Studies seem to find correlation, but has the causal link been established?
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There are different theories about the causes of depression. Three of them assume a cause that could be affected by aerobic exercise.
Neurochemical theory of depression:
It seems that depression might be caused by low serotonin and/or noradrenalin levels. Physical activity releases both neurotransmitters, thus alleviating depressive symptoms.
Cognitive theory of depression:
It seems that negative cognitive schemata cause depression. Sports improves self-efficacy and other schemata.
Diathesis–stress model of depression:
Depression is the result of a predispositional vulnerability combined with stress from adverse life experiences. Aerobic exercise relaxes the sympathetic nervous system thus alleviating stress and, as a consequence, depression.
Maybe someone else will come up with studies showing a causal connection. These (and other) theories might explain the connection.