Several lines of research have shown that depressive illnesses are disorders of the brain. But the exact causes for these illnesses are not yet clear and are still being studied. 

Imaging technologies show that the brains of people with depression look different or are working differently than those who do not have the illness. The areas of the brain that control moods, thinking, sleep, appetite, and behavior appear not to be functioning well. The scans also show very high or very low levels of important brain chemicals. But these images do not reveal WHY the depression has occurred.

In general, there is no one cause or risk factor for depression. It most likely results from many factors, such as family history, life experiences, and environment. Older adults with depression may have had it when they were younger, or they may have a family history of the illness. They may also be going through difficult life events, such as losing a loved one, a difficult relationship with a family member or friend, or financial troubles. 

For older adults who experience depression for the first time later in life, other factors may be at play. Depression may be related to changes that occur in the brain and body as a person ages. For example, some older adults who are at risk for illnesses such as heart disease or stroke may have hardening and inflammation of the blood vessels, and blood may not be able to flow normally to the body's organs, including the brain. Over time, this blood vessel disease and restricted blood flow can damage nearby brain tissue and harm the nerve connections that help different parts of the brain communicate with each other. If this happens, an older adult with no family history of depression may develop what some doctors call "vascular depression." 

Older adults may also experience depression as a result of brain changes caused by illnesses such as Alzheimer’s disease or Parkinson’s disease. This type of depression can appear in the early stages of these diseases, before many symptoms appear. 

Depression can also co-occur with other serious medical illnesses such as diabetes, cancer, and Parkinson's disease. Depression can make these conditions worse, and vice versa. Sometimes, medications taken for these illnesses may cause side effects that contribute to depression. 

Because many older adults face these illnesses along with various social and economic difficulties, some health care professionals may wrongly conclude that these problems are the cause of the depression -- an opinion often shared by patients themselves. 

All these factors can cause depression to go undiagnosed or untreated in older people. Yet, treating the depression will help an older adult better manage other conditions he or she may have.