The introduction of medical marijuana laws in the United States has led to a significant drop in violent crime.
The decrease is strongest in areas closest to Mexico, where counties near the border experienced a 12.5 percent drop in violent crime, including murders, robberies and violent assaults.
Most of the marijuana consumed in the US comes through Mexico, with US$6B of profit a year moving from the US to drug cartels in Mexico.
The rate of murder has dropped by 10 percent in US border states with medical marijuana laws. Murders directly linked to the drugs trade have fallen 41 percent.
The findings, published in The Economic Journal, appear to support the theory that decriminalisation of marijuana reduces violent crime in markets controlled by drug traffickers.
Medical marijuana laws allowing people to grow marijuana plants legally in the US mean people "don't need to buy illegal marijuana any more, so drug trafficking organisations have far fewer customers," Professor Evelina Gavrilova, one of the authors of the study, told The Independent.
Local farmers can legally grow marijuana to be sold to dispensaries, meaning drug cartels get less business.
She said the smuggling of drugs is directly linked to "extreme levels of violence". Cartels violently compete for territory and may steal drugs from competing cartels. Innocent bystanders who witness these acts of violence may be caught up in it.
The New Zealand Government's medical marijuana legislation will decriminalise cannabis use for those with terminal illness. It aims to kick-start a cannabis-growing industry in New Zealand, but doesn't go as far as some US states, which allow people to grow their own plants for personal or medical use.
A Member's Bill under Green MP Chloe Swarbrick's name would allow the terminally ill to grow their own cannabis. It would also allow those suffering a debilitating condition to use cannabis with the support of a medical practitioner.
As part of the Greens' confidence and supply agreement with Labour, it's been guaranteed a referendum on legalising the personal use of cannabis at or before the 2020 election.