An investigation into the relationship between digit length ratio (2D: 4D) and psychopathy

This study investigates whether a higher level of pre‐natal testosterone, as indicated by the 2D: 4D digit ratio, is a correlate of psychopathy and cognitive empathy. The main research hypothesis predicts that 2D: 4D digit ratios will correlate negatively with psychopathy scores and positively with cognitive empathy scores. The study facilitates a within‐subjects correlational design, using a quasi‐experimental method consisting of questionnaires and digit measurements. Fifty‐four participants volunteered and were gathered via purposive and convenience sampling. Each participant completed a self‐report psychopathy scale, a cognitive empathy test, a verbal IQ test and a scan of both hands. The scans were later measured and computed for digit ratios. T‐tests, multiple partial correlations and multiple regression found that, contrary to the research hypothesis, a higher level of pre‐natal oestrogen, as indicated by a larger 2D: 4D digit ratio, is a positive correlate of psychopathy in females, and a positive correlate of callous affect (psychopathy sub‐scale) in males, at less than 0.01 probability levels. Gender is also a correlate of psychopathy, as males scored significantly higher than females at 0.01 probability levels. This result sheds an interesting new light on the physiological correlates of psychopathy and the role that pre‐natal oestrogen plays in organisational effects on the brain during pre‐ and post‐natal development.