Crime rates in America have come down considerably in America since 1991, to which nationalreview.com in 2015 reported that: "Today, the national crime rate is about half of what it was at its height in 1991. Violent crime has fallen by 51 percent since 1991, and property crime by 43 percent." Most people then correlate that the credit for this significant and meaningful reduction in crime must be given to the increase in the numbers of our police forces along with their corresponding more sophisticated monitoring tools, that serve to stop crime before it is committed, and the well known fact that America incarcerates more people on a per capita basis than any other well developed country in the world, keeping the "bad" guys off of the streets.
Upon further reflection, this conclusion about why crime rates have fallen in America is not backed up by real evidence that actually supports such a finding. For instance, you can't find a country as similar to America and as close to America as Canada, our contiguous neighbor to the north, which both has a substantially lower overall crime rate than America's which is 43% higher than Canada's, while America's police officer manpower is 27% higher than Canada's as reported by nationmaster.com, but Canada incarcerates people at a rate of 114 per 100,000 persons in comparison to the United States which incarcerates people at a staggering 693 per 100,000 persons as reported by Wikipedia.com.
One of the most important reasons why crime rates in America have fallen so dramatically, is simply the demographics which demonstrates that America is an aging country that today has a median age of 36.8 years as compared to 1990 when it was just 32.9 years, very pertinent, as a significant portion of criminal activities are committed by young people, so that an older population will consequently have less crime. Additionally, many crimes are committed by people that grow up in impoverished environments, in which positive role models are sorely lacking, free time is spend unproductively, good educational opportunities are limited, delinquency is rampant, and parental supervision is anemic. The birthrate in America has declined markedly since 1990, especially telling is the significant decline in the birthrate of those in the lower socioeconomic level, indicating that less children overall are being born into families that have difficulty supporting them in a proper manner conducive towards their successful upbringing.
The fact of the matter is that crime is committed most often by those that believe that their opportunities are limited, are immature in their thought processes, and that have lost hope that they will ever succeed given their current condition. So too, successful social programs that provide the disenfranchised with enough food, shelter, and healthcare, has been successful in taking away some of that fuel to the fire of the angst that the underclass suffers from.
Although experts and social services have all sorts of reasons why crime has declined so markedly in America over the last twenty-five years, nobody seems to know for a certainty, exactly why. The thing about crime is that it is often a reflection of real frustration and anger, and so too it is done often in imitation of others, therefore when you are able to replace these things with hope, with good examples or viable alternatives, and with an inclusive society that lives the principle that it will not leave a child behind, there is that belief, that one's destiny may truly rest in one's own actions.