Researchers have found a link between the relative lengths of index and ring fingers and certain psychopathological personality traits, including antisocial behaviors and substance use disorders. The study, published in the Journal of Psychiatric Research, provides evidence that people with certain psychiatric disorders have a notably different finger length ratio compared to healthy individuals.
Previous studies have suggested that the ratio of the length of the index finger (2D) to the ring finger (4D), commonly known as the 2D:4D ratio, might be an indicator of various behavioral and personality traits. This ratio is believed to be influenced by the levels of testosterone and estrogen a fetus is exposed to in the womb. The higher the testosterone relative to estrogen, the lower the 2D:4D ratio, typically resulting in a longer ring finger compared to the index finger.
Motivated by these intriguing associations, the researchers aimed to deepen the understanding of how this finger length ratio correlates with specific psychiatric disorders and personality traits. The study’s primary focus was on individuals diagnosed with Amphetamine Use Disorder (AUD), Antisocial Personality Disorder (ASPD), a combination of both (AUD + ASPD), and a control group of healthy individuals.
“The topic is interesting, because the relation of the lengths of index finger (2D) and ring finger (4D) is one of the most robust biological markers formed during the prenatal stage with a remarkable impact later on an adult’s behavior,” said study author Serge Brand of the University of Basel and Kermanshah University of Medical Sciences.
“More specifically, a lower 2D:4D-ratio, that is to say, that the index finger is shorter compared to the ring finger, is strongly associated with an increased exposure to testosterone and with a decreased exposure to estrogen during the prenatal stage. Now, several original studies, meta-analyses and systematic reviews showed that a lower 2D:4D-ratio as a proxy of prenatal sex steroid exposure enables an adult organism to outperform in behavior in case of necessity. What was unclear so far was, if such sex steroid exposure during the prenatal stage might be mirrored in an adult’s behavior such as amphetamine use disorder and antisocial personality disorder.”
The researchers recruited 80 participants for their study, divided into two main groups: 44 individuals with clinical diagnoses (25 with AUD, 10 with ASPD, and 9 with both AUD and ASPD) and 36 healthy controls. Participants were thoroughly informed about the study and its objectives, and confidentiality was ensured. After obtaining their consent, the participants provided detailed sociodemographic information and underwent psychological assessments.
These psychological assessments included measures of Dark Triad traits (a combination of Machiavellianism, narcissism, and psychopathy), vulnerable narcissism, and intolerance of uncertainty. Additionally, a scan of the palm of their right hand was taken to precisely measure the lengths of their index and ring fingers, from which the 2D:4D ratio was calculated.
The researchers discovered that participants from the clinical group had significantly lower 2D:4D ratios compared to the healthy control group. This indicates that those with AUD, ASPD, and particularly those with both conditions, tend to have longer ring fingers relative to their index fingers. Furthermore, males across the groups showed lower 2D:4D ratios than females.
“We were surprised to observe such a linear association between higher symptoms of psychopathology and lower 2D:4D-ratios,” Brand told PsyPost. “That is to say: The more an adult participant had signs of psychopathology, the more it appeared that this adult has been exposed to higher testosterone concentrations and lower estrogen concentrations during the prenatal period of life.”
In terms of personality traits, the study found that lower 2D:4D ratios were associated with higher scores in Dark Triad traits, suggesting a link between prenatal hormone exposure and these socially aversive traits. However, there was no significant correlation between the 2D:4D ratio and measures of vulnerable narcissism or intolerance of uncertainty.
The study’s results are significant, as they suggest that the 2D:4D ratio, a simple physical measure, could potentially be used as a non-invasive biomarker for predicting certain personality traits and susceptibilities. This can offer insights into how prenatal development impacts individual differences in behavior and personality.
As Brand explained, the findings indicate that an adult person’s behavior could be understood as the result of biological traits and current psychological cognitive-emotional and behavioral processes and decisions.”
However, the study is not without its limitations. The sample size was relatively small, which might limit the generalizability of the findings. Additionally, the study primarily included participants from a single psychiatric facility, which may not represent the broader population. The researchers also noted the difficulty in keeping abreast of the vast amount of literature in this rapidly evolving field. Another limitation was the lack of comprehensive psychological data from the healthy control group, which could have provided a more rounded comparison between the clinical and non-clinical samples. Moreover, the study did not account for other comorbid conditions, which are common in clinical settings.
Future research in this area could focus on larger and more diverse populations to confirm these findings. Including participants with a wider range of psychological and physical conditions could also provide a more comprehensive understanding of the 2D:4D ratio’s implications. Additionally, further studies might explore the impact of other prenatal factors on personality development and behavior.
“It is important to understand that the finger lengths-ratio as a proxy of a specific exposure to prenatal sex steroids should not be understood as a person’s irrevocable fate! Rather, a lower 2D:4D-ratio and thus a higher exposure to testosterone during the prenatal period of physiological development might enable an adult person to show a specific pattern of behavior,” Brand noted.
“To make the case in point, compared to 117 age- and sex-matched sedentary females, 104 Swedish female Olympic athletes had statistically significantly lower 2D:4D- ratios, or simply put: The Swedish female Olympic athletes were exposed to higher testosterone and lower estrogen concentrations during their prenatal stage of development (Eklund et al., 2020).”
The study, “2D:4D-ratios among individuals with amphetamine use disorder, antisocial personality disorder and with both amphetamine use disorder and antisocial personality disorder“, was authored by Seyed Sepehr Hashemian, Senobar Golshani, Kimia Firoozabadi, Ali Firoozabadi, Christian Fichter, Kenneth M. Dürsteler, Annette B. Brühl, Habibolah Khazaie, and Serge Brand.