Why some Brazilian jailbirds can sing 'I'll be home for Christmas'

• A version of this post ran on the author's blog, Riogringa. The views expressed are the author's own.

Over 550,000 Brazilians are incarcerated, making Brazil home to the fourth-largest prison population in the world. The number of prisoners grew 30 percent over the past two decades, meaning that over half a million prisoners are occupying space meant for less than 300,000. So it's no wonder that there are legal mechanisms in place to alleviate overcrowded jails. Some of these rules happen to be implemented around Christmas.

First, there's the annual pardon. Each year, the president issues a decree ending the sentences of eligible prisoners. These are typically non-violent offenders, and last year it affected those with a sentence of 8-12 years. In 2011, around 4,500 prisoners received the Christmas pardon. In recent years, the pardon has freed around 2 percent of the country's prison population annually.

There's also what's called the saidão, or the "big leave." This can happen up to five times a year for eligible prisoners: at Christmas/New Year's, Mother's Day, Easter, and other holidays. In order to receive this right to leave for a maximum of 7 days, a judge must approve. And it's not just any prisoner who can benefit: prisoners must be in semi-open prisons, have good behavior, and have served a sixth of their sentence if they're first-time offenders or a quarter if they're repeat offenders. Last year, over 34,000 prisoners were eligible for Christmas leave.

During their week away, prisoners are expected to be on their best behavior, and are not supposed to go out partying, for example. Plus, prison agents are supposed to conduct random home visits to make sure prisoners are playing by the rules. Sometimes, electronic ankle bracelets are used: In 2010, around 18 percent of those on temporary Christmas leave had to use them.

Despite the threat of increased sentences and further punishments for disobeying the conditions of temporary leave, in recent years around 5 to 7 percent of prisoners failed to return to jail. In 2012, more than 2,400 prisoners nationwide didn't come back from Christmas leave, around 5.1 percent of those who earned the saidão. That year, Sergipe had the largest desertion rate of any other state: around 21 percent. São Paulo – which has the country's biggest prison population – saw over 1,300 prisoners avoid returning to prison last year.

Over time, the numbers add up. From 2003 to 2012, over 50,000 Brazilian prisoners on temporary leave failed to return to jail, according to a Folha de São Paulo report. Among those who went on the lam include several suspected murderers and a major drug trafficker.