One of the main arguments against legalising marijuana - which is what the ACT decided to do this week - is the idea that more people would want to get high if they weren't worrying about getting caught.

The ACT reforms make it legal to possess less than 50 grams of cannabis and grow two plants, which, while symbolically important, is not that different from the previous drug policy. 'Legalisation' doesn't mean weed cafes like in Vancouver or Amsterdam.

The ACT Liberals opposed getting rid of the $200 fine for recreational use partly on the ground it would increase usage.

The Labor Government, meanwhile, argued the opposite: Chief Minister Andrew Barr told Hack he expected usage to go down.

"Since we had the fine system and the decriminalisation that came in in the 1990s, self-identified cannabis use has fallen from around one in five people to about 8 per cent now," he said.

I think it will probably go down longer-term.

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Whatsapp A mural of Greens leader Senator Richard Di Natale in Sydney in 2018.

Though the ACT is the first in Australia to legalise, there's plenty of other jurisdictions in the US and Canada that have gone down that path and kept a close eye on usage rates.

Canada legalised weed for recreational use in October 2018, and saw a surge of people trying pot for the first time, many of them middle-aged.

Colorado did the same in 2014, and saw regular cannabis use increase a little, and occasional use increase quite a lot. The age of first use didn't change much, and neither did the proportion of high school students using cannabis. Treatment admission rates for cannabis decreased.

We can go on comparing with other jurisdictions, but the trouble with this is legalisation doesn't mean the same thing everywhere - in Canada, for example, you can take a bong hit in a licensed cafe.

You can't do that in Canberra.

'This isn't really legalisation - it's more of a homegrown model'

Dr Stephen Bright, a psychologist and harm reduction advocate with Curtin University told Hack the term 'legalisation' doesn't quite fit the ACT model.

"It's more a homegrown model," he said; Canberrans are allowed to own two plants.

True legalisation would mean legalising supply, which means weed dispensaries: the 'weed capitalism' model that's been embraced by California and other US states.

Experts say doing this would have a significant affect on usage rates.

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Whatsapp Don't expect this in Canberra: Kinds of marijuana on display at a California dispensary.

Dr Bright said the ACT reforms are so moderate they won't affect the public so much as the police, who can now focus on other crimes.

"It means that less policing resources and money is spent prosecuting people or following up on people to follow up on cannabis interventions," he said.

How many police resources are being spent on cannabis arrests?

Not that many. Over five years, ACT Police made 210 arrests for possession of 50g or less of cannabis, and issued 31 cautions and 420 fines.

In terms of cultivation, the figures are even lower: 20 fines and two cautions.

What these figures mean is that, even when the consequences of being caught with some weed are relatively low (either a caution or a $200 fine), use isn't that high.

It's hard to see how removing the fine will see Canberra vanish in a haze of blue smoke - a scenario that critics of the reforms are trying to conjure.

Marijuana use is declining in ever state and territory

Indeed, across Australia, rates of getting high are going down.

Fifteen years of cannabis use surveys show rates declining in every state and territory, regardless of how much authorities threaten to fine or arrest people.

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Whatsapp Cannabis use has gone down over the past 15 years.

What's causing this change? Young people are using less. In 2001, more than one-third of 20-29 year olds had recently used cannabis. In 2016, about a quarter had.

In fact, while millenials are saying no, boomers are passing the joint around: Cannabis use among people aged 60 and over has increased by 208 per cent in the past 10 years.

"There's a cohort effect where baby boomers are entering the age bracket and they've got time and money now that they've retired," Dr Bright said.

Illicit drug use across the board is decreasing among young people.

And the cause of this isn't any 'tough on drugs' policy.

Professor Wayne Hall from the Centre for Youth Substance Abuse at the University of Queensland told Hack that penalties for possession tend to have little effect on usage rates.

"People aren't that well informed about what the penalties are anyway," he said.

"Young people aren't deterred either way."

Even states that have criminal penalties - that haven't decriminalised recreational use like the ACT had - effectively have a model of "informal decriminalisation". Police avoid enforcing the laws, and judges given lenient sentences.

"If the police took a person before the courts the magistrates would give them a fine so there's not much incentive for police," Professor Hall said.

Only legalising supply could be expected to affect usage rates, he said.

"The reason for the outrage from politicians is the symbolism of the ACT's decision - they can now say they've legalised marijuana."

But despite the symbolism of this week's news, there doesn't seem to be the political appetite for hash cookies in your neighbourhood cafe, Dr Bright said.

"I'm not sure other states would go to the degree the ACT has gone to," he said.

"But I'm optimistic those states might look at decriminalisation."