Although the influence of existentialism on literature, art, and philosophy is not very surprising, its influence on psychology might be to some people. Understanding it better should help make it clear, however, that existentialism is less a philosophical school than a broader cultural movement that has involved a number of intellectual pursuits all focused on the existence and importance of the individual human being.
Traditional psychological research has been involved primarily with trying to understand the various drives and motivations behind human actions. This attention to discrete substances and relationships means that we arrive at generalizations about humanity, but we can lose sight of the human being we were looking at to be begin with the human being who makes things happen and who acts in the world.
This is not to say that existentialist psychologists arent interested in looking at things like hormone levels, neurological scans, and similar data their point, however, is that such information is very limited in its ability to give you information about any single individual person. For existential psychologists, individuals humans are both beings and becoming always in flux, always more than the sum of their parts.
There is quite a lot of variety in the research, practices, and orientations of existentialist psychologists. One of the reasons for this is the fact that existentialists in any field tend to eschew systematization of any sort. Existentialist philosophers dont build grand philosophical systems and existentialist psychologists dont build grand psychological systems. Instead, they are concerned with human beings as individuals, something that would be hindered if they applied systems which forced them to generalize and make too many assumptions.
Nevertheless, there are a number of common features which can be found in the work of most existentialist psychologists. For one thing, they typically reject the assumption of a subject/object distinction which has long been a basic feature in Western science generally and Western psychology specifically. This is understandable because, after all, human beings are both doing the studying and being studied just how much separation can there be?
Existentialist psychologists also take great pains to avoid treating a person as if they were isolated from events and situations in the of the world. Philosopher Martin Heidegger used the concept of being-in-the-world to describe how humans do not exist passively, but rather are actively engaged in their environment and with other people. Existentialist psychology takes this conception of humans very seriously and describes the world around us as one pregnant with meaning and structure and that world must be understood in order to understand the people in it.
Of particular importance in existential psychology are the features of choice, responsibility, and freedom in human lives. Patients are expected to seize their freedom and take responsibility for the choices they make in their lives, both good and bad. People may be an object of socialization and other factors which condition behavior, but they are also just as much a subject, a person who can become conscious of the influences on their life and thus capable of moving in their own direction.
Choice is, of course, one of the most fundamental concerns in existentialism generally. Existentialists regard human freedom as not merely significant, but even radical we may not choose to be born but, once born, we are responsible for the lives we lead. The choices we make create the structure of the world around us, even as other factors contribute in ways that arent entirely within our control.
Existentialist psychology doesnt ignore these factors; instead, it strives to help people become more aware of them and thus increase the sphere in which we have the freedom to make choices about how we will relate to deterministic forces. Accepting the existence of limitations is not a negative process but rather a liberating one, insofar as we are able to understand the limitations and deal with them on our own terms rather than someone elses.