"Something like 80% of women in prostitution are controlled by their drug dealer, or their pimp or their trafficker," according to former Home Office minister Fiona MacTaggart MP.
The idea that women who work in prostitution are largely in forced labour situations is one of the justifications behind a government proposal to introduce a strict liability offence of paying for sex with a woman who has been trafficked or is under the control of a pimp or her drug dealer.
It will be no defence for an accused man to say that he didn't realise that the woman had been trafficked or was "controlled for another person's gain".
Men accused of this crime will be charged with rape on the basis that the woman can't be deemed to have consented.
Given the underground nature of prostitution, how do researchers know the proportion of women who are controlled by others?
Presenter Tim Harford finds out how the research is done and the reliability of the statistics.
Interviewees include Professor Julia O'Connell-Davidson of Nottingham University, Hilary Kinnell of the UK Network of Sex Work Projects and Fiona MacTaggart MP.
more about the figures on trafficking.
to Ruth Alexander discussing the statistics surrounding prostitution on the Today programme.
Babylon: Myth and Reality is the theme of one of a British Museum special exhibition which runs until 15th March.
A Babylonian mathematical tablet
The ancient civilization is famed for the Tower of Babel, the Hanging Gardens and Belshazzar's Feast - but also for the great advances it made in mathematics.
The ability to write large numbers, the way we measure time, the number of degrees in a circle and the 7 day week - we apparently owe much to Babylonian mathematics.
To find out more, Tim Harford visits the British Museum's Babylon exhibition and talks to Eleanor Robson of Cambridge University's Department of History and Philosophy of Science.
Maths of the Credit Crunch
Catch up on
all our stories about the financial crisis
This week we continue the series.
Where was the media when we most needed it?
Gillian Tett of the
explains the moment she realised that nine tenths of financial activity was being largely ignored by journalists, politicians and regulators.
Until recently, media coverage of the financial markets was mainly limited to equities and mergers and acquisitions.
But the credit markets, derivative markets and the market in loans repackaged into bonds were worth far more.
The maths of bankers' bonuses
Last week, financial mathematician explained how the maths of the bankers' bonus system might have encouraged traders to make the same mistakes rather than diversify. Many of you have questioned the validity of his claim - but he is right and here is why.
He gave a theoretical example. A trader enters a bank where everyone else is doing exactly the same trade and that trade has a 50% chance of making a profit. The trader knows of a trade that has a 75% chance of making a profit.
But in order for the individual trader to get his bonus, two things have to happen: the bank has to make a profit and the individual trader has to make a profit.
If the trader does his good trade, the probability of him getting his bonus is the chance of his trade making a profit - ie 75% - multiplied by the chance of the bank making a profit - ie 50%.
His chance of getting a bonus is therefore 37.5%. However if he does the same as everyone else, his chance of getting a bonus is 50%.
Some listeners asked why the trader's chance of making a profit if he did the same trade as everyone else was not 25% (ie 50% x 50%).
The answer is that the 50 per cent strategy is perfectly correlated with the bank's profitability, while the 75 per cent strategy is perfectly uncorrelated.
When the probabilities are perfectly uncorrelated, 75% x 50% is the right calculation to work out if the trader gets his bonus.
When the strategies are perfectly correlated, either both the trader and the bank make money, or neither the trader nor the bank make money.
BBC Radio 4's
More or Less
is broadcast on Friday, 9 January at 1330 GMT and repeated on Sunday, 11 January at 2000 GMT.
Subscribe to the
More or Less
You can e-mail us by using the form below:
Disclaimer: The BBC may edit your comments and cannot guarantee that all emails will be published.