Subtle differences in brain function, personality and social development can determine whether you’re a lion or a pussycat when you drink.

Alcohol-related violence has been making headlines with increasing frequency, but not everyone who drinks alcohol, even to excess, becomes aggressive.

In fact it’s only a small minority, says psychologist and Stanford University substance abuse researcher, Dr Adrienne Heinz.

“Very few people when they drink actually become aggressive,” says Heinz.

But their behaviour can have far-reaching consequences. Families and friends can be the targets of alcohol-fuelled outbursts, as can other unsuspecting members of the public.

“Alcohol remains clearly the most important drug to be addressed as far as harm is concerned,” says Dr David Caldicott, emergency medicine consultant at the Calgary Hospital in Canberra.

Out of control

There is increasing evidence that subtle variations in brain function mean some people behave worse than others when they have a few drinks.

One way in which alcohol’s effects on brain functioning have been measured is to look at how people use what’s known as the brain’s executive system. Decision-making, problem solving and reasoning are all jobs the executive system takes control of. As Heinz explains, it is like the command centre of the brain, that “tells you when to put on the brakes, think about the consequences, steer yourself towards a better long-term outcome.”

But when we drink alcohol, executive control flags, making it harder to reflect on our behaviour and self-regulate. Instead of taking a few deep breaths when we feel slighted or insulted, we give in to our impulses, which for some are violent.

Importantly, some people naturally have poorer executive control than others, and these people, particularly if they are male, are more likely to be aggressive after drinking alcohol.

A lack of executive control could also help to explain why adolescents and young adults are so frequently the perpetrators of violent behaviour when drunk. It has been shown that our brains continue to develop well into our 20s and that one of the last parts of the brain to develop is the prefrontal lobe, the region responsible for reigning in impulses through executive control.

People who have a dependence on alcohol have a “double whammy” when it comes to executive control, according to Heinz. Each time they consume alcohol, their executive functioning is impaired due to the alcohol in their system. But their consistent use of alcohol also leads to poorer executive function even when they aren’t drinking, an effect that can last for up to a year after they stop drinking.

Studies on rats have shown that, as in humans, only a small proportion of individuals become aggressive when inebriated. The studies also show that rats with lower levels of the brain signalling chemical serotonin, and higher levels of another called dopamine, are more likely to be aggressive when given alcohol. (Such brain signalling chemicals are known as neurotransmitters.)

Similar changes in both of these neurotransmitters have been found in chronic alcohol drinkers, and it is believed likely they play a role in violence in non-alcoholic binge drinkers too. People with lower serotonin levels are also known to be more likely to consume alcohol to the point of excess. Since early life trauma and adversity can alter serotonin signalling, these factors have potential to raise the odds of a person having a short temper when drunk.