The Institute for Population and Social Research (IPSR) yesterday called on authorities to improve living conditions for female inmates in overcrowded prisons.
The call was made during a meeting with the National Human Rights Commission and the Justice Ministry.
"We found most prisons hosting female inmates, especially resting shelters, are holding prisoners in numbers between two and six times over-capacity," said Kulapa Vajanasara, an IPSR researcher.
The IPSR sent staff to visit more than 10 prisons last year as part of a study pushing for reform in the female prison system, said Kritaya Archavanitkul, one of the study team's members.
A Corrections Department regulation says each prisoner is entitled to a living space of at least 2.25 square metres, but the IPSR found available space was far smaller, she said.
In June there were 44,204 female inmates in jails across the country in female-only prisons, correctional institutions for drug addicts and mixed prisons.
The IPSR visited a prison in the South and found as many as 45 female inmates packed into single rooms.
A resting shelter where prisoners spent 14 hours a day was similarly overcrowded.
In some prisons, extra floors were added inside cells.
IPSR officials found during a visit to a northern prison that 150 female inmates were forced to share one toilet.
"Long police investigations and court procedures have led to overcrowding in prisons as facilities are used to detain people awaiting trial," Ms Kulapa said.
Chanchao Chaiyanukit, acting permanent secretary for justice, conceded that overcrowding was a serious issue.
He said 78% of the female inmates were jailed for drug-related offences. The state's policy on narcotics crimes has led to large numbers of offenders being sent to prison, exacerbating problems of overcrowding, he added.
The IPSR proposed the Justice Ministry and the Corrections Department introduce a "ticket-to-leave" policy.
Under the scheme, prisoners could be set free after serving three-quarters or two-thirds of their jail terms for good or excellent behaviour.
The IPSR estimated some 50,000 prisoners would be eligible for this privilege if the policy was adopted.
Other ideas to ease overcrowding include putting prisoners convicted for petty crimes on probation; assigning them to community service; and putting them under house arrest.
A suggestion by the National Health Security Office to use prisons only for detaining dangerous criminals is another option, the IPSR said.
According to the IPSR study published in June, the United States has the world's highest number of females behind bars (201,200), followed by China (84,600), Russia (59,200), Thailand (44,204) and Brazil (35,596).
In terms of population, Thailand comes first with 68.2 female inmates in every 100,000 people.