No one wants to go to prison, but too many in the United States end up there. According to the International Centre for Prison Studies, 707 of every 100,000 people are locked up the United States. In Germany, it's 76 per 100,000. Spain is at about 140. China's at 124. Out of wealthy countries, America's number one—the best—at putting people behind bars. With only about 5 percent of the world's population, the United States hold 25 percent of the world's prison population. How did we get here?

1. Excessive punishment for non-violent crimes

It's helpful to look at examples. California has the second largest prison population in the United States. The state has built 22 prisons since 1980 while it built only one university campus in that time. Not exactly an attractive figure, and it's even worse when you consider that it costs the state $8,667 per year to educate a college student at a state university compared to the $45,006 annual cost of housing a prisoner, according to CNN.

Recently, though, California has made serious efforts to reduce its prison population, partially in response to being ordered by federal judges to lessen prison overcrowding after complaints from many inmates about the conditions of prisons. Today, its prison population of 689 people per 100,000 is below the national average. California lowered its prison population by 23 percent between 2006 and 2012, compared to an average nationwide decrease of 1 percent in that time.

One way it did this was by reforming its “three-strikes law,” which consisted of mandating a 25 years-to-life prison sentence for anyone who received three felonies in their lifetime. In 2012, an initiative changed the law so those with three non-violent or non-serious felonies do not automatically serve the minimum they previously would have under the old law. Essentially, if you get caught three times stealing expensive appliances from Sur La Table, you don't end up in prison for the rest of your life.

California's Proposition 47, which was approved in 2014, has helped the state so far release 2,700 inmates whose crimes were changed from felonies to misdemeanors if they were nonviolent drug offenses or property offenses. One major reason the prison population is getting so high is that politicians trying to be tough on crime end up extending sentences for crimes that don't deserve such long sentences, Nazgol Ghandoosh, a research analyst for a criminal justice reform organization The Sentencing Project, told me.

2. The War on Drugs

We all know what's coming next: The War on Drugs, which our current president called an "utter failure" in 2004. Before the War on Drugs ramped up, the prison population ratio in the United States was about 150 inmates to every 100,000 people in 1980—much more comparable to other western countries.

“You'll notice that rising crime rates is not part of the explanation [for the increase in prisoners in the United States]. A recent comprehensive report by the National Research Council concluded that 'changes in crime trends or in police effectiveness as measured by arrests per crime contributed virtually nothing to the increase in incarceration rates over the 30-year period' from 1980 to 2010,” said Ghandnoosh. Crime has not seriously increased, but the war on drugs and some other factors have changed to put more people in prison for longer periods of time.

3. Long sentences

Ghandnoosh also says that long sentences for serious crimes have also gotten out-of-hand. This is due, in part, to mandatory minimums, which are minimum sentences for specific crimes. These are created by Congress or by state legislatures, and they essentially strip judges of the power to issue lenient sentences.

“The next important frontier in criminal justice reform is to curb excessive punishment for people with violent convictions,” Ghandnoosh said.

While many American citizens agree that someone caught with a joint shouldn't go to prison, it's harder to convince someone that a murderer shouldn't be sent to jail for the rest of their life or be executed. However, Ghandnoosh points out that someone who serves a long sentence and has changed their ways over time might not need to be in prison forever.

Marc Mauer, the executive director of The Sentencing Project, has advocated that “federal sentencing structures should establish an upper limit of no more than 20 years in prison, except for exceptional circumstances.” Perhaps someone who blows up a building, for example, might be an exception. He says the older someone gets, the less likely they are to commit serious crimes, according to his research. He also notes that “analysis by The Sentencing Project found that individuals released from life sentences were less than one third as likely to be rearrested within three years compared to other formerly incarcerated individuals.”

There is precedent for getting rid of life sentences. Norway has set a maximum prison sentence of 21 years and prioritizes rehabilitation, without seeing any negative effects. Countries like Spain and Portugal have abolished life-long prison terms, sometimes referring to them as “cruel and unusual punishment.”

4. Private prisons

Another topic that concerns people regarding the prison system is the privatization of prisons. That is, for-profit businesses based on taking in convicted criminals. While there have been many abuses of the system, it is not yet as large of a contributor to the prison population as some think.

“Many people think that private prisons are the main drivers of our over-incarceration problem,”Ghandnoosh said. “They're not. Nationwide, less than 10 percent of prisoners are in private facilities, though some states like New Mexico and Idaho hold over a third of their prisoners in private institutions. In California, the rate is under 2 percent. So overall, mass incarceration is a symptom of politics run amok, rather than capitalism run amok.”

On the other hand, private prisons are becoming increasingly common, so they will likely become more of a widespread problem in the future. Even more worrying, powerful prison guard unions often lobby to put more people behind bars to support their livelihoods. No matter what business someone has, they'll often fight to keep it at any cost -- even if the cost is a stranger's freedom.

Here's a video from Hank Green on mass incarceration: