# U.S. prison population

I recently finished reading: The author writes that 6.6% of U.S. American residents will find themselves at some point in their life incarcerated, about 20 million people. A big number on anyone’s scale. You can also find disturbing figures in Wikipedia: figure. Are these facts misleading? we need to account for population growth. The prison population should naturally rise even if the proportion of crime in the general population is constant, since the population itself is growing. Here I show that these facts are NOT misleading, and that the system is indeed not fulfilling its purpose.

Data for population growth can be found here, data for prison population can be found here. Time span of the data is from 1980 till 2005. I transform the data into percentage growth. What should we expect? The growth rate of prison population should be the same as the growth rate in the general population, under the assumption of a constant crime proportion. Is this the case?That is not the case. Every point here represents one year. The general population growth is around 1% per year, the x lab vary from 0.009 to 0.014, while the prison population growth rate averages at about 5% per year, more volatile with y lab that vary from 0.00 to 0.15 (15%). Some years are better, with only 2% increase or so, but in general, it is clear that things are not the way they should be and that there is no correction over time.

On the positive side, this unproportional growth rate seems to be declining over time:

We can see that the rate of growth is decreasing, more so for the state level, less so for the big fish in the federal level. In this figure, this can be seen by the declining slope of the red line.

In sum, maybe if we check more recent data, we will see that prison population has stopped growing in these worrying rates. The main point of the author in the aforementioned book, who spent more than two decades inside, is that the system has lots of sticks but almost no carrots. prisoners are not rewarded but only punished which creates (among other troubling issues) a volatile subculture that is easy to get sucked into and hard to leave. As a result, many prisoners have years added to their sentence as a result of misconduct and many find their way back after their release. After this short empirical analysis, I agree that indeed something is not working as it was intended.