If we just forget about the murder thing, why not "two strikes you're out" then? Because baseball?
I don't know where you got "more than half of the 20 are first-timers." That is simply impossible: by definition, no one is in the 20% if they are a first-timer.
You can read more about the Pareto Principle here:
https://en.wikipedia.org/wiki/Pareto_principle
From Wikipedia:
"The Pareto principle (also known as the 80–20 rule, the law of the vital few, and the principle of factor sparsity) states that, for many events, roughly 80% of the effects come from 20% of the causes."
This has been proven to apply to crime. Roughly 80% of crimes are committed by 20% of the people who break the law.
"You needed to claim something like "the vast majority of murderers have non-murder priors""
I am making a claim about whether someone should get a relatively light sentence for the given crime, versus whether someone should get a severe sentence for the given crime. I am not making any claims about which specific sentence applies to which specific crime. My claim applies to all categories of crime, from jaywalking to murder.
Your point about murder is irrelevant to the point I was making. However, I have noticed that when I mention "murder" people assume I must mean first degree murder, even though less than 20% of all murders are first degree. What's much more common is 3rd degree murder, such as drunk driving, or driving while sleepy, or over confidence in one's ability to control a camp fire at a camp grounds, or failure to put out a cigarette properly, or any of the hundreds of other small accidents that people can do that then leads to someone else's death, and which an aggressive district attorney will treat as 3rd degree murder.
> A roughly Pareto law (80/20) applies to criminals.
Okay, criminals can be divided into groups 80 and 20.
> The majority of all murders are done by a small group.
Presumably, since it's the next sentence, the murders are done by group 20.
> Meanwhile, the majority of all murderers never commit another crime.
[^] The majority of group 20 commits only one offense. (Unless of course you state that murderers have a non-murder prior, which was the clarification you refuted.)
> Jail sentences should be relatively light for the 80 and fairly severe for the 20.
[+] The non-murderers in 80 should get light sentences and murderers in 20 should get severe sentences.
> That means first-time offenders should get minimal sentences, but repeat offenders should get long prison terms.
Because of the statement marked [^], the majority of murderers should get minimal sentences, which conflicts with the statement marked [+], unless you state that all murderers have a non-murder prior.