If genetics accounts for better than 50 percent of the risk, what accounts for the remaining percentage? A lot of things do. Going back to our infectious model, comparison of addiction and infectious disease, we can see that there are environmental factors such as the availability of the drug and the permissiveness of the culture, as well as agent factors such as the addictiveness of the substance.

But psychological factors play an enormous role in the development of addiction. Some of these factors, as we’ve seen, have a genetic basis. Traits such as aggressiveness, impulsiveness, and antisocial tendencies tend to run in families independently of addiction. They put people at risk for using alcohol and drugs, especially at a young age. And the only way to develop an addiction is to be exposed to the drug in the first place.

Beyond these traits, though, lie other psychological factors that promote the development of addiction. Remember that each of these drugs stimulates the pleasure center of the brain, and they all affect mood. Being able to change one’s mood instantly is tempting to a person plagued with chronic depression or anxiety. Medical treatment of depression and anxiety may not be completely effective for everyone, and psychotherapy is time consuming and difficult. Many people who are depressed or anxious don’t have ready access to psychiatric services, and they come to feel that the use of alcohol or drugs as a coping mechanism is a useful compromise in the short term.

A history of psychological trauma also puts a person at risk for addiction. Posttraumatic stress disorder, or PTSD, is a reactive state that occurs after a severe, life-threatening trauma, and we know that people with PTSD are at increased risk of addiction. We saw this clearly illustrated during the years of the war in Vietnam. Many returning servicemen had begun using alcohol, marijuana, or heroin while overseas as a way to tolerate the severe stress of combat. Although many veterans quit alcohol or drugs after coming home, a number continued to suffer chronic anxiety and tension as a reaction to their combat experiences. And many developed addiction as a result, even though they were no longer in a combat setting.

One problem with PTSD is that the physiological reaction to the trauma does not subside with time.

When confronted with a sudden life-or-death situation, the body reacts by secreting adrenaline and gearing up to either defend itself or run. We call this the “fight or flight” reaction; it is usually followed by a brief emotional collapse and then quick recuperation from the experience. But in those with PTSD it’s as though this reaction cannot be turned off. The person is jittery, fearful, and hyperalert. Sleep is disrupted, sometimes by vivid nightmares, and the individual experiences a general emotional numbing towards other people and situations.

Sometimes the traumatic event seems to occur over and over again, like an instant replay, even while the person is awake; these flashbacks are painful and isolating. If the traumatic situation is ongoing, or the reaction to it persists for a long time, depression eventually sets in. Mood-altering drugs override the brain’s emotional centers, and people with PTSD often discover that drinking, smoking marijuana, or using other types of drugs is an effective way to temporarily change mood and get some respite from their misery. Unfortunately, addiction often follows, and even this way of managing painful feelings no longer helps but only makes things worse.
The disorder is overrepresented in people with addiction.