So, mood disorders are the most common type of mental illness and they come in two general forms: depression and bipolar disorder. So, depression is also known as major depressive disorder and this is to distinguish it from sadness, from our everyday usage of the term when saying that, "Oh, it's been a bad day, I'm pretty depressed." To have major depressive disorder is a more serious thing, and clinicians define it as a severely depressed mood that last two or more weeks, which means that a couple of days of deep sadness doesn't make you depressed, and is accompanied by feelings of worthlessness and lack of pleasure, lethargy and sleep and appetite disturbances. In order to be counted as having major depressive disorder, it can't have a reasonable cause. So, for instance, if the love of your life dies terribly, you're going to be really miserable for a couple of weeks, but that doesn't count as major depressive disorder. While the minimum amount of time you need to get counted as having major depressive disorder is two weeks. The average episode is about 12 weeks on average, that's how long it typically lasts. As I said, it's common, about 15 percent of people are going to be depressed at some point in their life. It's much more common in women than men and there's different theories as to why. So, one theory is that women in certain ways have harder lives, they make less money, they're more prone to be in poverty and poverty contributes to likelihood of becoming mentally ill. It could be due to hormonal differences and in fact, one type of depression, postpartum depression which is depression after the birth of a child, plainly is due to hormonal changes in a woman's body and brain. It could be because men are less willing than women to report depression and depressive symptoms because part of us sort of ideal notion of what is to be a man might include not going for help for this sort of thing. Finally, as a fourth theory, a promising theory is developed by my colleague, Susan Nolan Huxley and my late colleague Ariel, who suggests that men and women have different ways of dealing with stressful events. So, a man and a woman who are prone to this sort of mental illness, when they receive a life stressor, the man might deny that anything is going on and try to distract themselves, while women may accept what's going on and in fact, ruminate about it, think about it, mall over their sadness which may well turn everyday sadness into major depressive disorder. Depression is heritable. Much more so for severe depression and what this means is that if your identical twin has major depressive disorder you're likely to have it if your biological parents have it, it makes you more prone to have it. We know that neurotransmitters are involved in depression and so one theory of depression is that it's due to low levels of the neurotransmitters norepinephrine and serotonin. One mid of support for this is that when you give patients drugs, these drugs often work by increasing the level of norepinephrine and serotonin in your brain and sometimes they work, sometimes they make the depression go away. But just as with schizophrenia, we know it's not as simple as this and here's one reason why it's not a simple, suppose you get the drugs and you take them. It turns out that they have their effects on neurotransmitters right away. But your depression doesn't go away right away, it often takes a couple of weeks even if they're going to work and that's suggests that the neurochemical story is going to be more complicated. From a psychological level, what seems to go on in depression, what seems to be the core problem is some sort of negatively biased thoughts, some sort of negative and counterproductive patterns of thoughts. If a non-depressed person fails an exam, he or she might say, "Well, that's too bad. I'll do better next time, wasn't a fair exam." But if a depressed person might fail an exam, they might catastrophizing," This is the end of the world, this is terrible, my life will never be the same". They might view it as telling them something terrible about themselves, "I'm an idiot, I'm a fool." They may believe that things can never change. "I'll never get any smarter, I'll never do any better, this is the way my life will always be". Now, you might be tempted to think, well, this is why some people have major depressive episodes and others don't because they have these different patterns of thoughts, but it turns out there's not much evidence that people who become depressed before they were depressed, had a bad pattern of thought. So, this pattern of thought though maybe an integral part of depression, seems to be caused by depression, but it doesn't seem to be why people get depressed in the first place. The most extreme manifestation of depression and most serious manifestation is suicide. People who are not depressed do kill themselves, but people have mood disorders are much more likely to kill themselves about eight times more likely. In fact, about half of people who take their own lives are in the midst of a depressive episode. You'll see here some statistics about suicide. There's a sex difference, which is one way of thinking about it is that women attempt suicide a lot more. But men are more likely to succeed usually because they use more lethal methods. So instead of pills, they might use a gun and the demographics here are telling as well. So, very early on, black men are proportionally more likely to kill themselves. But then for the rest of life, there is an enormous difference where for some reason suicide is extremely likely relative to other people in older white men. Suicide is contagious, hard to believed to be contagious. There was Von Goethe, who wrote a book a long time ago about a young romantic who shot himself over a lost love. Then the Werther effect, the character was named murder, led to many young men killing themselves in fact, inspired by the book, and we know that there's often it was prominent suicide. In a high school for instance, it may lead to other suicides. Its as if it it gives people the idea where they haven't had it, it seems like an existence proof, and people are troubled by this. As I give this lecture, there's a television show called 13 Reasons Why, which is all about somebody's suicide and different groups are protesting the show arguing that it's going to motivate more people to commit suicide. On the other hand, it's not clear this contagion effect always occurs. So, Kurt Cobain killed himself and Kurt Cobain had many young fans. But for whatever reason that Kurt Cobain suicide didn't lead to other suicides on the part of young people. Now, I've talked about major depressive disorder which is pretty common. But the other form of mood disorder, a lot less common, about one percent of the population is bipolar depression. Bipolar depression isn't just sadness, but there's actually swings back and forth from mania to depression. A mania is an extremely high mood often with delusions and insane behavior and incredible extraordinary energy. We know bipolar depression is highly heritable, much more heritable than major depression. It seems to have a different brain basis, a different brain chemistry, is treated with different drugs. Very controversial leaders in association with creativity, where a lot of very creative people like Vincent Van Gogh or Virginia Woolf suffer from bipolar depression. One argument is that there are manic episodes, there are manic periods lead to the creation of great work. On the other hand, some people and I'm probably one of them, believe that this is sort of more of a coincidence than anything else and in fact, great artists and writers would be better off if they didn't suffer from bipolar depression and mania, might be a good thing. But it's not clear, you do your best work under a manic state and of course, the depression is a terrible thing and we should keep in mind that Virginia Woolf killed herself.