Are there really 21 million slaves worldwide?

Some Caribbean nations are demanding reparations from Europe over the Atlantic slave trade. But slavery still happens today - film director Steve McQueen said at the Academy Awards there are 21 million slaves worldwide. Is that figure correct?

The figure mentioned by McQueen in his Best Picture acceptance speech at the Academy Awards comes from the UN's International Labour Organisation (ILO), which has been producing a global figure for nearly 10 years.

The kind of slavery depicted in McQueen's film, 12 Years a Slave, is long gone, although the legacy of the slave trade lives on - claims for reparations are reportedly being made by 15 former colonies against eight European countries.

Slavery experts believe there is nowhere in the world where people are legally shackled, beaten and sold as if they were property, but the ILO's report "2012 Global estimate of forced labour" estimates that 20.9 million people are victims of forced labour.

Its definition of this is "work or service which is exacted from any person under the threat of a penalty and for which the person has not offered himself or herself voluntarily".

The ILO provides estimates for different world regions and its report suggests Asia has the most slaves - 11.7m

Another measurement of global slavery comes out with a much higher worldwide total - the Global Slavery Index says there are 29.8m, half of which are in India.

"There is trafficking of young women and boys into commercial sexual exploitation," says lead author Kevin Bales, a professor of contemporary slavery at the University of Hull. "There's slavery in carpet weaving, mining and ship breaking."

The most common form is called collateral debt bondage, which involves people who have borrowed money pledging themselves and their family as bonded labourers to the loan shark or slaveholder, which can carry on through generations until the debt is paid.

The index, which ranks 162 countries, puts China, Pakistan and Nigeria behind India as the four countries with the largest numbers of slaves. Meanwhile, the top spot per head of population is Mauritania in West Africa, followed by Haiti.

Even the US is estimated to have around 60,000 slaves, among them temporary visa holders and domestic servants. There are 4,426 in the UK, which currently has an anti-slavery bill going through Parliament.

Alex Balch of the Centre for the Study of International Slavery at the University of Liverpool, says: "The Global Slavery Index is pushing this idea that we should be recasting trafficking and forced labour and call them both - modern slavery. Calling these people slaves and calling it slavery is in order to garner political action."

It is an approach which seems to be having some results. In a speech in 2012, President Obama was keen to spell out his views on the subject. "I'm talking about the injustice, the outrage, of human trafficking, which must be called by its true name, modern slavery."

But how have Bales and his colleagues arrived at the figures?

The Global Slavery Index in common with the ILO uses what are called secondary sources. These can be government figures, NGO research and reports in the media. But Kevin Bales says another key tool is the random sample survey where researchers carry out interviews with real people to collect data on slavery.

They currently have fewer than a dozen of these country surveys. They then mathematically extrapolate this data to other countries where they have no survey data.

"I believe it is reasonable," says Bales. "We called in a very large number of statisticians who independently assessed our methodology before we went public and they also agree that it was acceptable."

However he does admit that the interviewing people to collect statistics is a method that risks double counting when people are enslaved in a different country to the one they are from.

So can we trust these figures? Or is there too much guess work involved to take them seriously?

"We may be completely off the rails, I can't deny that," says Bales. "When you are confronting a crisis and you are coming up with the best numbers you can, of course it's right to put that number out there."

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