Answer the following question given this paragraph: 

Some people have low levels of the neurotransmitter called serotonin in their brain. Scientists think that this is one cause of depression. Medications called antidepressants help bring serotonin levels back to normal. For many people with depression, antidepressants control the symptoms of their depression and help them lead happy, productive lives.


Q: What according to scientists is the major cause of depression?


A:
low serotonin levels