Yes, our mental well-being is dependent on the brain's 100 billion neurons and how they connect and interact with other neurons, but it takes a push from the outside environment to start the process, creating a stimulus and response relationship that defines who we are. Later in the course of development, established patterns of connections still can be altered by the environment - through learning. One indicator of environment at play is depression in teen-age girls, which is significantly higher than in boys. According to the Report, one explanation may be that girls are more socially oriented, more dependent on positive social relationships, and more vulnerable to losses of social relationships than boys. Then there are stressful life events, such as marriage breakups, death in the family, economic hardships, or racism and discrimination.
Single mothers face twice the risk of depression as do married mothers.
Victims of domestic violence must often bear their considerable emotional burdens while battling incapacitating depression. But the brain doesn't merely react through its neuronic pathways or, more specifically, the neurotransmitters such as dopamine and serotonin that service them. More complex interactions are thought to be at play. The hippocampus and amygdala are major nuclei of the brain's limbic system which underlies emotions. Then there is the neuropeptide CRH which is hypersecreted by the hypothalamus during depression, which results in the release of the stress hormone cortisol.