Today's teens are drinking less alcohol, smoking less and having less sex than 10 or 20 years ago. Dr Jude Ball has been investigating why.

Over the last 20 years, teenagers have been drinking less alcohol, smoking less and having less sex.

Dr Jude Ball has been investigating why.

Listen to the interview here

In the 1990s, teen drinking, smoking and sexual activity rose steeply, then it fell through the 2000s, Dr Ball says.

When it comes to tobacco, a different attitude to smoking is the biggest change she could measure.

"About a third of in 2001 said 'yep, it's okay for people my age to smoke' and that halved between 2001 and 2007 and halved again between 2007 and 2012.'"

Tobacco taxes don't seem to have played a role - the decline in smoking was steepest between 2000 and 2010 and eased off as taxes came in, she says.

Increased health awareness isn't necessarily a factor either, as despite anti-smoking campaigns in the 1990s rates went up over that period.

Dr Jude Ball's doctoral thesis: Sex, drugs, smokes and booze: What's driving teen trends?

Through the 2000s, teenagers drank less and less alcohol without any policy change at all, Dr Ball says.

One big factor seems to be young people don't go out at night the way they used to - and therefore spend less time around alcohol, cigarettes, cannabis and potential sexual partners.

Different parental behaviour also seem to play a big role - the current generation of parents are less permissive of teen drinking and less likely to supply teens with alcohol, she says.

Despite the general view that family life is deteriorating, data doesn't support that. Parents are actually spending more time with their kids and family life seems to be improving, Dr Ball says.

Yet despite this, young people are not getting healthier and happier, she says.

Mental wellbeing is strongly linked to physical activity - and teens aren't as active as they used to be.

"Physical activity is not improving, nutrition is not improving, mental health is getting worse."

Less partying and less physical activity may be two sides of the same coin, Dr Ball says.

"Some of the forces that are discouraging people from getting less involved in substance use could have a shadow side to them."

After three years of research, Dr Ball has found no simple answers to why teen behaviour has changed.

"It's a complex interplay of factors. The glib answers that we might want to put forward - 'oh, its education' or 'oh, its digital media' - the qualitative and quantitative data don't really support those theories."

Go to this episode on rnz.co.nz for more details