It can show as a change in sexual energy. Depression can be expressed in other ways such as through drug use, gambling, alcohol abuse, or other addictions. The cause of depression can be explained in several ways. There has been much written in the past about the genetic component- that it is passed down through generations. In doing a family history, it would be important to know, for example, if there has been drug or alcohol abuse in the family since this might be a way of self-medication to try to lessen a mood disorder that a relative may have had.
Depression can be environmental.
The person who is depressed often will dominate the family. For example, if the mother suffers from depression, the family will be affected by this and will always be reacting to her mood. This mood can unwittingly be passed down through the family and indeed effect all members to the point that depression may be the hallmark of family dynamics without anyone even realizing it. Also, if someone is raised by a depressed person, and that is the person they love and care about, in their socialization they may be attracted to depressed people and continue the dynamics through generations. Finally, depression can be the result of a child's reaction to the environment that he is part of.