However, the most common causes include abuse, medications, conflicts, death or loss, genetic factors, major events, serious illnesses, substance abuse, and other personal problems. Abuse, which could be physical, sexual, or emotional, early in life can cause depression in the final years of someone's life. Researchers believe that abuse damages parts of the hippocampus, a part of the brain, making it more vulnerable to depression. Specific medications have side effects that can lead to depression. Drugs can unbalance chemical levels in the brain, producing depression, an example would be the acne treatment, Accutane, or the antiviral drug, interferon alpha. Someone may also develop depression due to the emotional damage caused by arguing with family members or a loved one. Experiencing someone close walking out of your life usually always makes a person sad, but depressed people are more than just sad; they believe their life is unimportant and hopeless. Scientists know that depression is hereditary by studying disease patterns in families to determine a percentage of how much of the cause of depression is caused by genes, a phenomenon called heredity. Specifically they look at one twin who has depression, see if the other has depression, and come to the conclusion that the heritability is 40-50% (Levinson 1). Genes can cause depression, but so can other stressful and emotional factors, such as when a major event stresses a person. This could mean a new job, graduating from college, getting married, losing your job, getting divorced, and retiring. Sometimes depression is a reaction to a serious illness or coexists. Substance abuse and depression are one of those situations that came first. Sometimes depression causes substance abuse, while substance abuse can cause depression. In addition to this long list, any other personal problems, such as being bullied or