Prevalence rates tell us how frequently a disorder exists within a population.  Although from year to year prevalence rates are relatively stable, they may differ significantly between groups within a population. For example, the prevalence of major depressive disorder is higher among women than men in Western societies. In addition, globalization appears to be increasing the prevalence of disorders such as major depressive disorder and eating disorders in cultures where these disorders were previously insignificant.  Finally, prevalence rates can generate questions to be researched by psychologists.

One reason for differences in prevalence rates is that there are different levels of risk factors for different populations. Brown and Harris carried out an extensive set of interviews with a large population of women in central London.  The goal of the study was to determine what factors may play a role in the higher prevalence of depression in English women.  They found that 90% of the women who had become depressed in the past year had suffered an adverse life event, compared to only 30% of women that were not depressed.   The study found that it was the presence of a significant number of risk factors that appears to play a role of prevalence of MDD in a given population.  The presence of these risk factors may change over time, which is why prevalence rates may change as well.   

One of the problems of such studies is that the data is self-reported.  There is no official diagnosis of all the women in the Brown and Harris study.  However, this is a general limitation of prevalence rates.  Prevalence rates of disorders may be calculated through the use of hospital records or through carrying out interviews within a population.  One of the problems with relying on hospital records is that it may actually underestimate the prevalence of a disorder, since many people who may suffer from a psychological disorder may not seek out medical help.  When using interviews, there is the problem that although depressive symptoms may be identified, there is often no formal diagnosis, which may lead to an inflated prevalence rate.  Even in cases where only diagnosed patients are used, because we know that there are problems with the validity and reliability of diagnosis, prevalence rates may always be open to question.

Another aspect of prevalence rates is the role of globalization.  Ethan Watters writes about the globalization of the Western medical model of depression.  Through the marketing of drugs such as Prozac and the influence of Western media, Japan has seen an increase in the prevalence of MDD.  A study that attempted to look at how this could happen was carried out by Becker. She wanted to see if the introduction of Western television in Fiji would lead to an increase in disordered eating behaviours.  In this natural experiment, she studied two groups of girls between the age of 16 and 18.  Each group was asked about their eating behaviour – the first group before television was introduced and the second one three years later.  She found a significant increase in the rate of disordered eating behaviours in the second group – including dieting, low body image and self-induced vomiting.

The researchers used two different groups of girls so that they could focus on a specific age group – before and after the introduction of television. The use of two different groups makes it difficult to know if individual differences in the two samples may be the reason for the results of the study.  As with the Brown and Harris study, the symptoms are self-reported and no formal diagnosis is made.  Therefore, it is not really possible to argue that the prevalence of eating disorders has increased in Fiji during the time of the study.  Triangulating this data with hospital records would be one way to confirm that such an increase had taken place.  The use of a natural experiment has the strength that it is done under naturally occurring conditions, but they are often cannot be replicated, making the reliability of the research questionable.

Changes in prevalence rates of disorders may lead psychologists to generate new theories about the origins of a disorder.  An example of this is the growth in technology and social media.  In this case, a variable has been introduced into society which may have lead to a higher rate of anxiety and depression – or it may be that the use of social media has led to more people seeking help for the disorders. Chou and Edge carried out a study of university students to see if they thought that their friends were happier than they were.  They also asked them how many hours they spent on social media each week.  The results showed a direct correlation between the number of hours spent on social media and one’s belief that others have a better life than they do.  Skakya et al found a negative correlation between time on social media and mental health. The problem with this research is that we are not sure if those with poor mental health spend more time on Facebook, or whether Facebook leads to poor mental health.  As the prevalence of anxiety and depression in teens has risen, psychologists are searching for variables in society that may have led to that change.  Technology is being researched as a possible cause.

Prevalence rates help psychologists by indicating which groups appear to suffer most from a disorder which may lead to prevention strategies or better treatment.  Changes in prevalence rates can also be used to generate hypotheses about the aetiologies of disorders. However, the accuracy of prevalence rates in abnormal psychology needs to be treated with caution as they are not an accurate measure because of the limitations in the way in which data about prevalence is gathered.