I have no answer for the causes of "mental illness", and it is very likely and probable that there are multiple causes.
I am going to try to work through my nascent understanding of mental health, depression, and American culture and claim that it has emerged as an epidemic from the destruction of community relations and the sense of loss and feelings of isolation that serve as foundational component of American culture and capitalism.This thesis is nothing new and is built up from the theories of Deleuze and Guattari in Anti-Oedipus: Capitalism and Schizophrenia.
First off, the basis of western culture is the belief in a body/mind dualism- in which the two are separate and treated unequally. In this worldview, the mind serves the same role that a captain does on the ship. The mind is responsible for steering the ship, for deciding its course, and for keeping things together. The mind does not experience the world in the same way that the body does; the experiences of the material are mediated through your sense of touch, smell, sight, and hearing. For anyone who has lived in the real world it is obvious that these two are not separate and that what happens to your mind impacts your body and what impacts your body greatly affects your mental well-being.
Coming from this dualism though, illnesses of the body are given priority- they can be mended- while "disease" of the mind are the responsibility of the individual. We mend broken bones; we treat for cancers and other "life threatening" diseases; but we never examine mental health with the same concern and enthusiasm. For much of western history those with depression, bi-polar disorder, schizophrenia, etc where seen as being the result of morally weakness and spiritual deficiencies.
American culture- a culture that emphasizes individualism, self-sufficiency, and pulling oneself up by the bootstraps (which is something that is physically impossible but that might say more about the American dream then does any sociological research agenda)- compounds the body mind dualism as does the culturally constructed gender roles and rampant consumerism that people are forced to live with. By isolating individuals into atomized selves, destroying the basis of community that has sustained human life for hundreds of thousands- if not millions- of years, and then creating normative definitions of success, masculinity, and happiness that are all rooted in financial and material success. American culture both looks down and marginalizes those who fail -mental, physical, but often financially- and diffuses any societal imperative to help. Forced into an artificial feeling of aloneness and believing that you are to blame for your feelings we all come to internalize our psychosis. If you are poor and unable to pay rent you feel depressed and pathetic. You have failed to meet societies assumptions for success- you are not wealthy. Likewise, if you are a wealthy CEO you still feel isolated and alone. You have everything that society claims is needed for happiness- money- but you still feel like slitting your wrists. How do you deal with the situation in either case? You consume. If its good for General Motors its good for you.
When you feel hollow inside, devoid of any sense of worth or joy, the simplest way to create a feeling of happiness in this culture is to consume, even if that happiness is fleeting. Why is that? There are many reasons but one important one is that consumptions and consumerism are the only way to feel apart of any greater community. If I get the cool shoes I will be a part of something, same if I see that new hit movie, or have a nice car. In America one of the only things we feel we all have in common are the material objects/commodities in our lives. The easiest way of removing the feeling of aloneness and to connect with others in this culture is to consume.
Maybe we realize that we are trading our labor (in the form of money) for some one else's labor. In this way, at least subconsciously, we have at some relationships- between producer and consumer, between costumer and consumer, between worker and work. At the same time, as a consumer, I have a relationship with all other consumers. I can talk about the newest video game with people and feel a sense of connection, or I can join a Hummer fan group discuss the awesomeness of my brand new H2.
In addition, we place in those objects a relationship that is unhealthy and unnatural; the commodity becomes a surrogate for interpersonal bonds. Anyone who has ever collected something- from comics, to stamps, to coke products- you will know the sense of accomplishment you feel when you expand or complete a collection. This relationship with material objects takes the place of interpersonal human relationships. I feel the same joy, if not more, when I get that latest comic, or complete my collection of 19th century French stamps, or buy a new car as I do when I hangout with my friends or make love with someone I cherish. These cheap consumer goods become a repository for my sense of yearning and desire for relationships.
Some of you might say, well I am not alone: I have a family who loves me, a wonderful committed partner, or a loving dog or cat. I ask you this in response? Does the addition of one more person remove this feeling of being alone? Does it replace the loss of a community (both animal and human) that has sustained human life for the vast majority of its existence? I contend that marriage, partnerships, families and even friendships do not replace this feeling of loss. These relationships instead become nothing more then another form of consumption. We cling to relationships and to individuals in much the same way that the stamp collector clings to his stamps. This is not universally true, but is commonly the case. Some people- both those in monogamous and in polyamorous relationships- have found a way to be happy but in most cases marriage, partnerships, and families do not free us from the unhealthy happiness of consumerism but instead exacerbate the problem. You often turn to buying ones love from another; fearing that without them you are alone, and that only material gifts will make them happy and stay with you. This is why even though proclaim our love for others from hilltops, in front of priests, and on state issued documents current conceptions of family and friends does nothing to replace the loss of community.
Finally, the medical answer to most mental healthy problems in the United States is through consumptions as well. Do you not sleep at night? Do you feel anxious? Do you sometimes have a hard time getting an erection? If so take this pill. It will remove your sense of longing, make you numb to your surroundings, and most importantly make the pharmaceutical and psychiatric industries wealthy and successful (therefore they are at least happy?). This does not mean that medication is not an important component to mental health for some people. There are millions for whom these pills have saved their lives and I would never tell someone who has improved their lives through chemistry to stop. Nor do I believe that therapists are evil, misinformed, or make bad choices. Therapists want to help their "patients" and these the fact is that these pills work; they allow an individual to function in society, live, and hopefully address some of their own core psychological problems and improve their life.
What they do not do is confront the root of the problem- a loss of community and a break down of human relationships with each other and the natural world. They do not question the foundational philosophy of capitalism, the state, or civilization, and as Gilles Delueze and Felix Guattari claim psychology does not attempt to free us from our existence as machines; consuming, producing, eating, and fucking machines. They do not try to re-create and re-imagine life, but assume that life is dead and the machine, or at least the cyborg, is all that exists.
One group that is at addressing mental health in a holistic, egalitarian, and radical way is the Icarus Project. They are attempting to "call for new space and freedom for extreme states of consciousness, and alternatives to the medical model and the traumatic legacy of psychiatric abuse" while embracing an openly anti-capitalist, anti-statist, anti-racist, feminist, and non-hierarchal foundation. Instead of calling for increased medicalization of mental health they call for a holistic community sense of healing. Instead of viewing those with bi-polar disorder or depression as being "mentally ill" they question the basic assumption and power dynamics that exist within that assertion. The do not consider anyone to be " essentially diseased, disordered, broken, faulty, [or] existing within the bounds of DSM-IV diagnosis." Finally, they embrace an empowering vision of health that includes individual realization and empowerment with a strong sense of community and friendship. The Icarus Project might not have "the" answer to mental health and there is no one answer to this problem. But they are treating people as people- not machines and not as defective humans- and realize that we are all, even those who do not realize it, suffering from life under capitalism and the state. In fact, in a world so fucked up- with colonization being called liberation, with ecocide called progress, and with our strongest emotional connections existing towards characters on TV, the only sane person is the schizophrenic; they are the only one seeing the world for what it is- confused, destructive, irrational, and fake.
Addressing the root problem, as I have laid it out, can never be done through medication, or therapy. The only way to address the root cause of "mental illness" is through the re-creation of communities, the abolition of capitalism and collapse of western civilization and our post-modern existence. It requires a radical re-articulation of what it means to be happy, free, and successful and requires us to re-connect with the natural world and to destroy the body/mind dualism that has plagued western thought since the Greeks. If we, as a species and a culture want to be free we need to address and confront the sources of domestic abuses, exploitative relationships, and interpersonal violence. To do so we need to confront the systemic reasons for depression, schizophrenia, and the general feelings of malaise and hopelessness that are ever present in society. This means more then just revolution, more then just the destruction of the current order- the end to capitalism, religion, and the state- though this is essential. It requires the creation of communities and healthy relationships.