According to the World Health Organization, mental health disorders are one of the leading causes of disability worldwide. Data from the WHO from 2014 suggests that 1 out of 15 people suffer from major depression in Europe. If anxiety and all forms of depression are included, nearly 4 out of 15 people are affected.  WHO estimates that around 350 million people are affected by depression. More women than men are affected. Numbers such as these tell us that the study of mental disorders is important. Psychologists want to understand why some people develop mental disorders. By understanding the origins of psychological disorders we may be able to develop appropriate treatments and interventions.

When discussing abnormal behaviour, psychiatrists and psychologists use a common vocabulary. An important thing to consider is what symptoms an individual exhibits. These are important in making a diagnosis. As seen in the previous chapter, clinicians often use a diagnostic manual (e.g. DSM-V) to identify symptoms when they diagnose psychological disorders. These manuals do not deal with causes of psychological disorders but only describe clusters of symptoms that are characteristic of specific psychological disorders. Symptomology refers to common behaviours and physiological signs of mental illness. It is also important to find out why people suffer from a disorder—that is, the etiology - but this is much more difficult to establish for a psychological disorder than for physical illness; often it is not possible to establish a clear cause. In order to understand the complexities of etiology, the IB focuses on how biological, cognitive, and sociocultural factors may contribute to the onset of disorders.

For this unit, you are required to know the etiology of one disorder. The disorders are based on the DSM-V classifications.  You may choose from the following types of disorders:

• Anxiety disorders: Generalized Anxiety Disorder, Social Anxiety Disorder, Panic Disorder
• Depressive disorders: Major Depressive Disorder
• Obsessive-compulsive disorder
• Trauma and stress-related disorders: PTSD
• Eating disorders: Anorexia or bulimia

This chapter will look at two of these disorders – Major Depressive Disorder and Post-traumatic Stress Disorder.  If you are interested in other disorders, you should use the same framework of biological, cognitive (psychological), and sociocultural (environmental) factors that are used in this chapter.

When discussing a disorder, it is important to consider the prevalence rate, which is the measure of the total number of cases of the disorder in a given population. Prevalence rates may seem like simple statistical data, but one of the goals of this unit is to understand some of the complexities of prevalence rates.  Usually, a prevalence rate is given for an entire population.  In 2014, the US National Institute for Mental Health estimated that 6.7% of all U.S. adults suffered from Major Depressive Disorder. However, as we will see, this statistic does not give us the whole picture.  In this chapter, we will be examining cultural, socioeconomic, and gender differences that make this figure look less precise.  In addition, we will see that prevalence rates are not stable. Psychologists want to know why prevalence changes over time.

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Depressive disorders