Second-to-fourth digit (2D:4D) ratio and criminal involvement in schizophrenia

Some studies have reported that a subset of individuals diagnosed with schizophrenia may be involved in aggressive behaviors or legal incidents. However, it is important to emphasize that the vast majority of individuals with schizophrenia are not violent. The second-to-fourth digit ratio (2D:4D), considered an indirect marker of prenatal testosterone exposure, has been associated with aggression and violent behavior. This study investigated the associations between 2D:4D digit ratios, clinical symptoms, and aggression levels by comparing schizophrenia patients with and without criminal involvement. The study included 164 participants: 58 schizophrenia patients with a history of criminal involvement, 52 schizophrenia patients without criminal involvement, and 54 healthy controls. The lengths of the second (2D) and fourth (4D) digits of both hands were measured using digital calipers, and separate 2D:4D ratios were calculated for each hand. Clinical symptom severity was assessed using the Positive and Negative Syndrome Scale (PANSS), depressive symptoms using the Calgary Depression Scale for Schizophrenia (CDSS), and aggression levels using the Buss–Perry Aggression Questionnaire (BPAQ). Group differences were evaluated using analysis of variance (ANOVA), chi-square tests, and Pearson correlation analyses. To complement conventional statistics, a machine learning framework using Gradient Boosting and Random Forest classifiers was implemented, and feature contributions were examined using Explainable Artificial Intelligence (SHAP). Patients with criminal involvement had significantly higher PANSS scores compared with patients without criminal involvement (p 0.05), although both groups had higher CDSS scores than the control group (p < 0.001). In the BPAQ subscales, patients involved in crime showed higher physical aggression and anger scores but lower verbal aggression scores. Finger measurements revealed longer fourth digits bilaterally and lower 2D:4D ratios in patients with criminal involvement, particularly compared with non-offending patients (p < 0.05), while second-digit lengths did not differ between groups. Machine learning models showed high discriminative performance (AUC up to 0.99). SHAP analysis identified BPAQ anger, psychiatric treatment duration, PANSS scores, and the left-hand 2D:4D ratio as important contributors to the classification of criminal involvement. Schizophrenia patients with criminal involvement exhibit lower 2D:4D ratios and distinct aggression profiles. These findings suggest that lower 2D:4D ratios may be associated with criminal involvement in schizophrenia, particularly relative to non-offending patients. The 2D:4D finger ratio may warrant further investigation as a potential neurodevelopmental correlate alongside established biopsychosocial risk factors.