Can aggression be measured?

What our fingers say about us

It has been said that a large forehead indicates a proclivity for criminal behaviour. Unfortunately, the research claiming this is outdated and therefore completely wrong. But, says RUG PhD candidate Alvaro Mailhos, the questions remains interesting: can physical characteristics be linked to behaviour? His research concludes that a long ring finger indicates that a person is more aggressive. But at the same time he wonders: ‘What’s the point of this?’
By Tim Bakker / Translation by Sarah van Steenderen / Illustration by René Lapoutre

Social and physical dominance can be inferred from both behaviour and appearance, Alvaro Mailhos says. He will receive his PhD for his research on Thursday.

Football players whose ring fingers are significantly longer than their others show aggressive behaviour more often. They were given yellow and red cards more often during matches.

And the most competitive players also overestimate their own height, just like animals puff themselves up to show dominance.

The link between social dominance and puffing yourself up can also be deduced from someone’s signature.

And in women, the size of their signature is associated with narcissism.

But we should not be too hasty in drawing conclusions, warns Mailhos: other factors play a much more important role in people’s behaviour.

Reading time: 10 minutes (1,350 words)

The past has shown that theories about the connections between biological and psychological characteristics are not always up to snuff (see boxes). More often than not they were complete nonsense, says Alvaro Mailhos (behavioural and social sciences). But that does not mean there is no connection whatsoever, a fact he proved with a set of studies that will earn him his PhD at the RUG this Thursday.

In the nineties, Mailhos got his PhD in physical chemistry. Now, he works as an instructor of molecular biology at Universidad de la Repúplica in Uruguay. It was there, several years ago, that he met RUG professor of social psychology Bram Buunk, with whom he started this PhD research.

‘I know a lot about evolutionary psychology and sexual selection mechanisms, which is a sadly underdeveloped field in Uruguay. I wanted to contribute to broadening it. Moreover, my way of research is especially cheap. Not needing much in the way of research funds is always advantageous.’


Located as they are on the intersection between evolutionary and social psychology, Mailhos researched the indicators of dominance, including the ‘softer’ social dominance (‘leadership, in other words’), aggressive dominance, and intersexual competitiveness between men and women.

In men, Mailhos used the ratio between the index and ring fingers as a physical clue for aggressive dominance. He had good reason for this. ‘A foetus that has been exposed to a high testosterone level in utero often has a ring finger that is longer than the index finger’, he explains. ‘The 2D:4D ratio, the ratio between the index and the ring fingers, is an indication of those testosterone levels. And testosterone levels in turn influence aggressive behaviour.’

The direct connection between the finger ratio and aggressive behaviour is difficult to measure. Mailhos came up with an inventive, more objective way of measuring: football. ‘Football is the most popular sport in Uruguay, and your skill level directly influences your social status’, he says. A football match, therefore, is an ideal situation in which to study competitiveness. The pressure to get the ball, as well as the constant physical contact: these are perfect breeding grounds for emerging aggression.

Mailhos followed several youth teams in a Uruguayan football league and counted how many yellow and red cards the players received, using them as in indicator of the frequency and degree of aggressive behaviour. He then compared this information to the players’ 2D:4D ratio. It turned out that the players who received one or more red cards per season had a significant lower 2D:4D ratio – relatively longer ring fingers – than their team mates who had not received any cards. The number of weighed cards per match was also higher (red cards are heavier than yellow ones).

Unfortunately, a person’s hands do not tell us everything. Or perhaps that is a good thing. But that is why Mailhos also studied psychological characteristics as a yardstick for dominance. To do this, he was inspired by nature. ‘We often see animals that encounter opponents puff themselves up’, he explains. ‘Dogs and cats raise their hackles, and bears raise themselves up on their hind legs.’


‘In our society, height plays a similar role and can indicate health, wealth, social environment, and finally, a person’s ability to physically harm others.’ That is why Mailhos asked football players who had been tested on their intersexual competitiveness about their height, without telling them he would measure it himself later. According to him, the more competitive players were likely to overestimate their height. The fact that this actually happened is striking. After all, there was no direct confrontation or threat that would cause the players to ‘puff’ themselves up.

‘So I wondered if there were other ways in which more competitive people tried to make themselves look bigger. I realised that a signature is something that represents ourselves in a way, and that we might be able to use the size of signatures as a barometer for social dominance. Inspired by a different study, we also tested for narcissism, which is a form of social dominance.’

Adrenaline en criminality

Professor of criminology Wouter Buikhuisen, who worked at the RUG in the seventies, said there was a link between criminal disposition and biological factors.

Buikhuisen thought that people who produced less of the stress hormone adrenaline would feel less stress. This led to the supposition that they would not be frightened as easily and therefore be more prone to criminal behaviour.

Buikhuisen was greatly criticised for his theory. No person is a born criminal, his opponents said much in the same way Lombroso’s had, they are a product of society.

The discussion about this is still ongoing. Several years ago, for instance, American professor of neurocriminology Adrian Raine concluded that serial killers often have an abnormally low heart rate and that malnourished children more often grow up to be gang leaders.

Mailhos’ hypothesis was correct once again. People who are socially dominant often have larger signatures, although the differences were difficult to identify with the naked eye. Interestingly enough, only in women was there any direct link to the size of their signature and narcissism, even after the results had been tested for various factors.

‘Perhaps there’s a difference in the way men and women express narcissism’, says Mailhos. ‘Earlier studies into narcissism in women have shown that they often express it through physical displays: through their appearance, by wearing jewellery, etc. And it was also expressed in larger signatures with more curlicues and lines.’


Does that mean we should be wary of women who have large signatures and men who have large ring fingers? Not exactly, says Mailhos. ‘The results are mainly theoretical, and only statistically relevant. You have to take them with a pinch of salt. There are so many other, more important factors that play a role in the expression of aggression and dominance. Only when those other factors are constant can you show a connection. So I can’t use my results to make any predictions.’

Can he give an example, though? ‘It would only be anecdotal. I’d look like a clown.’ Mailhos chooses his words carefully, for a reason. ‘It’s a sensitive subject. Too often the tabloid press uses it to write big headlines. They’ll make up that the ratio between people’s index and ring fingers can determine your luck in love or how successful your career will be, for example. Which is nonsense, obviously.’

‘So I want to be cautious, but I don’t want to debunk my own work. These connections are a fact. And isn’t it interesting that something that happens in the second trimester of pregnancy can have a measurable influence on what happens to a person when they’ve reached adulthood? It shines a new light on the role of biological mechanisms on neural networks. The brain is truly a marvel.’


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