Nearly a third of New Zealanders now in their early 40s have tried meth at least once, suggests new research exploring the link between the drug and violence in society.
The Otago University researchers behind the study also found that those who used methamphetamine at least once a week were between two and five times more likely to be involved in violence – either as a perpetrator or victim – than those who'd never tried it.
In a similar finding, those who had used the drug once or twice were more than 60 per cent more likely to be involved in violence during the same time period they were using the drug, compared with non-users.
The findings were drawn from the longitudinal Christchurch Health and Development Study (CHDS), which has tracked more than 1000 people since their birth in Canterbury in 1977.
Researchers asked participants at ages 21, 25, 30 and 35 about their use of amphetamine and methamphetamine as well as involvement in violence, either as a victim or perpetrator.
Twenty-eight per cent reported using meth at least once between the ages of 18 and 35 - and one in 10 had used the drug at least monthly at some point.
A smaller proportion – about 5 per cent – reported using meth at least weekly for a period.
Compared with people who had not used the drug at all, those who used it at least weekly were more than twice as likely to be the victim of intimate partner victimisation; and almost five times more likely to be the perpetrators of violence.
CHDS director Professor Joe Boden said this was the first time meth use and its links to violence had been studied in a longitudinal group.
Harm from the drug had increased in step with global supply networks broadening their reach out of Asia and trafficking in more pure crystalline forms of meth.
"It has long been suggested that amphetamines increase the risk of violence perpetration and victimisation, but evidence thus far has shown only evidence of association, rather than a direct causal link," Boden said.
There was a need for robust evidence on the link between meth and violence in our New Zealand communities.
Other studies had been done in those with mental illnesses or those addicted to the drug, but not in the general population, he said.
One of the benefits of CHDS data was its wide range and depth, allowing researchers to more precisely estimate what harm could be pinned to meth.
The study also suggested meth was the third most common illicit drug in New Zealand, after cannabis and ecstasy.
"Some violence is likely associated with involvement in the drug trade, but our study findings indicate that reducing rates of methamphetamine use in the population overall would result in decreased incidence of violent assaults."
Brain link to antisocial behaviour?
Meanwhile, another much larger longitudinal research group – the world-famous and long-running Dunedin Study – has drawn an intriguing link between persistent antisocial behaviour and certain brain structures.
Findings published this week in The Lancet found that those in the study who went on to engage in life-long antisocial behaviour like stealing, aggression and violence, bullying, and lying had structural differences in their brains that were not seen in people who were only troublesome in their youth.
The authors say their findings cannot prove causation, but offer the first robust evidence of a link – and have implications for the way we treat juvenile offenders.
The observational study of 672 participants suggested those ongoing issues might be related to a thinner cortex and smaller surface area in regions of the brain that have already been identified in other research.
But for the larger group of people who were once antisocial but then grew out of it, no such traits were found.
"Our findings support the idea that, for the small proportion of individuals with life-course-persistent antisocial behaviour, there may be differences in their brain structure that make it difficult for them to develop social skills that prevent them from engaging in antisocial behaviour," said the study's lead author, Dr Christina Carlisi, of University College London.
"These people could benefit from more support throughout their lives."