictor of Darwinian fitness, depressing fertility of women nearly universally in both modern and developing societies [2][3][4].
Such genetic correlations are informative about the past selection pressures that have led to clustering of lifehistory and behavioural trait values along the fast-slow continuum of the life-history speed. According to the theory of life-history evolution, traits characteristic for slow pace of lifelate maturation and reproduction, slow development, high somatic investment into body and brain growth, intensive parental care, low birth rates and long life-spanhave coevolved with high intelligence and conscientious personality traits [16][17][18].
Although consistent with the life-history theory, these findings leave open the question of whether and how environment modifies the conversion of genetic correlations between the educational attainment and morphometric traits into the phenotype. Environmental exposures may either exaggerate or attenuate translation of genetic predispositions into phenotypes [19]. For instance, genetic influences on educational attainment may vary across the context of childhood SEP [14,20,21]. Access to education may modify the relationship between height and cognitive function [22]. Further, variation in morphology of different brain structures can mediate the associations between family income and school achievement during adolescence [23] and memory performance in adulthood [24].
Aim of this study is to describe the relationship between three anthropometric traits and educational attainment among the large sample of Estonian schoolchildren born between 1937 and 1962. In particular, we ask whether height, cranial volume and face width, measured in childhood predict later educational attainment independently of each other, family SEP and sex.
These questions are relevant for understanding the aetiology of individual differences in educational attainment that are sensitive to genetic as well as societal factors and growth environment [14,25,26]. For instance, although numerous studies have recorded positive associations between cranial volume or height vs educational attainment, only few have simultaneously tested the effect of both [27]. At proximate level, distinguishing the effects of height vs cranial volume on education is important because of strong and sex-specific allometric relationships between these traits [28,29]. Ultimately, accounting for such allometric relationships between anthropometric traits is also important for predicting the rates and directions of evolution of human phenotype. For instance, the question whether the contribution of height to educational attainment is independent or caused by allometric correlation with cranial volume (a proxy for cognitive ability) is relevant for understanding the evolutionary forces acting on morphometric traits via correlated selection on education [see, e.g., 30].
Another morphometric trait that is interesting in the context of educational attainment is face width, an important component of face masculinity [30,31]. Face width is sensitive to testosterone exposure in utero [32] and during puberty [30]. Men and women with masculine faces are perceived as more aggressive, dominant and strong reviewed by [33][34][35][36]. Associations between adult testosterone levels and face masculinity are less clear [37][38][39]. Notably, however, a small number of studies have found that facial masculinity correlates with another proxy of circulating testosteronephysical strength among both men [40,41] and women [35,42]. It is highly likely that all the above-described associations relate to genetic differences in testosterone production as genetic variants that are associated to higher testosterone levels in the body are also associated with facial masculinity [43].
We had no predictions about the direction of association between facial width and educational attainment. On the one hand, higher fetal testosterone levels may be associated with compromised development of cognitive abilities in early childhood (at that, affecting different components of cognition among boys [44,45] and girls [46,47]). On the other hand, facial masculinity in men seems to be commonly associated with success and goal attainment not only in in various competitive social contexts but also in prosaically oriented settings [reviewed by 49]. Endogenous male testosterone positively predicts cognitive performance in both cross-sectional and longitudinal analyses reviewed in [48] and testosterone administration often (but not always) improves visuospatial cognitive function [49]. As regards academic performance, Kausel et al. [50] showed that among both male and female university students, relative face width predicted positively achievement in non-quantitative (i.e., orally examined) courses while grade points earned on the basis of written tests showed no associations with face morphology.
We start with testing the assumption that face width can be considered as a testosterone-dependent trait by examining the correlations between face width and handgrip strength. Next we ask if morphometric traitsheight, cranial volume, and face width measured during school years predict the highest educational level the children were going to obtain. Specifically, we aim to test whether previously described associations between height and educational outcomes can be ascribed to allometric relationship between cranial volume (a proxy of cognitive ability) in which case we predicted that inclusion of cranial volume as a predictor variable will cancel the effect of height on educational attainment. Alternatively, if height has an independent effect on education, we expected that both predictors will remain simultaneously significant in the model. Next we test for the occurrence of significant interaction terms between morphometric traits vs sex and two markers of growth conditionsparental SEP and rural vs urban origin. This enables to establish whether the associations between body parameters and education are similar among boys and girls and in children growing up with differential access to material and cognitive resources.

Methods
Data on morphometric measurements and family background were obtained from the anthropometric study performed by Juhan Aul between 1956 and 1969. The historical background of this sample is described by Hõrak and Valge [51]. Face width was measured as a bizygomatic distance, i.e., maximal distance between the most lateral points on the zygomatic arches. Cranial volume was calculated according to Rushton [52]: 7.884*(head length-11) + (10.842*head width-11)-1593.96 for girls and 6.752*(head length-11) + (11.421*head width-11)-1434.06 for boys (units in mm).
The dataset involves 15,205 girls and 11,757 boys (for both, average age = 12.7, SD = 3.1, range = 7-19 years); this dataset 16 times larger than the one used in the previous study of Hõrak and Valge [51] and involves both girls and boys. These data were used for calculation of age-and sex-specific residuals for height, cranial volume, face width and handgrip strength and correlations between these (Additional file 1: Figure S2-S3). Residuals were calculated with smooth non-parametric regression line and automatic smoothing parameter selection using package 'gam' for R [53] and transformed to z-scores within sexes. In addition to age in days, regression included birth date as a predictor variable to account for secular increase in body dimensions during the study period [see 53].
From this sample, we identified a subset of 11,032 individuals whose educational attainment was recorded in the Estonian Population Registry (https://e-estonia.com/ solutions/interoperability-services/population-registry/). Data on the highest level of education obtained were based on self-reported data from the last Estonian population census in 2011 (https://www.stat.ee/phc2011); by that time all the subjects had completed their maximum level of education. Educational attainment was divided into three categories: primary (8 years of schooling or less), secondary (including secondary vocational) and tertiary (> 11 years of schooling). On the basis of parental professions recorded during data collection, participants were assigned to parental SEP values (highest in the family) as unskilled manual workers, skilled manual workers or non-manual workers.
Ordinal logistic regression [54] was used for testing the effects of morphometric traits, biosocial variables and their interaction on the cumulative probability of obtaining education beyond primary level. Additionally we run the same models in binary logistic regressions where the dependent variable was either obtaining tertiary vs secondary or secondary vs primary education. All models were tested for the interactions between birth year vs sex, parental SEP, and urban/rural origin to account for confounding secular trends in educational attainment during the study period. All tests are twotailed with a P-level below 0.05 as a criterion for significance.

Results
Higher levels of education were more prevalent among girls and in general, children of urban origin (Additional file 1: Figure S1, Figs. 1-2). All anthropometric variables were positively correlated (Additional file 1: Figure S2-S3). Face width predicted handgrip strength better (girls: r = 0.26; boys: r = 0.35) than another measure of head size, cranial volume (girls: r = 0.20; boys: r = 0.27), confirming our assumption that face width can be regarded as a marker of testosterone exposure.
Parental SEP was the strongest predictor of educational attainment (Figs. 1-4, Additional file 1: Table S1). Yet after adjusting for the covariates in the model, all three anthropometric parameters independently predicted the educational attainment that children were going to obtain. Taller children with larger heads and narrower faces had higher cumulative probability of obtaining either secondary or tertiary education (Figs 3, 4 and 5). None of the interaction terms between morphological variables and factors (sex, rural/urban, SEP) improved the fit of the model in Additional file 1: Table  S1 significantly (p = 0.13-0.81; likelihood ratio tests).
To illustrate the results, we apply the odds ratios presented in Fig. 1 to the quantile values of raw measurements  Table S1 for the estimates and p-values of interaction terms. Nagelkerke R 2 for the model = 0.19 among the most common age class, 13-years old children (1577 girls and 1276 boys). For girls, being 4.5 cm (2.8%) taller than the median of 155 cm (5. 2 cm above 153 cm for boys; 3.3%) increased the chances of proceeding from primary to secondary or secondary to tertiary education by 10%. Girls with cranial volume 54 cm 3 (4.1%) above the median of 1262 cm 3 (boys: 52 cm 3 above 1367 cm 3 ; 3.6%) had 15% higher chances of obtaining the secondary (over primary) or tertiary (over secondary) education. Having a 0.3 cm (2.3%) narrower face than the median of 13 cm increased the chance for proceeding to secondary or tertiary education by 5% for both sexes.
Next we tested in binary logistic analysis whether the same variables as in Fig. 1 predicted the chances of proceeding from secondary to tertiary education. The best model was almost identical to that in Additional file 1: Table S1 with an exception that threeway interaction between sex, urban/rural origin and year of birth (YOB) has lost its significance (Additional file 1: Table S2).
Examination of model predicting the chances of proceeding from primary to secondary education revealed that face width was not significant any more (P = 0.063 in Wald test). The best model (Additional file 1: Table S3) also suggested that the effect of cranial volume on proceeding to the secondary education depended on sex. Analysing the data for boys and girls separately revealed that in the case of boys, none of the morphometric variables predicted education (cranial volume: OR = 1.04, 95% CI = 0.99-1.10; height: OR = 1.05, 95% CI = 0.99-1.10). In the case of girls, both morphological variables remained significant although notably, the confidence intervals showed some overlap with these of boys (cranial volume: OR = 1.13, 95% CI = 1.06-1.19; height: OR = 1.07, 95% CI = 1.01-1.14). Fig. 3 Predicted probabilities ±95% CI of obtaining primary, secondary or tertiary education in relation to cranial volume. Based on the model described in Additional file 1: Table S1 and Figure 1 Discussion Parental SEP was the most important determinant of educational attainment. Children whose parents were in non-manual profession had 3.6 times higher and children whose parents were in skilled manual profession had 1.5 times higher cumulative odds of obtaining educational level beyond primary (Fig. 1). This is consistent with previous findings demonstrating that parental SEP (besides intelligence) is the most important predictor of educational attainment worldwide [55,56], including Estonia [57]. The mechanism of this connection may involve both social and genetic inheritance of both SEP and education [14] and the genetic correlations between the two are not trivial. For instance, social deprivation can genetically strongly correlate with educational attainment (r g = − 0.55 in UK Biobank sample [58];).
Being of rural (vs urban) origin reduced the odds of obtaining education beyond primary level by 44% and being a boy vs girl by 47%. Rural-urban educational gap is consistent with the global evidence [59]. However, the higher female to male ratio in secondary and tertiary education seems to be specific peculiarity of some Soviet and socialist regimes [60] as compared to the global patterns in the middle of the twentieth century [61]. Figures 1 and 2 also indicate that after accounting for socioeconomic and sex differences, number of siblings had still independent (though small) effect on education. This pattern, too, is consistent with previous evidence e.g., [62].
After adjusting for biosocial variables, all morphometric traits still significantly predicted educational attainment. Thus, within each category of parental SEP, rural vs urban origin and sex, taller children and those with larger heads and narrower faces were more likely to proceed to secondary and/or tertiary education. Assuming that measured morphometric traits reflect some intrinsic qualities of individuals, our results provide anthropometric evidence that within socioeconomic strata, a detectable meritocracy Fig. 4 Predicted probabilities ±95% CI of obtaining primary, secondary or tertiary education in relation to height. Based on the model described in Additional file 1: Table S1 and Figure 1 in the Estonian educational system under the Soviet regime existed. Our assumption that cranial volume and height reflect qualities related to educational achievement is supported by previous findings from the Juhan Aul's dataset, showing that girls progressing slowly at school (those that were > 1.5 years old for their grade) were shorter and had smaller heads than those progressing at school at an appropriate rate for their age [63].
Our finding that cranial volume predicted educational attainment (controlling for social stratification and height) is similar to that of Nave et al. [27] who assessed the brain volume on the basis of MRI scans of 13,608 40-69 year old participants of UK Biobank. The similarity of the findings to these two large-scale studies indicates that head size is a robust predictor of educational attainment, irrespective of the method of measurement and whether using adult population experiencing market economy or children pursuing education under the Soviet regime. Nave et al. [27] did not find any sex differences in associations between brain volume and cognitive ability or educational attainment. Results of our study are generally consistent with this finding with an exception that odds of obtaining secondary education depended on head size among girls but not in boys.  Our study confirms earlier findings that height is positively associated with educational attainment [6,71,72]. One of the likely reasons for this association is genetic correlation (r g = 0.16-0.65, depending on age) between height and IQ [73]. This may occur either because the sets of genes affecting these traits partially overlap (pleiotropy) and/or because positive assortative mating for height and education (as components of overall attractiveness) causes cross-trait assortative mating, leading to genetic covariation due to gametic phase disequilibrium [10].
Importantly, this study also shows that the effect of height on education was independent of cranial volume. This means that taller children did not obtain more educations because their brains were larger than those of shorter children; height per se was important [see 6]. One possible explanation for this independent height effect is that taller children possess more non-cognitive skills. For instance taller children may more likely participate in sport activities that enable to accumulate social skills that favour continuation of education [72]. It is also possible that active gene-environment correlation may play a role, for example, if tall children generate intellectually more stimulating environments than short children [73]. Yet another, not mutually exclusive explanation would be discrimination or social stigma against shorter individuals. For instance Vågerö and Modin [74] shoved that among Swedish men with similar cognitive abilities; those who were taller were more likely to obtain university education than shorter ones. Lastly, the finding that the effect of height on education was independent of cranial volume is consistent with an idea that variation in these traits reflects genome-wide mutational loads. I.e., height, cranial volume and educational attainment may stand as independent markers of overall genetic quality see [10,75,76].

Face width
We a found a small effect of face width on the odds of obtaining tertiary education. Although face width correlated positively with height and cranial volume, children with narrower faces (for fixed height and cranial volume) had higher odds for continuation of education beyond the secondary level. Assuming that face width is a proxy for testosterone exposure in utero and/or during adolescence, our results suggest that both boys and girls with higher testosterone exposure or levels were less likely to progress on educational path.
The result that both boys and girls with narrower, less masculine faces were more likely to obtain higher education is notable as it contrasts with previous finding that both male and female university students with relatively wider faces gained better grades in orally examined courses [50] and that face masculinity predicts success in various competitive and pro-socially oriented settings [77]. Our finding is also not compatible with studies showing the positive association between circulating testosterone levels and cognitive abilities (see Introduction). On the other hand, assuming that face width is testosterone-dependent trait, our results compare favourably with those of a study of 4462 US military veterans, whose serum testosterone levels correlated negatively with the years of education (r = − 0.11) and intelligence (r = − 0.07) [78]. Mediation analysis of veteran data showed that the effects of testosterone on education were mediated similarly by high antisocial behaviour and low intelligence. Dabbs Jr. [78] suggests that characteristic interests could lead high-testosterone individuals away from school and towards a world of action: given that testosterone is related to simple perseverant responding, "high-testosterone individuals may find little satisfaction spending hours sitting in classrooms and considering ideas". If such reasoning is correct then proposed relationship between high testosterone and low perseverance might also explain our finding that children with narrower faces were more likely to pursue tertiary education. Consistent with such an explanation are several studies showing that high levels of testosterone are linked to higher impulsivity and weaker behavioural control reviewed in [79], i.e., traits characteristic to the fast life histories and low educational attainment [80].

Strengths and limitations
Current study is the second largest one after that of Nave and co-authors [27], showing that cranial dimensions and height predict the educational attainment, controlling for the biosocial background. Its strengths are that due to historical reasons, the participation was not voluntary, which considerably reduces the recruitment selection and that all the participants were younger than 20, which almost entirely eliminates the mortality selection in the sample see [81]. For instance, the sample of Nave et al. [27] consisted almost entirely of adults and elderly that overrepresented individuals of higher SEP.
On the other hand, measuring children poses a limitation because they grow at individually different rates. E.g., a child might be small for age at age 13 but still end up in above average size if his/her pubertal growth spurt starts later than average and vice versa. This is an unavoidable problem of all crosssectional morphometric studies of children; however such measurement errors are conservative as they do not introduce any systematic bias. Another limitation is that the study was based on secondary data, thus its design was not under the control of the authors. Nevertheless, we consider that the quality, uniqueness and richness of the dataset collected by Prof Juhan Aul weighs up against possible shortcomings of the study design (e.g., randomization issues). Importantly for quality, all the morphometric measurements were collected by the same person and recording of sufficient amount of background variables enables statistically control for the main biosocial confounders.

Conclusions and implications
Our main findingthat adjusting for other morphometric traits and biosocial variables, morphometric traits still robustly predicted educational attainment, is relevant for understanding the current patterns of evolution of human body size. Across western populations, female education is almost universally negatively related to reproductive success [82][83][84] while among men, typically stabilizing or positive selection on education prevails [reviewed in 84,85,86]. According to the census data, Estonian women with only primary education bore 0.5 to 0.75 more children on average than women with tertiary education throughout the twentieth century [85]. In parallel, natural selection in developed countries usually favours shorter women and taller men [86][87][88][89]. These parallel patterns refer to the possibility that selection acting on educational attainment could be at least partly responsible for the concurrent selection for smaller stature and cranial volume in women and opposite trends in men. So far the phenotypic response to this correlated selection has been masked by secular increase in body dimensions [see 53]. However, one might predict that with general improvement and equalisation of growth conditions, genetic between-individual differences in body dimensions will become more clearly detectable in the phenotype [see also 82,86,89].
That evolutionary changes can be rapid is confirmed by a study in Iceland, showing that selection against genetic variants associated with educational attainment can lead to change in the genetic composition of population in few generations [90]. Hence, the possibility that traits genetically correlated with educational attainment (such as height and cranial volume) will show a correlated response in the phenotype in the future generations, should not be underestimated. At the same time, the effect of education on reproduction involves a substantial non-genetic component too [14,26]. In this context, our findings suggest that patterns of current selection on human body dimensions constitute an example of geneculture coevolution.
As for practical concerns, the finding of this study that height predicts educational attainment independently of cranial volume, sex and SEP calls for the research on possible issues of discrimination or social stigma against shorter individuals at school. Behavioural pathways (and associated genetic underpinnings) that lead to educational advantage of taller children deserve further investigation.
Abbreviations CI: Confidence interval; IQ: Intelligence quotient; SEP: Socioeconomic position; YOB: Year of birth

Funding
The study was funded by Estonian Research Council grant IUT34-8. The funding body had no role in the design of the study and collection, analysis, and interpretation of data and in writing the manuscript.

Availability of data and materials
The dataset supporting the conclusions of this article is available in the https://cran.r-project.org/package=ormPlot. Commented R codes for statistical analyses (including tests for the proportional odds assumption) and graphics have been uploaded in ESM 2 (Additional file 2).
Ethics approval and consent to participate Data processing was carried on anonymously under the licence of the Research Ethics Committee of the University of Tartu (protocol # 275/T-1, issued on 20.11.1017). The permission was granted to the authors to use any of the datasets collected by Prof Juhan Aul. The need for informed consent from participants had been waived due to the retrospective nature of the study.

Consent for publication
Not applicable.