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Even though depression and anxiety are different types of disorders, they tend to go together. But why can it happen?

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[♪ INTRO ].

If you've ever experienced anxiety and depression — in the clinical sense, I mean — you'll know that they can feel really different. With anxiety, you're all ramped up.

And with depression, you're very, very down. Yet they tend to go together. And a lot of medications, especially certain types of antidepressants, can be used to treat both.

We still don't know a ton about how exactly anxiety and depression work in the brain — or how antidepressants work to treat them. But over time, psychologists have come to realize that the two types of conditions are surprisingly similar. They may feel very different in the moment.

But they actually have a lot of symptoms in common, and involve some very similar thought patterns. They might even have similar brain chemistries. So if you're looking to understand a little more about how anxiety and depression manifest themselves — whether for yourself or for someone else in your life — those connections are a good place to start.

Depression and anxiety aren't really specific disorders — they're generic terms for types of disorders. But the most common, and most closely linked, are major depressive disorder, or MDD, and generalized anxiety disorder, or GAD. In any given year in the U.

S., where it's easiest to find detailed statistics, about 7% of the population will have MDD, and about 3% will have GAD. Lots of those people have both: About 2/3 of people with major depression also have some kind of anxiety disorder, and about 2/3 of people with generalized anxiety disorder also have major depression. And whether you have one or the other or both, the same medications are often at the top of the list to help treat it — usually antidepressants.

Unsurprisingly, psychologists have noticed these statistics. But for a long time, we've thought of generalized anxiety and major depression as very different things, and understandably so. Probably the most noticeable symptom of anxiety is arousal, which in psychology is a technical term rather than a specifically sexual thing.

It basically just means being on high alert — whether psychologically, with increased awareness, or physically, with things like a racing heart and sweaty palms. Arousal isn't part of major depression, though. And there's a key symptom of MDD that doesn't usually show up in generalized anxiety: low positive affect, which is the technical term for not getting much pleasure out of life and feeling lethargic and just kind of … blah.

So there are important differences between anxiety and depression, which is part of why they're still considered separate classes of disorders. But when you look at the other symptoms, you start to realize that major depression and generalized anxiety have almost everything else in common. There's restlessness, fatigue, irritability, problems with concentration, sleep disturbances … the list goes on.

And that's just in the official diagnostic criteria. So for decades, psychologists have been examining the models they use to describe anxiety and depression in the brain to see if they point to a similar source for both types of disorders. They've come up with lots of different ideas, as researchers do, but the most common ones tend to center around the fight or flight response to stress.

Fight or flight kicks in when you're confronted with something your mind sees as a threat, and it automatically prepares you to either fight or run away. And when you think about it, anxiety and depression are just different types of flight. Psychologists often characterize anxiety as a sense of helplessness, at its core, and depression as a sense of hopelessness.

Anxiety might feel like you're looking for ways to fight back. But part of what makes it a disorder is that it's not a short-lived feeling that's easily resolved once you have a plan. Of course, as with all things mental health, anxiety disorders can be deeply personal and won't feel the same for everybody.

But clinical anxiety does tend to be more pervasive. The worry sticks around and starts to take over your life because it doesn't feel like something you can conquer. So anxiety and depression might just be slightly different ways of expressing the same flight response: helplessness or hopelessness.

And maybe that's part of why they so often go together. That connection also shows up on the biochemical side of the stress response. There are a lot of hormones involved in this response, and their effects interact in super complex ways that scientists still don't fully understand.

But both depressive and anxiety disorders are closely associated with an oversensitive stress response system. Researchers think that's one reason both of these types of disorders are so much more common in people who've experienced major stresses like trauma or childhood abuse. Those stressors could make their stress response system more sensitive.

The main hormones involved aren't always the same, but the changes can cause some of the same symptoms — problems with sleep, for example. So anxiety and depression seem to be two sides of a similar reaction to stress, in terms of both thought processes and hormones. Still, that doesn't really explain why some antidepressants can treat both anxiety and depression.

Because those medications primarily affect neurotransmitters, the molecules your brain cells use to send messages to each other. If you thought we had a lot left to learn about how the stress response works, we know even less about what the brain chemistry of anxiety and depression looks like, or how antidepressants help. But if the thought processes and physical responses that go along with these disorders aren't quite as different as they seem on the surface, it makes sense that the brain chemistry would be similar, too.

And that's exactly what scientists have found. More specifically, lots of studies have pointed to lower levels of the neurotransmitter known as serotonin as a major factor in both anxiety and depression. Researchers have even identified some more specific cellular receptors that seem to be involved in both.

There's also some evidence that the way the brain handles another neurotransmitter, norepinephrine, can be similar in both anxiety and depression. Since most antidepressants work by increasing serotonin levels, and some of them also affect norepinephrine, that could explain why they're so helpful for both anxiety and depression. Although again, there's a lot we don't know about their exact mechanisms.

Ultimately, there's no denying that in the moment, anxiety and depression can seem like very different feelings. And if someone has both types of disorders — well, it's easy to see how that could feel overwhelming. Like, it's hard enough treating generalized anxiety or major depression on their own.

And it's true that it is often harder to treat these conditions when someone has both. But maybe not twice as hard. After all, anxiety and depressive disorders have a lot in common, from their symptoms to the basic brain chemistry behind them to some of the treatments that can help.

The fact that they often go together can be really tough. But understanding more about why that is has also pointed us toward better treatments and more effective therapies, that really can help. Thanks for watching this episode of SciShow Psych.

If you're looking for someone to talk to about your mental health, we left a few resources in this video's description. And if you'd like to learn more general info about treatments, you can watch our episode on misconceptions about antidepressants. [♪ OUTRO ].