Incarceration rates are one factor in a society's violent crime patterns. Given how few countries there are in the world, especially in the US's democratic/socioeconomic peer group, there's limited value in focussing on just one variable and saying: "Ha ha, that is it!" There are just too many variables and too few countries to compare with to get any kind of certainty.
Other factors include:
Gun ownership certainly has an effect, whether one sees it as deterrent or promoter of violent crime. The US is an outlier there.
Poverty. The nature of crime shifts as a society gets richer.
Society and culture. Japan has an enviably low rate of violent crime.
Immigration rates. Canada has a much higher legal one, with nowhere near the violence (but, before one reads too much into the legal bit, do illegal immigrants commit a lot of crimes? at the risk of deportation?) However, immigration factors can't always be pooh-poohed away as "just racism". Sweden has admitted lots of refugees lately and encountered some problems. Here's the govt's response, which tries to defuse the alarm, but acknowledges some grounds concerns. And, while this article does not address violent crime, murder rates have trended up in Sweden in the recent past.
According to the most recent study, people born abroad are 2.5 times as likely to be registered as a crime suspect as people born in Sweden to two native-born parents. In relation to this latter group, therefore, the relative risk of being suspected of crime for people born abroad is 2.5. For those born in Sweden to two non-native parents, the relative risk is 3.2, which means people in this group are slightly more than three times as likely to be registered as a suspected offender as those born in Sweden to two native-born parents. However, the magnitude of this excess risk decreases when differences in age, gender and living conditions are taken into account, from 2.5 to 1.8 and 3.2 to 1.7 respectively.
To be clear, immigration-related problems are often also an indictment of the unwillingness of a host society to adapt to and integrate immigrants: if you let people come in, you have to find a way to make them feel welcome and allow them to succeed in their new home. Europe could learn a lot from the US, and Canada, in that regard.
Aim of incarceration. This can be retribution, keeping the rest of society safe, deterrence. And finally, rehabilitation and reinsertion. Norway is a country scoring high on the last 2, the US is definitely not, while retribution and deterrence seem a big motivation.
Police and justice system crime solving and condemnation rates. If no one gets caught... The US is fairly good there. But this is also why some places like Mexico or Brazil are so dangerous: a robber knows they likely won't get caught. Police trust rates: will the community support policing and report offenders? The US has a strong lack of trust, at least in certain communities.
Besides police investigation competency, it would also be rather foolish to think no effect would result from longer prison terms. Note this 2012 article from The Guardian, a trusty left-wing stalwart from the UK (but those stats mostly concern non-violent and for-profit crime however).
The researchers concluded that prison was particularly effective in reducing property crime when targeted at serious and repeat offenders. They concluded that an increase of just one month in the average sentence length for burglaries – from 15.4 to 16.4 months – would reduce burglaries in the following year by 4,800, out of an annual total of 962,700.
For fraud, an increase in sentences from 9.7 to 10.7 months would result in a reduction of 4,700 offences a year, out of 242,400. The report declares this to be "a substantial effect, especially when we consider that the length of sentence usually corresponds to approximately half the actual time spent in custody".
Historical, clustered, crime rates. Many parts of the US are quite safe, except for gun happy police (I had a gun pulled on me for not having my car lights on, so, yes, I feel I am entitled to that opinion). But certain parts are not, at all. Look at Chicago murder rates. If you grew up in an area where poverty and violence is the norm, you're more likely to be violent. Yes, as per another, fairly insinuating, answer, some Black communities are a part of that. But is that because they're Black? Or because they stayed poor as most of the country got richer? And have ready access to guns, unlike say minorities in French ghetto suburbs? Italy sees tremendous violence from its Mafia affiliated communities, yet, no one would suggest that is a factor of white Italian men in general.
Disagreements over how to distribute the spoils led to the First 'Ndrangheta war (1974-77) killing 233 people.
The Second 'Ndrangheta war raged from 1985 to 1991. The bloody six-year war between the Condello-Imerti-Serraino-Rosmini clans and the De Stefano-Tegano-Libri-Latella clans led to more than 600 deaths.
Jail can also drive up criminality. France's Islamic terrorists have often started out as low level property criminals who get radicalized. "Learning the trade" in jail is a thing. This is a strong enough concern that even the UN warns of the risk. Though I will also quote a warning from that booklet:
For religiously inspired extremism, it is very important not to confuse people who might have (re)
discovered their faith with people who have developed radical views. Most people who convert or
revert, e.g. to Islam, during imprisonment are doing so for peaceful individual motives or to bond
with a group of other prisoners. However, prejudices around the linkage between religion and
extremism remain very present and may hamper good risk assessment
In communities exposed to both violent crime and prejudiced policing, many children have grown up without their fathers who were in jail. Last, jails in the US are an industry and sometimes people who end up in them really should not have.
The general violent crime decline from the 1990s to the mid 2010s in the US has been theorized endlessly about:
- hard criminals were in jail, kept away from the community.
removal of lead in gas
availability of abortions, giving women the option to raise children when in better conditions
men coming of age in the 70s-80s having been raised by traumatized veterans of WW2.
These explanations are all over the map, and incarceration rates, despite going up under 3 strike laws, are not an obvious winner. Especially not as other countries also saw similar drops.
So, it's a complex thing. Being "soft on crime" is a sure way to lose votes in the US. Not always without reason, the whole "defund the police" thing was a joke, even as the country needs better trained and accountable cops.
But very high incarceration rates are generally not a good sign in a country. And regulatory capture by the prison-police industrial complex has not been good to California's budget for example:
California spends $900 million a year incarcerating parole violators and $465 million on supervising parolees, it said. The bulk of the supervision costs are ''for parole agents who spend much of their time filling out paperwork to send parolees back to prison.''
That is almost $1.4 billion spent on parole. Because two-thirds of new inmates are being returned for parole violations, ''this means that California's parole system is a billion-dollar failure,'' said Nancy Lyons, the deputy executive director of the commission.
A primary reason, experts say, is that the prison guards' union, the California Correctional Peace Officers Association, is the most powerful union in the state and contributes the largest amount of any political action committee to politicians in the state. The union also represents parole officers, and many of them began their careers as prison guards. Experts say the guards and parole officers have a financial incentive to keep the number of inmates high, helping preserve their jobs and ensure high salaries.
P.S. I am trying to avoid being dogmatic. El Salvador's president has introduced an extreme system to move "gang-affiliated criminals" into jail. For all the concerns about human rights, with El Salvador's homicide rate dropping nearly 60%, from its horrific previous rates, this is something to follow. Still, El Salvador is not the US and gang-affiliation tattoos would seem to provide identification advantages...
(El Salvador's recent drop does not appear in this 2023 data. It's now 18, not 52, which is why I mentioned the phenomenon)