What factors cause clinical depression to arise in one person but not another?

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We don't know what causes mental illness.

That doesn't mean the question is now moot. It's not necessarily the established facts that make the causation question interesting, but rather the exploration of (and between) different factors.

Before we dig into those factors, let's acknowledge how broad the term "mental illness" is. It may bring to mind a severe psychotic disorder, like schizophrenia, but mental illness has many shades of severity, both between diagnoses and within them. To start out, let's look at two common categories.

  1. Mood disorders affect how a person feels emotionally. This group encompasses depression and bipolar disorder, among others.
  2. Anxiety disorders are known for the fear and uncertainty they instill in people living with them. Phobias, obsessive-compulsive disorder and post-traumatic stress disorder all fall here.

Of course, this is just a small sample. Conditions as disparate as Alzheimer's and anorexia are also mental disorders. And while we can't make a comprehensive list of every potential cause of every known mental disorder, we can discuss how one condition can illustrate both genetic and environmental factors, while still not entirely revealing much about why mental illness affects one person and not another.

And maybe it's something that could nearly be called bad luck. According to the journal Pediatrics, adults who were spanked as children (also shoved, slapped or grabbed -- actions short of harsh physical or sexual abuse, in other words) were between 2 and 7 percent more likely to develop a psychiatric disorder (including mood disorders like depression and alcohol or drug dependencies, as well as more severe psychotic disorders) [source: Healy].

That might make you think that our mental health is extremely sensitive and subject to vulnerability not by a huge crisis, but by a less significant event. Let's read on to find out why we shouldn't be too worried that a small action will hurt us badly in the long run.