Japan has the world's oldest population, with more than a quarter of its citizens aged 65 or older.

The ageing population has already put a strain on Japan's financial system and retail industry. But in recent years, another unexpected trend has been unfolding: In record numbers, elderly people in Japan are committing petty crimes so they can spend the rest of their days in prison.

According to Bloomberg, complaints and arrests involving older citizens are outpacing those of any other demographic in Japan, and the elderly crime rate has quadrupled over the past couple of decades.

In prisons, one out of every five inmates is a senior citizen. And in many cases — nine out of 10, for senior women — the crime that lands them in jail is petty shoplifting.

The unusual phenomenon stems from the difficulties of caring for the country's elderly population. The number of Japanese seniors living alone increased by 600% between 1985 and 2015, Bloomberg reported. Half of the seniors caught shoplifting reported living alone, the government discovered last year, and 40% of them said they either don't have family or rarely speak to them.

For these seniors, a life in jail is better than the alternative.

"They may have a house. They may have a family. But that doesn't mean they have a place they feel at home," Yumi Muranaka, head warden of Iwakuni Women's Prison, told Bloomberg.

It costs more than $20,000 a year to keep an inmate in jail, according to Bloomberg, and elderly inmates drive that cost even higher with special care and medical needs. Prison staff members are increasingly finding themselves preforming the duties of a nursing home attendant. But female inmates interviewed by Bloomberg suggested they feel a sense of community in prison that they never felt on the outside.

"I enjoy my life in prison more. There are always people around, and I don't feel lonely here. When I got out the second time, I promised that I wouldn't go back. But when I was out, I couldn't help feeling nostalgic," one of the women told Bloomberg.

Intentionally getting arrested isn't necessarily unique to Japan. In the United States, for example, there have been cases of people deliberately getting locked up to gain access to healthcare, avoid harsh weather conditions, or force themselves to quit a drug habit.

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But the scale of Japan's problem is alarming authorities. The government is trying to combat its senior crime problem by improving its welfare system and social services program, according to Bloomberg, but the wave of senior criminals doesn't appear to be ending any time soon.

"Life inside is never easy," social worker Takeshi Izumaru said. "But for some, outside, it's worse."