Longer Prison Sentences Make Incarceration More “Contagious”

When people receive prison sentences about 17 months or longer, their peers become significantly more likely to be locked up

Jason Edwards/National Geographic Society/Corbis

Social scientists have known for some time that imprisonment can act like a disease. Prison is cathcing—friends and family members of people sent away are more likely to be jailed themselves, just as friends and family members of a sick person are more likely to catch their viral flu. 

To carry the analogy further, imprisonment has become like an epidemic amongst some minority communities: one in every three African American males born today can expect to go to prison at some point in their lives. Black people "are six times more likely than whites to be incarcerated, making up 38 percent of the 1.6 million Americans behind bars while accounting for only 13 percent of the U.S. population," Science magazine noted.

Looking at imprisonment this way, researchers hypothesized that there may be a "tipping point" whereupon imprisonment reaches epidemic proportions. Virginia Tech statistician Kristian Lum and colleagues looked into the possibility that the longer sentences black people usually receive (on average) could represent that tipping point. For drug possession sentences, for example, white people are given an average sentence of about 14 months, whereas black Americans usually get about 17 months.

In a study published in the Journal of the Royal Society Interface, Lum and colleagues created a virtual world of imaginary people modeled after our own, assigning each a gender and age, and drawing on data from the Census Bureau and other federal agencies. At the beginning, one percent of the population was in jail. The scientists then ran a simulation twice, first using a 14-month sentence and then a 17-month sentence, and observed what happened over the next 50 years. The differences were dramatic, as Science explained

The disparity in sentencing between whites and blacks emerged as the single factor making the disease of incarceration a true epidemic among blacks... With no differences between the two hypothetical groups other than the length of the sentence, even three extra months in prison led to higher incarceration rates over time. In the test for which the shorter sentence was used, incarceration rates actually declined from the starting point of one percent... to 0.725 percent, for the 50-year duration.

In the second simulation, however, the longer sentence clearly fell on the other side of the tipping point. When the virtual inmates were imprisoned for 17 months, incarceration rates throughout the community climbed steadily until just under 3 percent of the population was in jail 50 years later.

When the scientists kept the simulation running indefinitely, the incarceration rate leveled off at one percent for the shorter sentence, and seven percent for the longer one. That figure, amazingly, "mirrors the difference in rates of white and black incarceration in the United States today."