Nearly one in every hundred Americans is in prison. Our rate of incarceration is one of the highest in the world, with new penitentiaries becoming overcrowded as soon as they open. Certain environmental issues, such as poverty, racism and poor parenting, increase criminality. Now the focus of research has shifted to biological and genetic factors.
A certain amount of aggression seems to be an instinctual survival trait in humans. It is only when this aggression turns to anger and is directed at causing harm to others does it become violence. Harvard psychologist Richard Herrnstein, co-author of the 1985 book Crime and Human Nature (Simon and Schuster), states, “Criminals are, on the whole, angry people. For reasons we’re still trying to understand, they tend to express that anger in antisocial ways.”
High testosterone levels have been blamed for turning normal aggressive urges into violence. This helps explain why, in all societies, 90 percent of violent criminals are men, with many being young. Even among other species, young males are the troublemakers.
Subtle brain and nervous system damage can lead to learning or attention-span problems that increase chances of criminality. These can involve head injury before or during birth, while drug and alcohol use during pregnancy can cause fetal brain damage. Lead poisoning causes brain injury and has been linked to disciplinary problems and crime.
There is some evidence that genetic factors may be involved. A Danish study of identical male twins, separated at birth and reared apart, found that if one committed a crime, the other was five times more likely than average to commit a crime. The comparison figure for fraternal twin boys reared apart is three times the average.
Studies have indicated low verbal IQ and a tendency toward impulsivity, both demonstrable in childhood, increase the chances of criminality. A low IQ may make it difficult to fully comprehend the rules of society, and a tendency toward impulsivity makes it hard to think of long-term consequences of behavior.
Shortly after birth, infants display unique personalities. Some seem shy and retiring, while others are bold and inquisitive. Inborn traits, when coupled with all the experiences of growing up, lead to the eventual personality and behavior of the adult.
Each of us is a mixture of our biology (nature) and our environment (nurture). The same is true whether one turns out to be an artist, a scientist, a bank robber or a murderer. Research continues, but much more is needed before we can stop building more prisons and successfully address the issue of prevention.