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Finger length sign of psychopath

Think you know how to spot a psychopath? Researchers say the answer could lie in their hands, specifically, the length of their fingers. 

When it comes to true crime, many of us fancy ourselves modern-day versions of Nancy Drew. 

Despite much experience with actual criminology or psychological studies, fans of Mindhunter, Dahmer and the My Favourite Murder podcast will have a pretty comprehensive understanding of the makings of a psychopath. 

But while quickfire knowledge of the Macdonald Triad and most common serial killer star signs are no doubt useful pieces of trivia, sometimes it pays to have some concrete warning signs to fall back on. 

Fortunately, researchers in Canada think they might’ve found one. 

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According to the new study, published in the February 2024 edition of the Journal of Psychiatric Research, you can tell someone is a possible psychopath by looking closely at their hands. 

The research involved analysing the finger lengths of people with clinically diagnosed psychopathy or other psychiatric issues, to determine whether there were any physical commonalities. 

As it turns out, there are, and it comes down to something called the 2D:4D-ratio, which is “a shorter index finger, compared to a longer ring finger.” 

Understanding the 2D:4D-ratio 

The researchers noted that it is already proven that a shorter 2D:4D-ratio correlates with higher instances of “Dark Triad” personality traits, aggressive behaviour, internet use disorder and more competitive athletic performances. 

Dark Triad traits include narcissism, Machiavellianism and psychopathy – a potent cocktail often found amongst murderers and serial killers. 

The new study set out to test the correlation between a shorter 2D:4D-ratio and “individuals with amphetamine use disorder (AUD), antisocial personality disorder (ASPD), or both AUD and ASPD (AUD + ASPD), and when compared to healthy controls.” 

The researchers recruited 44 individuals (10 female, 34 male) who had previously been diagnosed with either AUD, ASPD or both AUD and ASPD, along with 36 ‘healthy’ controls, with no previously diagnosed conditions. 

The individuals provided scans of their hands and the test sample also completed a series of surveys about “ Dark Triad traits, narcissism sensitivity, and intolerance of uncertainty.

Further, participants with AUD, ASPD and both AUD + ASPD completed “a series of self-rating questionnaires on Dark Triad traits, narcissism sensitivity, and intolerance of uncertainty.” 

The results

The study confirmed that “compared to healthy controls, individuals with a clinically diagnosed psychiatric issue [AUD, ASPD, or AUD + ASPD] showed lower 2D:4D-ratios” – i.e. a shorter index finger and longer ring finger. 

Those in the AUD + ASPD segment scored the lowest 2D:4D-ratios, meaning more psychiatric disorders compounded in higher scores of psychopathology. 

When it came to the Dark Triad, the report also confirmed a correlation between a lower 2D:4D-ratio and higher Dark Triad traits. In fact, “The lower the 2D:4D-ratio, the higher the [Dark Triad] scores among the clinical sample.” 

Those with Dark Triad rates were also likely to have higher scores of narcissism sensitivity and intolerance of uncertainty, but that was not specifically related to the 2D:4D-ratios.

The researchers did note that having a short 2D:4D-ratio isn’t uncommon, so having a shorter ratio isn’t necessarily a future set in stone. 

How does a low 2D:4D-ratio occur?

So what leads someone to have a shorter index finger, and thus a higher likelihood of malignant behavioural conditions? 

According to the study, “Individuals with amphetamine use disorder and concomitant antisocial personality disorder (AUD + ASPD) appeared to have been exposed to particularly high prenatal testosterone and particularly low oestrogen concentrations” during “the first trimester of the foetal development”. 

The effect of which can be behavioural quirks as the person grows up, including “highly problematic” social behaviour and “mental health issues”. 

The study also notes this may be “biologically rooted and understood as a fast life history strategy” – which refers to a being’s rapid development and maturation, which may include risky behaviour, early and rapid reproduction and high adaptability, as a result of challenging environments. 

While there is still much to be learnt about the makings of a murderer, this study is a useful aid in ongoing discussions and research around whether psychopaths are born or made, and how ideally, these scenarios could be avoided. 

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