Subtitles section Play video Print subtitles With over 350 million people affected by depression worldwide, it's no doubt an incredibly real and serious issue. But, what exactly is going on inside of a depressed person? Is there a biological basis for these intense feelings of sadness? In the past, depression was often described as simply a "chemical imbalance" in the brain. Specifically, scientists believed that a lack of the neurotransmitter, serotonin, was to blame, which is often referred to as the "feel good" chemical. However, the only real evidence for this was that when some depressed people were prescribed drugs which increased serotonin levels, it helped alleviate their symptoms. But while chemicals most certainly are involved, this view doesn't really capture just how complex depression is. In recent years, scientists began to notice that the brain cell growth and connections may actually play a larger role. When we look at the brain of a depressed person, studies show that the hippocampus tends to be much smaller than average. Other areas of the brain are also physically affected, but this region in particular controls memory and emotion. And the longer a person has been depressed, the smaller the hippocampus becomes. The cells and networks literally deteriorate. It turns out that stress may actually be a main trigger in the decrease of new neurons in this area of the brain. In fact, studies have shown that when this region of the brain is regenerated and new neurons are stimulated, mood improves. Interestingly, many modern drugs, including those which affect serotonin levels, have an indirect effect on the growth of brain cells. This is likely why serotonin-based drugs seem to help some patients - but not for the reasons we once thought. Instead, they promote the release of other chemicals, which ultimately stimulate neurogenesis, or the growth of new neurons. Knowing this, some scientists now believe focus should be on drugs which directly affect neurogenesis. But while your neurons and chemicals may be the direct influencers, many genetic factors have been discovered as well. One particular study found that a variation in the serotonin transporter gene leaves individuals more vulnerable to depression. Every individual has two copies of the gene - one from each parent. And this gene can either be short or long. After tracking 800 young adults over 5 years, the studies revealed that 33% of individuals with one short version became depressed after stressful life events and people with two short genes fared even worse. On the other hand, those with two "long" genes were much less likely to become depressed with similar life stress. Many other genes have been identified which increase the likelihood of depression, too. And it makes sense when you consider that depression and bipolar disorder both run in families. Studies of identical twins show that if one has bipolar disorder, the other has a 60-80% chance of developing it, too. So while the true cause or causes of depression have yet to be pinned down precisely, and trust us, there is a HUGE list of other variables that studies suggest may come in to play. It's important to remember that depression is a disease with a biological basis, along with psychological and social implications. It's not simply a weakness that somebody should get over, or even something that we have a say in. And just like heart disease, or cancer, shedding light onto the subject is of the utmost importance, in order to bring funding and proper research. But, is depression only a human phenomenon? We look into the question "Do Dogs Get Depressed?" in our latest AsapTHOUGHT video, and discover the many studies done to understand depression among other species, including your pets at home. You can click the link in the description for that video. You can also check out the book “Animal Madness” by Laurel Braitman which touches on the subject. In fact, you can get it for free from our friends at Audible by going to audible.com/asap. Audible is the leading provider of audiobooks with over 150,000 downloadable titles across all types of literature. You can download this audio book or another of your choice, for free, at audible.com/asap. And with a subscription you get one free book a month! Special thanks Audible for making these videos possible! If you think you're suffering from depression, we have included some resources and more information in the description. And subscribe for more weekly science videos!