Women from minority hill tribes in Thailand are increasingly being lured into prostitution, an anthropologist investigating the issue has told the BBC.
Girls are often forced into prostitution through unpaid debts
David Feingold, who has recently made a film on the trafficking of women in East Asia, found disturbing evidence of women lured into the Thai sex industry, not just from the remote highland regions but also from neighbouring countries such as Burma, Laos and China.
Mr Feingold told the East Asia Today programme that, within Thailand itself, the greatest factor influencing the trafficking and exploitation of girls was their lack of citizenship.
Because they used to be nomadic, and therefore were effectively status, they have never been recognised as Thai, even though they now live in settled communities in Thailand.
"The single most important thing is to give all of the hill tribes a legal status," he said.
These people - nearly 500,000 in all - are unable to get educational qualifications, own their own land or travel outside their districts to look for work, Mr Feingold said.
This increases the girls' vulnerability, and many get into debt and are then forced into prostitution, he said.
The reasons why Burmese girls end up as prostitutes in Thailand are slightly different, the film-maker said.
Many are fleeing forced labour, displacement from fighting and economic degradation.
Others see their choice as staying at home and getting raped for free by the Burmese army, or going to Thailand for sex work.
"They are making a choice, but it's not a choice anyone should have to make," Mr Feingold stressed.
For Chinese girls, the situation is different again.
"Some girls are literally kidnapped, but there are others who see the streets of Thailand as being paved with gold," he said.
These girls go to Thailand voluntarily, with an economic incentive.
Mr Feingold's film - which is narrated by the actress Angelina Jolie - also contains rare interviews with Thai brothel owners.
Mr Feingold said that many of these owners did not see themselves as evil, but just as people running a business.
He also said that the documentary exposed the high levels of corruption among the Thai police, with the brothel owners treating corruption as a daily fact of life.
"They didn't whisper about it - they simply said we pay this much to the immigration officials, this much to the police, this much on beer, and this much on towels," Mr Feingold said.
His film, Trading Women, will be shown in London, Paris, and Washington in October.