Depression, neuroticism and 2D:4D ratio: evidence from a large, representative sample

A body of literature reports higher rates of depression and neuroticism in female samples compared to male samples. Numerous studies have investigated the role of prenatal sex hormone exposure in this sex difference, using the ratio between the second and fourth digit of the hand (“2D:4D”) as a putative marker. However, the sample sizes of those studies were mostly small and results remained inconclusive. The aim of the present study is to test the suggested associations between depression, neuroticism and the 2D:4D ratio in a large, representative sample of over 3,000 German individuals. It was hypothesized that a higher 2D:4D (supposedly representing a more “feminine” prenatal hormone exposure) would positively predict (1) one’s history of depression as well as (2) neuroticism rates and (3) acute depressive symptom scores. Controlling for biological sex, we only found suggestive evidence for linear associations with neuroticism in the case of left hand 2D:4D ratios and the mean 2D:4D of both hands. However, additional analyses indicated that these results may have been spurious due to confounding. Our findings suggest that the 2D:4D ratio is not a relevant predictor of depression, while there was mixed evidence in the case of neuroticism.


Supplementary
23 + * + + * + [16] 448 − + * − + * Note: + = positive association, i.e., a more "feminine" digit ratio was associated with higher depression / neuroticism rates, the direction hypothesized in the present study; − = negative association, opposite to the direction hypothesized; ? = the effect direction is not known to the authors of the present study, but it is clear that the association was tested due to statements about statistical significance. For empty cells, no corresponding results could be found. An asterisk ( * ) indicates that the effect was considered statistically significant (p < 0.05). All associations presented without asterisks were nonsignificant (p > 0.05). Austin and colleagues [2] reported separate results for two samples which are differentiated as [2] a and [2] b . The 23 observations listed as the sample size for Manning & Fink [11] represent aggregated mean data for 23 nations. The results of Li et al. [6] are restricted to cynomolgus monkeys.
2 Additional analyses: differences depending on consent to 2D:4D measurement We used data from the Innovation Sample of the German Socio-Economic Panel Study up to the survey year 2018 [14].
In 2018, 3,509 of 3,958 individuals gave their consent to having their digit ratios measured. On the variables we studied, there were no significant differences between participants who consented to getting their digit ratios measured and those who declined consent. The corresponding tests are reported in Supplementary Table S2. For biological sex and history of depression, chi-squared (χ 2 ) is reported. For neuroticism values and scores on the Patient Health Questionnaire-2 (PHQ-2) [9], t-values are presented with p-values for two-tailed tests. Note: HIST = history of depression, NEUR = neuroticism, measured using the Big Five Inventory-SOEP, PHQ-2 = Patient Health Questionnaire-2, a measure of acute depressive symptoms. All tests compared those participants who consented to getting their digit ratios measured with those who declined consent.

Supplementary
3 Additional analyses: controlling for age Due to observing small but consistent negative pairwise correlations of age with all of the 2D:4D measures and with two of the outcome measures, exploratory analyses were conducted which included age as an additional predictor. The sample of 3,389 individuals had an age range between 18 and 98 years (M = 55, SD = 18). For the regression analyses, the age variable was standardized as well to improve comparability of effect sizes. The results of these additional exploratory analyses for right hand 2D:4D, left hand 2D:4D and the mean 2D:4D of both hands are shown in Supplementary Table S3, Supplementary Table   S4 and Supplementary Table S5, respectively. Most notably, no association with any 2D:4D measure had a p-value below our threshold for suggestive evidence when controlling for age.
As several previous studies analyzed university students, we also repeated the exploratory analyses with an interaction term on a subsample restricted to participants under the age of 30 to account for a possible age-specific effect in younger Note: CI = 95% confidence interval, FEM = female sex, HIST = history of depression, NEUR = neuroticism, OR = odds ratio, PHQ-2 = Patient Health Questionnaire-2, R2D:4D = the length of the second digit divided by the length of the fourth digit of the right hand. β indicates the change in the z-standardized outcome variables for a change of one standard deviation in 2D:4D (or age) while controlling for biological sex and age (or 2D:4D) or the change in the case of female instead of male participants while controlling for age and 2D:4D. Note: CI = 95% confidence interval, FEM = female sex, HIST = history of depression, L2D:4D = the length of the second digit divided by the length of the fourth digit of the left hand, NEUR = neuroticism, OR = odds ratio, PHQ-2 = Patient Health Questionnaire-2. β indicates the change in the z-standardized outcome variables for a change of one standard deviation in 2D:4D (or age) while controlling for biological sex and age (or 2D:4D) or the change in the case of female instead of male participants while controlling for age and 2D:4D. Note: CI = 95% confidence interval, FEM = female sex, HIST = history of depression, M2D:4D = the mean of the digit ratios of the right hand and left hand, NEUR = neuroticism, OR = odds ratio, PHQ-2 = Patient Health Questionnaire-2. β indicates the change in the z-standardized outcome variables for a change of one standard deviation in 2D:4D (or age) while controlling for biological sex and age (or 2D:4D) or the change in the case of female instead of male participants while controlling for age and 2D:4D. Note: CI = 95% confidence interval, FEM = female sex, HIST = history of depression, NEUR = neuroticism, OR = odds ratio, PHQ-2 = Patient Health Questionnaire-2, R2D:4D = the length of the second digit divided by the length of the fourth digit of the right hand. β indicates the change in the z-standardized outcome variables for a change of one standard deviation in 2D:4D while controlling for biological sex or the change in the case of female instead of male participants while controlling for 2D:4D. Note: CI = 95% confidence interval, FEM = female sex, HIST = history of depression, L2D:4D = the length of the second digit divided by the length of the fourth digit of the left hand, NEUR = neuroticism, OR = odds ratio, PHQ-2 = Patient Health Questionnaire-2. β indicates the change in the z-standardized outcome variables for a change of one standard deviation in 2D:4D while controlling for biological sex or the change in the case of female instead of male participants while controlling for 2D:4D. Note: CI = 95% confidence interval, FEM = female sex, HIST = history of depression, M2D:4D = the mean of the digit ratios of the right hand and left hand, NEUR = neuroticism, OR = odds ratio, PHQ-2 = Patient Health Questionnaire-2. β indicates the change in the z-standardized outcome variables for a change of one standard deviation in 2D:4D while controlling for biological sex or the change in the case of female instead of male participants while controlling for 2D:4D.

Additional analyses: including quadratic terms
Further analyses using quadratic terms yielded similarly small effect sizes as the analyses reported in the main article. The results of these additional analyses with quadratic terms for right hand 2D:4D and left hand 2D:4D are shown in Supplementary   Table S9 and Supplementary Table S10, respectively. The results for the quadratic term of right hand 2D:4D reached our threshold for suggestive evidence (history of depression: p = 0.03; neuroticism: p = 0.04; PHQ-2: p = 0.048). The results are visualized in the following section ("Illustrative figures") of these supplementary materials.

Illustrative figures
Supplementary Figure S1 illustrates the null result for the interaction effect in the exploratory logistic regression model. As the exploratory logistic regression results were very similar across 2D:4D measures, the figure only illustrates the right hand 2D:4D data for the sake of space. The results for the regression models with quadratic terms (R2D:4D 2 and L2D:4D 2 ) are visualized in Supplementary Fig. S2 and Supplementary Fig. S3. There was suggestive evidence for quadratic associations with the right hand 2D:4D data, but it is barely visible, illustrating the very small effect sizes involved.
Supplementary Figure S1. Marginal effects in the exploratory logistic regression, using right hand 2D:4D. Predicted Probability of History of Depression  Note: 2D:4D = the length of the second digit divided by the length of the fourth digit of the hand. The y-axis was capped in each graph for illustrative purposes. The area around a line illustrates the 95% confidence interval for the corresponding estimate. The range of standardized 2D:4D values displayed includes 99% of the sample used for the regression models.

Additional analyses: multilevel modeling accounting for interviewer effects
To investigate the influence of systematic differences between interviewers, we also repeated the exploratory analyses with an interaction term using multilevel modeling, grouping 2D:4D observations by interviewers. Multilevel modeling was performed using the meglm command in Stata, estimating normally distributed random effects. Robust standard errors were estimated for neuroticism and PHQ-2 scores. The family-link combination of "bernoulli" and "logit" was specified for history  Note: CI = 95% confidence interval, FEM = female sex, HIST = history of depression, n 1 = number of individual participants, n 2 = number of groups based on interviewers, NEUR = neuroticism, OR = odds ratio, PHQ-2 = Patient Health Questionnaire-2, R2D:4D = the length of the second digit divided by the length of the fourth digit of the right hand. β indicates the change in the z-standardized outcome variables for a change of one standard deviation in 2D:4D while controlling for biological sex or the change in the case of female instead of male participants while controlling for 2D:4D.   Note: CI = 95% confidence interval, FEM = female sex, HIST = history of depression, M2D:4D = the mean of the digit ratios of the right hand and left hand, n 1 = number of individual participants, n 2 = number of groups based on interviewers, NEUR = neuroticism, OR = odds ratio, PHQ-2 = Patient Health Questionnaire-2. β indicates the change in the z-standardized outcome variables for a change of one standard deviation in 2D:4D while controlling for biological sex or the change in the case of female instead of male participants while controlling for 2D:4D.

Additional analyses: digit asymmetry measures
To inspect our 2D:4D data for anomalies, digit asymmetry measures were analyzed. For this, we focused on replicating three previous observations reported in the literature: a sex difference in left-and rightward asymmetry [19], a sex difference in composite asymmetry [10] and a quadratic relationship between 2D:4D and composite asymmetry [10].
Similar to the methodology of Voracek, Offenmüller & Dressler [19], participants were grouped in three categories depending on their signed asymmetry scores between right hand and left hand for the second and fourth digit: participants with scores above 0.25mm for both digits were categorized as having rightward asymmetry, participants with scores below -0.25mm for both digits were categorized as having leftward asymmetry. The remainder was categorized as an "all other" group. Using a chi-squared (χ 2 ) test, we analyzed whether there was a statistical dependency between biological sex and directional asymmetry. Our results replicate the finding of Voracek and colleagues [19]: there was a significant relationship (p < 0.005), with leftward asymmetry being more frequent in men and rightward asymmetry being more frequent in women.
The 3x2 data used for the analysis is shown in Supplementary Table S14.
Similar to the methodology of Manning & Fink [10], we computed the relative asymmetries for the second and fourth digit, then summed these and divided by two to obtain a composite asymmetry score (Asy 2,4 ). Then, as done in the previous study, we restricted the range of second digit and fourth digit asymmetry from perfect symmetry (0) to 10% asymmetry to remove extreme cases. Our analysis using a two-tailed t-test did not replicate the finding of a sex difference in composite asymmetry (d = −0.03, p = 0.45). The average composite asymmetry was approximately 3% in both female and male participants, compared to 2% in female and 1.8% in male participants of the previous study by Manning & Fink [10]. However, standard deviations for composite asymmetry were very similar compared to previously reported estimates [10]. Furthermore, the finding of a positive quadratic relationship between 2D:4D and composite asymmetry [10] was replicated in both right hand and left hand data (p < 0.005), using polynomial regression models. The results of these analyses on composite asymmetry are presented in Supplementary Table S15 and Supplementary Table S16.
The analyses of digit asymmetry measures did not indicate any major anomalies in our 2D:4D data compared to previous datasets, further supporting the notion that reasonably accurate 2D:4D measurements were collected. Note: 2D:4D = the length of the second digit divided by the length of the fourth digit of the hand (R = right hand, L = left hand), Asy 2,4 = composite asymmetry, CI = 95% confidence interval. β indicates the change in composite asymmetry for a change of one standard deviation in 2D:4D, e.g., a β of 0.01 would indicate a change of one standard deviation in 2D:4D being associated with a 1% increase in asymmetry.

Additional analyses: interaction between 2D:4D and history of depression
Based on the theorizing that higher prenatal testosterone exposure, associated with increased activation to positive stimuli [8], may be a protective factor in brain development, associations of 2D:4D with behavior within the clinical population could possibly differ from associations in the rest of the population. To investigate this, additional analyses including history of depression and an interaction term between 2D:4D and history of depression as predictors were performed for the neuroticism and PHQ-2 outcome variables. The results of these additional analyses including an interaction term with history of depression for right hand 2D:4D, left hand 2D:4D and the mean 2D:4D of both hands are shown in Supplementary Table S17,   Supplementary Table S18 and Supplementary Table S19, respectively.
There was suggestive evidence for one interaction effect, but it was restricted to the right hand data and the neuroticism outcome, with a p-value barely below the threshold (p = 0.046). The p-values for the left hand and mean 2D:4D interactions were considerably larger. Therefore, it seems plausible that this suggestive evidence for a single interaction effect in the right hand data could represent a false positive. Due to these limitations and the limited usefulness for informing the evaluation of our hypotheses, these results are not further discussed in the main article. It should be noted that the main effects on neuroticism in the right hand and left hand data were surprisingly similar, resulting in identical rounded values. Note: R2D:4D = the length of the second digit divided by the length of the fourth digit of the right hand, CI = 95% confidence interval, FEM = female sex, HIST = history of depression, NEUR = neuroticism, OR = odds ratio, PHQ-2 = Patient Health Questionnaire-2. β indicates the change in the z-standardized outcome variables for a change of one standard deviation in 2D:4D while controlling for biological sex and history of depression, the change in the case of female instead of male participants while controlling for 2D:4D and history of depression or the change in the case of cases instead of controls while controlling for 2D:4D and biological sex. Note: L2D:4D = the length of the second digit divided by the length of the fourth digit of the left hand, CI = 95% confidence interval, FEM = female sex, HIST = history of depression, NEUR = neuroticism, OR = odds ratio, PHQ-2 = Patient Health Questionnaire-2. β indicates the change in the z-standardized outcome variables for a change of one standard deviation in 2D:4D while controlling for biological sex and history of depression, the change in the case of female instead of male participants while controlling for 2D:4D and history of depression or the change in the case of cases instead of controls while controlling for 2D:4D and biological sex. Note: M2D:4D = the mean of the digit ratios of the right hand and left hand, CI = 95% confidence interval, FEM = female sex, HIST = history of depression, NEUR = neuroticism, OR = odds ratio, PHQ-2 = Patient Health Questionnaire-2. β indicates the change in the z-standardized outcome variables for a change of one standard deviation in 2D:4D while controlling for biological sex and history of depression, the change in the case of female instead of male participants while controlling for 2D:4D and history of depression or the change in the case of cases instead of controls while controlling for 2D:4D and biological sex.