Sweden has experienced such a sharp fall in the number of prison admissions in the past two years that it has decided to close down four prisons and a remand centre.

"We have seen an out-of-the-ordinary decline in the number of inmates," said Nils Öberg, the head of Sweden's prison and probation services. "Now we have the opportunity to close down a part of our infrastructure that we don't need at this point of time."

Prison numbers in Sweden, which have been falling by around 1% a year since 2004, dropped by 6% between 2011 and 2012 and are expected to do the same again both this year and next, Öberg said.

As a result, the prison service has this year closed down prisons in the towns of Åby, Håja, Båtshagen, and Kristianstad, two of which will probably be sold and two of which will be passed for temporary use to other government authorities.

Öberg said that while nobody knew for sure why prison numbers had dropped so steeply, he hoped that Sweden's liberal prison approach, with its strong focus on rehabilitating prisoners, had played a part.

"We certainly hope that the efforts we invest in rehabilitation and preventing relapse of crime has had an impact, but we don't think that this could explain the entire drop of 6%," he said.

In the opinion piece in Sweden's DN newspaper in which he announced the closures, Öberg said that Sweden needed to work even harder on rehabilitating prisoners, doing more to help them once they had returned to society.

One partial explanation for the sudden drop in admissions may be that Swedish courts have given more lenient sentences for drug offences following a ruling of the country's supreme court in 2011. According to Öberg, there were about 200 fewer people serving sentences for drug offences in Sweden last March than a year previously.

Sweden's prison services will retain the option to reopen two of the closed prisons should the number of inmates rise.

"We are not at the point of concluding that this is a long-term trend and that this is a change in paradigm," Öberg said. "What we are certain of is that the pressure on the criminal justice system has dropped markedly in recent years."

Hanns von Hofer, a criminology professor at Stockholm University, said that much of the fall in prison numbers could be attributed to a recent shift in policy towards probationary sanctions instead of short prison sentences for minor thefts, drugs offences and violent crimes.

Of the fall in prison population between 2004 and 2012, he pointed out, 36% related to theft, 25% to drugs offences and 12% to violent crimes.

According to official data, the Swedish prison population has dropped by nearly a sixth since it peaked at 5,722 in 2004. In 2012, there were 4,852 people in prison in Sweden, out of a population of 9.5 million.

How the rest of the world compares with Sweden

According to data collected by the International Centre for Prison Studies, the five countries with the highest prison population are the US, China, Russia, Brazil and India.

The US has a prison population of 2,239,751, equivalent to 716 people per 100,000. China ranks second with 1,640,000 people behind bars, or 121 people per 100,000, while Russia's inmates are 681,600, amounting to 475 individuals per 100,000.

Brazilian prisons hold 548,003 citizens, 274 people per 100,000; finally, India's prison population amounts to 385,135, with a per capita rate of just 30 inmates per 100,000 citizens.

Among the countries with the smallest prison populations are Malta, Equatorial Guinea, Luxembourg, French Guyana and Djibouti. Sweden ranked 112th for its prison population.

• This article was amended on 12/11/13 to clarify the figures regarding theft, drug offences and violent crime.