There are two types of depression: psychological depression and existential depression.
Psychological depression is what most everyone means when they talk about "depression." Psychological depression is a reaction to the stress and anxiety caused by a negative circumstance. It often clears up on its own, but sometimes it messes with brain chemistry and requires some help to pull out of, even after the original circumstances no longer exist.
Existential depression might also be understood as intellectual depression. It is the outcome of much careful consideration of life, its reality, and its meaning. To be sure, not everyone who ponders the mysteries of life comes to existential depression, but some do. One differentiating factor leading to existential depression is the experience and knowledge of circumstances that seem to contradict hopeful assessment of the human condition. Again, not everyone who ponders the problem of evil comes to existential depression, but some do. This brings us to probably the most particular differentiating factors leading to existential depression: high intelligence and emotional sensitivity. Emotional sensitivity combined with high intelligence create an individual that is both highly intuitive and empathic--a burden-bearer that eventually becomes consumed with the futility of life.
Existential crises arise when one's conception of the universe and life is no longer adequate. One example would be a believer in God who finds they can no longer believe in God. The facts as they see them point them to atheism, but the existential crisis comes when they realize that without God, there is no point. The resolution of this crisis comes when a person creates their own meaning, or borrows meaning from another belief system.
The core problem of those who experience existential depression is a lack of this existential meaning. This is not to say that these people have nothing meaningful in their lives; they may care greatly about a spouse and children, for example. However the ultimate meaning is missing. Awareness and/or experience of enough negative circumstances have convinced them that the situation is irredeemable or without meaning. (Their high level of intelligence commits them to the logical conclusion even if it ruins them.)
Ultimately, then, the task for those experiencing existential depression is to 1) join a religion or 2) create their own meaning. The high level of intelligence probably precludes joining a religion and very likely the person has already left religion in their personal quest for truth. So the more reasonable route is to create meaning.
The creation of meaning is a journey that begins with finding out oneself and applying those better principles and strengths in constructive ways that benefit the world around you. Candle in the darkness kind of stuff. It can be as simple as creating music or art. It could be creating a business or organization. It could be becoming a part of another organization that is doing or creating something. The core of this is creative, constructive, community-connected activity. People with existential depression have to realize that they could create the kinds of positive environments that demonstrate to others that life is something better than futile, if even just a little. The very fact that you have this existential depression means that you have just enough hope to not fill the existential void with the acts of selfishness and evil that make others despair and give up. Now just kick that up a notch or two!
It occurs to me that the freedom to create meaning is contingent on other life circumstances. For example, I am not going to be creating much "meaning" if I can't pay my bills. I am going to be worried about paying bills, or putting food on the table, or what have you. This is where Maslow's hierarchy of needs fits in.
Maslow's hierarchy of needs is a theory that claims that certain needs must always be fulfilled before higher needs can be attended to. In order from most pressing (lowest) to least pressing (highest), they are: physiological (bodily requirements), safety, love/belonging, esteem/respect, and self-actualization. Maslow's hierarchy of needs has been criticized on the grounds that these levels can be arranged differently or possibly even combined based on culture.
However the point to be made here is that self-actualization, or fulfilling one's personal potential, usually does take a back seat to the other needs, and it is precisely within this fulfillment of personal potential that those with existential depression may find their deliverance.
The question may arise that if self-actualization is precluded by other unmet needs then how it is that a person can even get to point of having existential depression. They wouldn't have the energy and time to think about it! The answer here is that the mind is not bound, and intelligence takes the mind far beyond mundane matters.
Lower level needs can provide a welcome distraction from existential depression. However, intelligence will continue to see beyond the distraction and the realization that the person is stuck in trying to just make it can make things worse. Usually in such a situation the depression goes underwater for a while then reappears for a while and so on. So it is important that such a person reach for self-actualization no matter what. I would dare venture that the majority of people with existential depression are no higher than lower-middle class for the simple reason that the need and opportunity for self-actualization are rather evident if the matter can be attended to without the distraction of other unmet needs. As a matter of fact the general lack of self-actualizing activity to those lower on the income scale may even precipitate an outcome of existential depression. It's like being fat because you have no idea what exercise is and all you've ever eaten is McDonald's.
A person stuck in existential depression has got to try to create and connect and self-actualize even if the cupboards are bare and the car is spewing smoke and the medical bills can't be paid.
Religion - I find it very interesting in Christianity, throughout the New Testament, that the message of Jesus Christ is held to be of little value to those with money or intelligence, specifically that these things are seen in the Bible as very nearly literal barriers to salvation. So then Christianity is for those who can't realize their own potential? This is analogous to Marx's conclusion that religion is the "opiate" of the people. Those with existential depression are likely to drop out of religion.
Politics - I think American right-wing politics is getting "responsibility" and self-actualization mixed up. There are plenty of "responsible" poor people who are so busy trying to make ends meet they can't seem to find the resources to bring out any greater inner potential. Should poor people try? Sure. However the deck is stacked against them. Those with existential depression are probably not right-wing in their political views.
Marriage - There have been studies showing that purpose for marriage over the years has changed from love to growth. In other words, people now seek a marriage partner that will allow both of them to grow (self-actualize). Younger people are getting married less, especially where they are farther down socioeconomically (less of a consciousness about self-actualization). Those with existential depression are married for love or necessity or not married at all.
Inequality only serves to exacerbate the negatives mentioned above. It's Maslow's hierarchy of needs in full force, especially with the phenomenon of poorly performing schools in low income neighborhoods (family and economic stability preclude an emphasis on educational attainment, or in teenage years a full existential crisis is being papered over with irresponsible behavior). Neighborhoods higher up on the socioeconomic ladder can transmit values of personal self-attainment better and better facilitate it as well.