Since the 1990s the number of prison inmates skyrocketed According to the article is that the cause of declining crime rates?

Answer 1

Since the 1990s, there have been new categories of crime added. The ever increasing types of drug trafficing and sales and the new laws to combat that; new types of laws to address domestic violence and stalking, just to name a few. Prior to the eighties, these last two were not criminalized and victims had to fend for themselves.

Still addressing the sharp rise in prison inmates over the past 20 years, many new effective tools have been developed, in particular, the sciences, computer investigation, tracking telephone and financial records. As the crime rate went down, authorities developed cold case squads to solve old crimes that they couldn't solve 15 and 20 years ago. These new methods have been very effective.

I don't know the source of your statistics; my trust in them goes only so far. Back in the '90s, I got into a "discussion" with a friend about whether the violent crime rate was up or down; she said up and I said stayed about the same. After looking into how to find those numbers and who reports them, I found out that the US Dept. of Justice uses reports from law enforcement agencies around the country and the FBI used telephone surveys, contacting a cross section of Americans and asking them about their experience with violent crime for the past 12 months. Sure enough, the Dept. of Justice numbers said it was up and the FBI said it was down. Why the difference? Because law enforcement agencies are government agencies depending on getting funding for the next fiscal year. Go figure. So always understand whose numbers you are quoting and what they're based on.