Major depressive disorder is characterized as persistent depressed mood that makes every day activities hard to complete and is the most common psychological disorders in the world. In the US alone, 16.9% of people will experience depression in their lifetime and people who have experienced at least one episode of depression are 50% more likely to have another episode. These episode can last anywhere from a few weeks to months at a time. Here are many types of depression and many causes and results of depression vary per person but depression is the most treatable mental illness. Depression is different for every person who experiences it and can be presented in many different types of symptoms. The most common symptoms are change in appetite which can affect weight, change in sleep patterns, fatigue, loss of energy, feelings of worthlessness and guilt, difficulty concentrating, and suicidal thoughts. A person’s gender and family heritage affect their chance of getting depression and how it affects them. According to the National Comorbidity Survey in 2007, depression affects, “approximately 20% of women and 13% of men at some point in their life.” Many factors, such as fluctuating hormones, being more invested in relationships, and more stress, can explain this 7% difference in people affected. Also, relatives of someone who has had depression are twice as likely to develop major depressive disorder. That seems like a high risk factor but, “The rate of concordance for major depressive disorder is higher among identical twins (50%).” This major increase in probability is believed to be because of nature and nurture. It also could be due to the chemicals in the brain which is the main source of developing major depressive disorder. When someone is depressed there are many chemical shifts in their brain that are the source of the depression. People with mood disorders, such as depression, most commonly have imbalances, “in certain neurotransmitters, particularly norepinephrine and serotonin.” Serotonin plays a role in the central nervous systems and maintains mood balance so when it is not present moods become out of control. Many factors can cause this imbalance in serotonin, including genes, according to some studies it suggested that, “an alteration in a specific gene that regulates serotonin (the 5-HTTLPR gene) might be one culprit.” Another culprit for causing major depression disorder is the prefrontal cortex which is a major part to personality development. When certain regions of the cortex are decreased in activation then is may disable its ability to control negative emotions which not only causes a person to be in a negative mood, but have trouble override and control these states of mind. So, people with depression cannot simply control how they feel and try to be more positive because their brain won’t let them. The exact science behind depression is defended by many scientists and psychologists, and there are many theories on what, combined with the chemical imbalances in the brain, causes depression. Research suggests that the source of depression is not one answer alone, rather a combination of genetic, biological, environmental, psychological components. One cognitive theory, known as “The Hopelessness Theory”, suggests that a style of thinking, particularly negative, leads to a sense of hopelessness which causes depression. According to this theory, “hopelessness is an expectation that unpleasant outcomes will occur or that desired outcomes will not occur, and there is nothing one can do to prevent such outcomes.” By thinking negatively on all aspects of the world and one’s life then they will become hopeless in all they do and begin to show symptoms of depression. Another theory focuses on rumination and is conveniently known as the “Rumination Theory”. According to this theory, the repetitive focus on the fact that one is in a depressed state, in a passive way, causes them to dwell on their symptoms and deepens their depression. The theory suggests that rather than focusing on the fact that one has major depression disorder, they should rather, “distract oneself from the symptoms or attempt to address them in an active, problem-solving manner.” It is unclear which of the many theories in the world about the cause of depression is the bets to believe, but it is very evident that there is not one cause for depression, rather a combination of many sources.