The human brain matures when we turn 18. Right?

Not so, it appears. Research made possible by scanners which map the brain shows that the organ continues to change and develop until the mid-20s - findings which have implications for teenagers and adults.

Professor Harlene Hayne of the University of Otago says this is a step from technology which limited researchers to studying brains of people who had died.

The development of non-invasive technology meant young people could be scanned safely and their brains observed while they performed mental tasks. By using functional magnetic resonance imaging (fMRI scanning), researchers realised adult functioning did not occur until a person was in their mid-20s.

Like early childhood, adolescence is a period of rapid brain development, Hayne says. "The brain is most vulnerable when it's undergoing very rapid periods of development, which is why the human fetal brain is so influenced by prenatal exposure to alcohol.

It's also why the infant and young-child brain is so receptive to new experience - because it is undergoing change.

And now those same issues are translated to another developmental period where we have a brain undergoing different kinds of change, but nonetheless dramatic.

"We see, in fact, the adolescent brain is highly susceptible to damage and it's also highly receptive to new experience, which is what makes teaching adolescents such a rewarding experience, because their brain is really poised to process information in a very new way. The developmental changes in the late-adolescent brain are both its biggest weakness and its greatest strength, and it's up to those of us who work with them on a daily basis to arrange circumstances that facilitate growth and minimise harm."

Adolescents behave differently from adults because their brains are different, Hayne says.

They are more risk-prone and less risk-averse; their brains are reward-driven because the dopamine system is rapidly developing. Their brains are also very responsive to certain classes of drugs, including alcohol, which can lead to problems of addiction and over-consumption.

Functional imaging has shown that the brains of adolescents who are sober but who have a long history of alcohol abuse no longer function as well as the brains of people of a similar age who have not had the same exposure to alcohol.

"We really are our brains, so it's not surprising adolescent brains like risky behaviour, driving too fast, unprotected sex, alcohol, because that's what they are programmed to do," Hayne says.

Something similar in the way of risk-taking behaviour happens in the adolescent brains of other mammals.

It may have been an evolutionary selection to get them out of the home nest and into the world, and to find a mate.

"The message for me has always been that understanding the immaturity of the adolescent brain makes us more compassionate with adolescents. If we think of an 18-year-old as an adult and they don't meet up to expectations we get angry with them, but if we think of an 18-year old as an adolescent then we are slightly more compassionate in trying to understand their behaviour."

In addition to changes in risk-taking behaviour, changes in the adolescent brain also influence their sleeping and waking cycle.

As anyone who has raised an adolescent will know, their circadian rhythms change, and their preferred bedtime and waking time gets later and later, but because of the constraints of schooling and jobs they still have to wake up early and so end up with cumulative sleep deprivation.

This sleep deprivation can have adverse consequences for their academic achievement and psychological wellbeing, she says.

Despite adolescents being at one of the most physically healthy stages of their lives - they have escaped the potentially deadly diseases of infancy and have not yet come up against the diseases of aging - there is a 200 per cent to 300 per cent increase in mortality and morbidity between late childhood and adolescence.

It turns out that adolescents engage in a lot of behaviour that is quite literally hazardous to their health, she says.

There used to be big differences between risk-taking behaviour between male and female adolescents, but that gap is narrowing.

Girls are now as much into alcohol, unprotected sex, drugs and other risky behaviour and are hurting themselves and dying at almost as high a rate as their male counterparts.

Hayne believes this is a result of the changes in the way girls are brought up.

The lesson we can learn is that girls haven't always been like this, so if we find what has changed in the way we deal with them, we should perhaps apply the lesson to both males and females, Haynes says.

New knowledge about the relationship between adolescent brain development and risk-taking puts the responsibility on the real adults in a community to design systems that protect adolescents from themselves just as we protect children from themselves, she says.

"The functional drinking age in any country is two or three years below the legal drinking age, so when we lowered the purchase age in New Zealand, we were virtually giving carte blanche to 15-, 16- and 17-year-olds. It's been an experiment; it's failed and it's probably time to rethink that."

Another issue affecting adolescents is driver licensing, with New Zealanders able to operate a motor vehicle from the age of 15, one of the lowest ages of any Western country.

Haynes says New Zealand's graduated licensing system - with learners', restricted, and full drivers' licences - sounds good on paper, but in reality drivers virtually ignore the built-in restrictions.

The argument that the age of conscription is 18, always comes up when Hayne advocates raising the drinking age.

"They say they can go and fight for their country but they can't have a beer. That's exactly right, but let's increase the age at which you can make the decision to fight for your country as well.

"It's a ridiculous benchmark to use for all these other things. I'm not absolutely convinced that an 18-year-old can actually make an informed decision about going to war," she says.