Depressive rumination, a negative thinking style characterized by repetitive and passive thoughts about the causes, meanings, and consequences of one's feelings and distress, is often described as being a habitual response tendency that forms a vulnerability to depression. Behavioural Activation (BA) is an effective treatment for depression but little is known of mechanisms of changes during a successful treatment completion and for whom the treatment benefits the most. The main purpose of the study is to investigate whether habit-like mood-reactive rumination will change during Behavioral Activation treatment for current depression and mediates symptom changes in the treatment. Important moderators of change will also be investigated (i.e. history of early life stress and cognitive flexibility). We aim to provide individual BA treatment for up to 120 currently depressed participants (from 90 to 120 participants) in 12 treatment sessions over 11 weeks. Measures are obtained at pre-treatment, during treatment, at post-treatment and at 6 month follow up.

Depressive rumination, a negative thinking style characterized by repetitive and passive thoughts about the causes, meanings, and consequences of one's feelings and distress, is often described as being a habitual response tendency that forms a vulnerability to depression. Behavioural Activation (BA) is an effective treatment for depression but little is known of mechanisms of changes during a successful treatment completion and for whom the treatment benefits the most. The main purpose of the study is to investigate whether habit-like mood-reactive rumination will change during Behavioral Activation treatment for current depression and mediates symptom changes in the treatment. Important moderators of change will also be investigated (i.e. history of early life stress and cognitive flexibility). We aim to provide individual BA treatment for up to 120 currently depressed participants (estimated number of participants is from 90 to 120) in 12 treatment sessions over 11 weeks. Measures are obtained at pre-treatment, during treatment (in sessions and during two assessment windows after session 4 and 8), at post-treatment and at 6 month follow up. Multimodal assessment of key constructs will be used in the study, including clinician ratings using semi-structured diagnostic interviews, self-report questionnaires, experimental tasks and ecological momentary assessment to capture moment-to-moment changes during the flow of daily life. Our main research questions are: 1) Does BA lead to reduction in depressive symptoms and diagnostic status, and are these symptom changes mediated by changes in habit-like mood-reactive ruminative thinking? 2) Are treatment gains and possible mediation of habit-like ruminative thinking, moderated by history of early-life stress and cognitive flexibility, that both have been established as predisposing factors for symptom onset in depression and are also know moderators of the development of rumination as a habit? 3) Does perceived control and reward-related responses increase during BA and are these changes associated with rumination as a habit? 4) Are gains during treatment maintained at 6-month follow up after treatment completion?

Moderators measured at pre-treatment: :

History of early life stess (total and physical/sexual/emotional abuse in particular), measured with the The Childhood Traumatic Event Scale (CTES) and The Adverse Childhood Experiences (ACEs) Questionnaire.

Cognitive flexibility: The Standard version of the Verbal Fluency Test (VFT), The Trail Making Test (TMT) and Digit-Span will be administered.

Mediators measured at pre-treatment, during treatment, post-treatment and follow up:

Depressive rumination will be measured with the brooding and reflective pondering subscales of the Ruminative Response Scale (RRS).

Habitual characteristics of ruminative thinking will be measured with the Habit Index of Negative Thinking (HINT).

Perceived control will be measured with the Pearlin Mastery Scale (PMS).

Reward-related responding will be measured with the Environmental Reward Observation Scale (EROS).

Level of activation will be measured with the Behavioral Activation for Depression Scale (BADS).

Selected items from the RRS, PMS, EROS and BADS are administered along the PHQ-9 at start of all treatment sessions in the study.

State ruminative thinking, perceived control, reward-related responding and level of activation will also be measured 8 times per day during ecological momentary assessment via smartphones for six days at pre-treatment and post-treatment and for three days at two assessment windows during treatment.

Inclusion Criteria:

1. Current major depressive episode according to the DSM-5 diagnostic criteria evaluated with the DIAMOND diagnostic interview, that is considered to be the primary diagnosis.
2. Sore of 14 or higher on Becks Depression Inventory-II (BDI-II) that measures severity of symptoms of depression past 2 weeks.
3. Participants are between 18 and 65 years of age at start of study.
4. Satisfactory understanding of the Icelandic language to complete measures in the study.
5. Completion of pre-treatment assessment that includes 2 visits to researchers and a 6-day ecological momentary assessment.

Exclusion Criteria:

1. Current or past manic or hypomanic episodes according to the DSM-5 diagnostic criteria evaluated with the DIAMOND diagnostic interview.
2. Current or past psychotic disorders according to the DSM-5 diagnostic criteria evaluated with the DIAMOND diagnostic interview.
3. Presence of substance abuse within the last 12 months according to the DSM-5 diagnostic criteria evaluated with the DIAMOND diagnostic interview.
4. Presence of active and serious suicidal thoughts or a suicidal attempt in the previous 2 months.
5. Unstable medical treatment for depression (type of drug and/or dosing) during past two months at pre-treatment assessment.
6. Recent psychotherapy within the past month at pre-treatment assessment or active psychotherapy during study participation.
7. Cognitive impairments or severe physical illness.