It's hard to predict what Britain's relationship with the EU will be like once the dust settles in April 2019, but the odds that free movement will continue in its current form are slim. I think that's real shame; as Sam Bowman points out, EU migration has been incredibly beneficial to the UK. EU migrants haven't lowered wages or increased unemployment, they've simply helped us pay down our eye-watering debts (by about £1.5bn a year).

The Government instead seem to prefer what they call a "bespoke immigration policy" with different rules applying to different sectors. In other words, a centrally planned labour market. That'd be a bitter disappointment: it'd mean increased bureaucracy and slower growth for Britain's businesses.

But, as the saying goes, every cloud has a silver lining. It gives us a chance to enact the most effective anti-poverty policy ever discovered.

One sector with a particularly pressing need for low-skill migration will be agriculture. Farmers rely on cheap, seasonal labour. If they can't get it many will be forced to close as they lose out to foreign competition. Most countries rely on seasonal migration programs, indeed Britain itself had one targeted at Romanians and Bulgarians as late as 2013.

But as much as farmers benefit from access to seasonal migrant labour, it's the migrant workers themselves that see the biggest benefit as a new study from the Centre for Global Development's Michael Clemens and Hannah Postel shows..

They looked at a program that loosened regulations in the wake of the Earthquake in Haiti to allow more Haitians to get seasonal worker visas. Clemens and Postel compared Haitians who got seasonal visas to those who were unsuccessful. The results were staggering.

Haitian migrants saw massive income increases. On average they saw their incomes multiply by around fifteen times. And they didn't just spend that on themselves, they helped those stuck back in Haiti by sending remittances. In fact, they were able to double their families annual income in a matter of months.

The paper's authors point out that these results are unprecedented. No other aid programme comes close to producing income benefits on this scale.

It's certainly proved to be a much more effective program than the many recovery projects targeted at Haitians. Haiti received $13bn in humanitarian aid after the 2010 Earthquake, but the results have been dismal. An NBC News report in 2015 found that five years on, there were still 85,000 Haitians living in displacement camps.

Migration beats traditional aid projects for three main reasons.

1. It goes straight to those who need it. A significant chunk of foreign aid is eaten up by overheads (29% on average according to Bill Easterly). Now that wouldn't be a problem if the aid itself (the other 71%) was effective. But too often it's not spent well. It's rare for a project to raise the incomes of the poorest by more than £1 for each £1 spent. It's different with migration, all of a seasonal worker's earnings end up in a Haitian household where nearly 85% is actually spent in Haiti. And once it's spent it Haiti it spurs on even more productive activity.
2. It's not a free lunch, it's a paid lunch. We're fond of saying there's no such thing as a free lunch, but in this case it's one better. We're not just lifting people out of poverty for free, we're actually benefitting from the exchange. These jobs simply wouldn't exist in the absence of seasonal migration programs. Locals wouldn't do it at a competitive wage - so farmers would have had a choice automate the jobs or lose out to foreign competition. When Haitians come to the US, they'll be spending and paying taxes boosting the local economy.
3. It delivers massive financial flows to the developing world. The big reason though is scale. Because a person's productivity varies massively based on where they are - compare an Uber driver in London to a rickshaw driver in India - moderately opening up to migration can deliver gigantic financial flows to the world's poorest. As Clemens put it once, we're leaving "trillion dollar bills on the sidewalk".

These schemes certainly have their critics. Many argue that workers are liable to be exploited on seasonal programs – some have gone as far to say that they're close to slavery. But Clemens and Postel spoke to the Haitians who got visas. They replied that they wouldn't change the program if they could and the only concern they had was that the program would be cancelled and they wouldn't be able to work in the US.

Given the current appetite for greater migration control, it's unlikely that we'll see the level of expanded migration that Clemens and myself would like. But, we can take action at the margin.

New Zealand shows the way. They amended their seasonal worker program to encourage greater migration from Tonga and Vanuatu. That boosted incomes of Tongan and Vanuatuan migrants by factors of 12 and 8 respectively. That's miles better than many (if not all) of the aid projects DFID currently funds and again, free. Let's take New Zealand's lead and use Brexit as an opportunity to try out the world's most effective anti-poverty policy.