Across epidemiologic studies, MDD is found to be a common psy-chiatric disorder. The lifetime risk for MDD in community samples vary from 10 to 25% for women and 5 to 12% for men (American Psychiatric Association, 2000). The point prevalence of MDD for adults in community samples has varied from 5 to 9% for women and from 2 to 3% for men (American Psychiatric Association, 2000). While the incidence rates of MDD in prepubertal boys and girls are equal, women over the course of their lifetime are two to three times more likely to have MDD after puberty. Whereas a strong relationship exists between low social class and schizophrenia, a weaker but nevertheless meaningful relationship may exist between low income status and the occurrence of MDD. Analyses of the Ep-idemiological Catchment Area (ECA) data indicated that the lowest income group manifested twice the risk of MDD than the highest income group while the National Comorbidity Survey (NCS) con-cluded that individuals with low socioeconomic status demonstrate higher risk for MDD than individuals who are economically well-off. The rates of MDD may also be influenced by childhood adver-sity including severe physical abuse, sexual abuse, neglect and poor care (Harkness and Monroe, 2002). The NCS identified the risk factors associated with having MDD comorbid with another men-tal disorder as opposed to MDD alone. These risk factors include younger age, lower level of education and lower income.
Lifetime risk refers to the proportion of individuals being studied who would go on to develop the disorder during their lifetime. Estimates of lifetime risk of MDD in community samples vary from 20 to 25% for women and 7 to 12% for men (Depression Guideline Panel, 1993). Lifetime prevalence refers to those indi-viduals who, up to the time of assessment, have had symptoms that met diagnostic criteria at some point in their lives. The NCS esti-mated overall lifetime prevalence of MDD as 17.1%. The estimated prevalence was twice as high in females than males. The ECA study and NCS identified higher lifetime prevalence in younger age groups consistent with the birth cohort effect and possible re-call bias. In the survey replication of the NCS (NCS-R) (Kessler et al., 2005), the lifetime prevalence estimate of major depression was 16.6%. Lifetime prevalence for all mood disorders was 20.8%. Both of these estimates were considered to be conservative.
Point prevalence or current prevalence refers to the proportion of the individuals that have the disorder being studied at a desig-nated time. The specific point prevalence of MDD in community samples has ranged from 5 to 9% for women and 2 to 3% for men. The current point prevalence estimates in the NCS were 4.9%. Of the more prevalent 12 month disorders, the NCS-R found that major depressive disorder was the third most common disorder (6.7%). Although more than one third of cases were found to be mild, the prevalence of moderate and serious cases was 14% of this population and mood disorders was considered the second most common disorder of those serious cases. The point preva-lence of MDD in primary care outpatient settings ranges from 4.8 to 8.6% (Depression Guideline Panel, 1993a). In hospitalized patients for all medical conditions, more than 14% had MDD.
For preschool children, the point prevalence is thought to be of 0.8% (Depression Guideline Panel, 1993). Point prevalences of major and minor depressive disorder of 1.8 and 2.5%, respec-tively, were found in a sample of 9-year-old children from the general population, based upon the use of a semistructured di-agnostic instrument (Kashani et al., 1983). A semistructured diagnostic instrument was used to find a 4.7% point prevalence rate of major depression in a community sample of 150 adoles-cents. Those adolescents diagnosed with MDD had symptoms that met criteria for dysthymia as well. A point prevalence rate of 3.3% was found for dysthymia. Weller and Weller (1990) have shown the prevalence of MDD in clinical samples of children and adolescents to be 58% in educational clinics, 28% in outpatient psychiatric clinics and 40 to 60% in psychiatric hospitals. By comparison, a prevalence of 7% is found in hospitalized pediat-ric patients. Emslie et al., (1990) assessed depressive symptoms by self-report in a large sample of high school students of mixed ethnic background in an urban school district. They found that hispanic females reported more severe depression whereas white males reported the least severe scores of depression. For males and females, African-Americans and hispanics reported signifi-cantly more depression than whites. Female gender, being behind in school and nonwhite ethnicity predicted higher self-report scores of depressive symptoms.
Weissman and colleagues (1991) found a 1% prevalence of MDD in adults 65 years and older who lived in the commu-nity. The data indicate that a lower lifetime prevalence of MDD was found in the oldest age group ($ age 65) in comparison to younger age groups. Women manifest an increased prevalence of MDD in comparison to men and no significant differences were found across racial or ethnic groups. However, other com-munity samples of older adults were found to have a high preva-lence (8–15%) of clinically significant depressive symptoms (but not a formal diagnosis of MDD). In a recent Stockholm group, the frequency of MDD was 5.9% and the rate of DD was 8.3% (Forsell et al., 1994).
In comparison to community settings, higher prevalence rates for MDD are found in treatment settings for older adults: 11% in hospitals, 5% in outpatient nonpsychiatric clinics and 12% in long-term care settings. There is also a higher preva-lence rate in treatment settings of clinically significant depres-sion that is not severe enough to warrant a formal diagnosis of MDD: 25% prevalence in hospitals and 30.5% in long-term care facilities.
Klerman and Weissman (1989) as well as The Cross-National Collaborative Group (1992) called attention to a changing rate of MDD for recent birth cohorts found in: North America, Puerto Rico, Western Europe, Middle East, Asia and the Pacific Rim. Specifically, earlier age of onset and increased rate of depression occur in individuals born in more recent decades. This finding was supported in NCS-R as well. Historical, social, economic, or biological events most likely account for the variability in rate of depression noted in different countries included in the study. However, an overall increase in the rate of depression was noted across many of the geographic locations.
Older adults continue to manifest a higher suicide rate than in younger age groups. However, suicide rates have increased in younger age groups as the changing rate of MDD is observed in younger cohorts. In keeping with the birth cohort effect, recur-rences of MDD in late life may become a significant health con-cern as the population ages.
Familiarity with risk factors for MDD may help the clinician rec-ognize or diagnose this common and serious psychiatric illness. Accordingly, The Depression Guideline Panel (1993) enumerated 10 primary risk factors for depression:
· History of prior episodes of depression;
· Family history of depressive disorder especially in first-de-gree relatives;
· History of suicide attempts;
· Female gender;
· Age of onset before age 40;
· Postpartum period;
· Comorbid medical illness;
· Absence of social support;
· Negative, stressful life events;
· Active alcohol or substance abuse
In the NCS, 4.9% of subjects were diagnosed as having a cur-rent episode of MDD. Of the subjects with depression, 43.7% had noncomorbid depression while 56.3% had comorbid conditions. This distribution of comorbid versus noncomorbid conditions is consistent with other reports of community and clinical samples examining the extent of cooccurrence. In the NCS, certain risk factors, including: 1) younger age, 2) lower level of education, and 3) lower income, were more associated with comorbid de-pression than noncomorbid depression.