However, the most common causes include abuse, medications, conflict, death or loss, genetics, major life events, serious illness, drug addiction and other personal problems. Abuse, which may be physical, sexual or emotional, during the early stages of life can cause depression in the later years of life. Researchers believe that abuse damages parts of the hippocampus, a part of the brain, making it more vulnerable to depression. Some medications have side effects that cause or can cause depression. Medications can unbalance the levels of chemicals in the brain, producing depression. An example would be the acne treatment, Accutane, or the antiviral drug, interferon alpha. A person may also develop depression because of the emotional harm caused by a fight with family members or a loved one. Seeing a loved one leave your life usually always makes a person sad, but depressed people are more than just sad; they believe their lives are unimportant and hopeless. Scientists know that depression is hereditary by studying patterns of illness within families to determine what percentage of the cause of depression is caused by genes, called heritability. Specifically, they look at one twin with depression, see if the other twin 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 more emotional factors, such as when a major event stresses a person out. This can mean a new job, graduation, marriage, job loss, divorce and retirement. Sometimes depression is a reaction to, or coexists with, a serious illness. Addiction and depression is one of those situations that came first. Sometimes depression causes substance abuse, while substance abuse can cause depression. Adding to this long list, any other personal issues, such as being bullied or