If everyone in the world could move to whatever country they wanted to, New Zealand's population would swell by 231 per cent.

United States-based research firm Gallup's most recent Potential Net Migration Index (PNMI), a 2015-2017 survey of more than 450,000 adults from 152 countries, has revealed that New Zealand would see a substantial influx of people if migration were free.

This is pertinent because New Zealand has voted to adopt the legally non-binding UN Migration Compact – the first-ever global agreement on a common approach to international migration – which was signed in Morocco on Wednesday.

The New Zealand Government was undecided on the agreement until Wednesday, when Foreign Affairs Minister Winston Peters announced the country would vote for the compact because legal advice suggested it would not compromise sovereignty. Legal advice suggested the compact is non-legally binding and does not create legal obligations.

READ MORE:

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* UN report: International migrant numbers increase by 49 per cent since 2000

* European Union proposes new asylum rules to stop migrants crossing Europe

* Govt still to decide on UN migration pact, but National says it wants NZ out

National Party leader Simon Bridges said National would oppose the compact if it were in power.

NASA/GODDARD/NPP New Zealand's PNMI is 231 per cent, and its brain gain/drain index is 333 per cent. (File Photo)

Many United Nations member states are expected to adopt the compact, but countries such as the United States and Australia opposed it because of the potential impact on their ability to determine their own immigration and foreign policy.

Getty Images Foreign Affairs Minister Winston Peters says New Zealand is voting for the UN Migration Compact.

THE SURVEY

Although the survey did "not predict migration patterns", it showed how much each country's population could grow or shrink if migration were a free-for-all.

The survey subtracted the number of people who wanted to leave their country from those who wanted to migrate to their country to determine the potential changes to each country's adult population.

It also showed what specific types of people were interested in migrating, and where, so global leaders could gauge what kinds of migrants to expect and find ways to keep existing talent in their countries.

GALLUP New Zealand's population would go through the roof if migration were free, a Gallup survey estimates.

The interviews were carried out with 453,122 people over the age of 15, either in-person or over the phone. Between 1000 and 4000 people from each country were interviewed.

The 152 countries surveyed represent about 99 per cent of the world's adult population.

If a country's PNMI were -100 per cent, that would mean the total adult population would leave if they had the opportunity.

GALLUP If everyone in the world could move to whatever country they wanted to, New Zealand's population would grow by 231 per cent while Sierra Leone's would drastically decrease.

The scale could go all the way to infinity in the opposite direction, meaning the number of people moving to that country was unlimited because it is so desirable.

The brain gain/drain index was measured on the same scale, but if it were -100 per cent, that would mean the total population of highly-educated residents (who have completed four years of education beyond high school or have the equivalent of a bachelor's degree) would leave if given the chance.

New Zealand's PNMI is 231 per cent, and its brain gain/drain index is 333 per cent.

WHAT QUESTIONS WERE ASKED?

The people interviewed for the migration index were asked:

* Ideally, if you had the opportunity, would you like to a) move permanently to another country, or b) would you prefer to continue living in this country?

* If "would like to move permanently to another country", to which country would you like to move? [Open-ended, one response allowed]

GALLUP This map shows the most desirable countries to migrate to in dark green, and the least desirable in light green.

The PNMI showed populations would mostly grow only in New Zealand, Australia, Oceania, North America and Europe if everyone moved wherever they wanted.

Populations in sub-Saharan Africa, the Middle East, North Africa, Asia, Latin America and the Caribbean would shrink.

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