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Major depression is a syndrome that affects 15 to 20 percent of the population. It is among the most prevalent of all psychiatric disorders. Moreover, twice as many women than men comprise the 15 to 20 percent of the population who will experience a clinically significant episode of depression at some point in their lives. Major Depressive Disorder, the diagnostic label for a clinically significant episode of depression, is characterized by at least a two-week period of persistent sad mood or a loss of interest or pleasure in daily activities, and four or more additional symptoms, such as marked changes in weight or appetite, sleep disturbance, restlessness or slowing of thoughts and movements, fatigue, feelings of guilt or worthlessness, concentration difficulties, and thoughts of suicide. Although there are clearly difficulties in attempting to study depression in different cultures (Tsai and Chentsova-Dutton 2002), the prevalence of depression varies widely across the world. In general, Asian countries, such as Japan and Taiwan, have the lowest documented lifetime prevalence rates of depression (both approximately 1.5%); poorer countries like Chile have the highest rates (27%); the United States and other Western countries have intermediate lifetime prevalence rates of depression (Tsai and Chentsova-Dutton 2002). It is interesting to note that studies have shown that Mexicans born in Mexico have lower rates of depression, while those born in the United States have rates the same as non-Hispanic whites (Golding, Karno, and Rutter 1990; Golding and Burnam 1990). In general, the more acculturated Mexican-Americans are, the less likely they are to experience depression. However, those with more acculturative stress (e.g., coping with a move from being high status in Mexico to being lower status in the United States) tend to experience more depression than those with less acculturative stress (Hovey 2000).

The relatively high rates of depression have led the World Health Organization Global Burden of Disease Study to rank this disorder as the single most burdensome disease in the world in terms of total disability-adjusted life years (Murray and Lopez 1996). More importantly, depression not only has a high prevalence rate, but also has a high rate of recurrence. Over 75 percent of depressed patients have more than one depressive episode (Boland and Keller 2002), often developing a relapse of depression within two years of recovery from a depressive episode. This high recurrence rate in depression suggests that there are specific factors that increase people's risk for developing repeated episodes of this disorder. In attempting to understand this elevated risk for depression, investigators have examined genetic and biological factors, and psychological and environmental characteristics, that may lead individuals to experience depressive episodes.

Some forms of depression have a strong genetic influence. Depression has been shown to run in biological families; indeed, having a biological Although there is no conclusive evidence that a specific, single gene for depression exists, vulnerability to depres sion seems to be inherited. An identical twin with a de pressed twin is 67 percent more likely to be depressed. DENNIS DEGNAN/CORBIS relative with a history of depression increases a person's risk for developing an episode of depression. Furthermore, twin research has consistently and reliably demonstrated that major depression is a heritable condition (e.g., Kendler and Aggen 2001). Research using broad definitions of depression suggests that men and women have different heritabilities for depression, with genetic factors proving more etiologically important for women than for men (Kendler and Aggen 2001). Gaining a better understanding of this difference in heritabilities may help to elucidate the reasons underlying the higher rates of depression in women than in men.

Although genetic factors are important, they do not fully explain the etiology of depression. For example, there are sets of identical (monozygotic) twins in which one is affected with depression and the other never becomes depressed. Because monozygotic twins have identical genetic makeups, these differences must be due to factors that the twins do not share. Some of these factors are biological (but not genetic). There is abundant evidence that biology can affect mood. For example, thyroid problems can often mimic depression and cause weight changes, sad mood, and other symptoms of depression. Similarly, investigators have demonstrated some drugs or medications (e.g., reserpine) can induce a depression-like syndrome, whereas other medications (e.g., antidepressants) are effective in alleviating depressed mood. These medications generally affect the neurotransmitters implicated in depression. Biological factors can also affect the risk for depression. For instance, obstetrical complications seem to increase the risk of developing depression later in life (Fan and Eaton 2000; Preti et al. 2000). In addition, because in virtually every culture women are at greater risk for depression than are men (cf. Nolen-Hoeksema 1990), it is likely that something about the biology of being female, such as hormonal functioning, may make depression more likely to occur.

There are also psychosocial influences in the development of depression. Some research suggests, for example, that a childhood history of abuse or neglect can put an adult at greater risk for depression (e.g., Bifulco, Brown, and Adler 1991). Moreover, there is evidence that a history of abuse may be related to suicidal thoughts and behavior both in patients and nonpatients, above and beyond the effects of having a diagnosis of depression (Read et al. 2001; Molnar, Berkman, and Buka 2001). Furthermore, social support (e.g., from friends or family) can mitigate depression, whereas a lack of support may increase the severity or length of a depressive episode (George et al. 1989; Goering, Lancee, and Freeman 1992). Finally, there appears to be a robust link between stressful life events (e.g., divorce, bankruptcy) and the onset of major depression, suggesting that such events may play a role in the etiology of some major depressive episodes (Stueve, Dohrenwend, and Skodol 1998). Recent studies have examined the impact of befriending as an intervention for women with chronic depression, and have found that the addition of such social support had a positive impact on the depression, further bolstering the importance of social support in depression (Harris, Brown, and Robinson 1999).

Temperamental factors have been found to increase people's risk for developing depression. For instance, there is a great deal of evidence linking neuroticism to depression (e.g., Duggan et al. 1995; Kendler et al. 1993). In fact, high levels of neuroticism have been found not only to be associated with current depression, but also to persist in people following recovery from their depressions. Some investigators have drawn on these data to suggest that neuroticism may be present prior to the first onset of depression, and may represent a vulnerability marker or risk factor for developing depression (Duggan et al. 1995).

Finally, there may be specific patterns of thinking that elevate people's risk for the development of depression. Research has demonstrated that certain cognitions or cognitive styles are strongly related to depression. For example, according to the reformulated learned helplessness model (Abramson, Seligman, and Teasdale 1978), people who believe that negative events result from stable, global, and internal factors are more likely to become depressed than are individuals who do not hold these views. For instance, if a person believes that he failed a math test because he is bad at math, rather than attributing the failure to the difficulty of the test or his having had a bad day, then he is attributing his failure to an internal factor. If he then says that he is bad at school more generally and has always been, then he is making stable and global attributions as well, putting him, according to this model, at increased risk for becoming depressed. Similarly, Aaron Beck (1976) has posited that individuals who attend to negative stimuli more readily than to positive stimuli, and who have dysfunctional beliefs about loss and failure (e.g. that others never fail, or that they should never fail), are also likely to become depressed. Although these negative cognitive styles may be longstanding and appear to be a part of someone's personality, it is still unclear whether these cognitive patterns cause depression, are a consequence of depression, or have a more complex relationship to this disorder (Gotlib and Abramson 1999).

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