Behavioral Activation (BA) is an idiographic and functional approach to depression. It argues that people with depression act in ways that maintain the depression such as avoidant behavior. Treatment revolves around discovering the contingencies that lead to maintenance of the depression and experimentation with changing one's behaviors according to an agreed upon plan. BA uses functional analysis to identify the three-term contingency of antecedents, behaviors, and consequences.
Practitioners using Cognitive-Behavioral Therapy (CBT) are likely to believe that depression has its roots in negative cognitions. Aaron Beck attributed depression to a cognitive triad: a negative view of self, the world, and the future. Cognitive biases may also exist which cause situations to be interpreted in skewed way (e.g. overgeneralization, magnification, personalization, selective abstraction, arbitrary inference, and categorical/dichotomous thinking). These result in a pessimistic explanatory style -- that is, seeing problems as one's fault, as permanent, and as pervasive. Depression can be resolved by changing one's pattern of thinking, although this requires both cognitive restructuring as well as some behavior modification such as desensitization or exposure therapy to overcome avoidance.