About a week ago I had a conversation with someone who thought that crime was on the rise in the U.S., not long before I coincidentally came across this video addressing that and other misconceptions. It's worth a few minutes of your time:
As the video notes, crime has fallen quite drastically, by about half in fact, since about 1993. (By some measures, violence peaked even earlier, in the 1970s; see this earlier post.)
The video is a bit misleading on one point, however: While the U.S. incarceration rate is indeed huge in comparison with that of other countries and much higher than it used to be, in the past few years the overall prison population has gone down (link) as the result falling incarceration rates at the state level, and in 2014 the number of federal prisoners likewise declined (link) and will likely continue to do so at least over the next couple of years (link). You can read more about this (and other misconceptions about incarceration) in a short article from a year ago by Stanford University professor Keith Humphreys at this link. See also this NPR report about a September talk by Attorney General Eric Holder.
Going back farther into history, the very-long-run trend across human history seems to be toward lower and lower levels of violence, as MIT's Stephen Pinker pointed out in his book The Better Angels of Our Nature (which I reviewed back in 2012 here). Pinker summarizes his thesis in a talk here. Pinker of course acknowledges that this decrease has not been steady or consistent, with violence spiking many times in certain regions and periods of time, and not just during wars. For example, crime and violence rose in the U.S. starting in the 1960s before the more recent fall.
The reason for the drop in crime rates since 1993 has no simple explanation. There is an interesting coincidence with the launch of anti-crime efforts by the Clinton administration that included federal funds to encourage cities to put more cops on patrol, especially in high-crime areas. Sending a lot of people to prison for long periods of time is also bound to have had some impact. But attempts to find clear reasons for the change haven't been successful, because an explanation that seems to work in one area falls apart in another.
One interesting possibility is the link between young children's lead exposure and their later problems with impulse control as adolescents and young adults. In particular, atmospheric lead (mainly from the use of lead antiknock compounds in gasoline) rose and then fell about 20 years ahead of the rise and fall in crime. Mother Jones magazine has a useful page on its website referencing a number of articles on that subject (link). From what I've read the idea sounds plausible at least as a contributing factor, but I don't know enough to judge how important the connection might be.
Incidentally, for more underreported good news, see this post from July of 2013).