The estimated lifetime prevalence of MDE in the total sample is 19.2%. () This estimate varies significantly across the four age groups considered here (χ23 = 70.4, p < .001) due to a much lower estimated prevalence among respondents in the oldest age group (65+; 9.8%) than in younger age groups (18-34, 35-49, 50-64; 19.4-22.7%). The same general pattern holds for 12-month and 30-day prevalence estimates (8.3% and 3.1%, respectively) in the total sample, where estimates vary significantly across the age groups (χ23 = 46.9-103.5, p < .001) due largely to much lower prevalence estimates among respondents ages 65+ (2.6% and 1.0%, respectively) than in younger age groups (7.7-10.4% and 3.0-3.7%, respectively). Very similar age differences are obtained separately for women and men despite prevalence estimates being consistently higher among women than men.
Thirty-day, 12-month, and lifetime prevalence estimates of DSM-IV/CIDI MDE by age and gender
It is noteworthy that the ratio of the prevalence estimates among 65+ year olds compared to the total sample is lower for recent (30-day and 12-month) prevalence (31-32%) than for lifetime prevalence (51%). The same pattern holds separately among women (36-38% vs. 57%) and men (9-19% vs. 35.1%). This gradient suggests that recall error is not responsible for the lower prevalence estimates among 65+ year olds, as recall failure would produce an opposite pattern, with high estimates in the shortest time frame and low estimates in lifetime prevalence. A related observation is that the ratio of 12-month prevalence to lifetime prevalence is consistently lower among respondents ages 65+ (22-28%) than younger respondents (37-57%). This is an expectable pattern on substantive grounds, but would be reversed if elderly recall failure drove differences in estimates..
Age-of-onset, lifetime course, and 12-month severity
Retrospectively reported age-of-onset (AOO) of MDE varies significantly across age groups in the total sample (χ23 = 696.8, p < .001). A monotonic increase exists in mean AOO from the youngest to oldest age groups: 17.8 (ages 18-34), 25.5 (ages 35-49), 33.1 (ages 50-64), and 43.0 (ages 65+). () The difference between the median age-at-interview in each age group and the mean AOO of MDE in that age group increases with increasing age. For example, in the youngest age group (ages 18-34 at interview), mean AOO is approximately 8 years earlier than the median age-at-interview (i.e., 17.8 vs. 26) compared to 16, 24, and over 30 years in the successively older age groups. This pattern is what we would expect based on substantive processes and, like the patterns in , argues against a methodological interpretation of the age differences. As would be expected based on these inter-cohort differences, the mean number of lifetime episodes of MDE reported by respondents with a lifetime history increases monotonically with age-at-interview, from a low of 15.4 among respondents in the 18-34 age group to 30.2 in the 65+ age group.
Dimensions of DSM-IV/CIDI MDE lifetime onset, course, and 12-month persistence and severity by age among NCS-R respondents with MDE
The mean self-reported duration of depressive episodes in the 12 months before interview, which was retrospectively reported to be 27.5 weeks, does not vary significantly across age groups (F3,692 = 1.6, p = .21). () However, symptom severity of these episodes, as assessed by the QIDS, does vary significantly with age, with the proportion of cases classified clinically mild higher in the 65+ age group (21.8%) than in younger age groups (6.8-10.3%) and the percent classified clinically severe lower in the 65+ age group (37.8%) than in the younger age groups (45.8-53.0%). Consistent with these results, the proportion of 12-month cases rating their depression as causing severe role impairments varies significantly with age (χ23 = 9.9, p = .020) and is lowest in the 65+ age group (56.5% vs. 61.3-72.2%). The mean number of days out of role in the past year due to depression among 12-month cases also varies significantly with age (F3,692 = 8.3, p < .001) and is again lowest in the 65+ age group (18.8 vs. 28.3-58.4 days).
Comorbidity of 12-month MDE with other 12-month DSM-IV disorders
All 14 DSM-IV disorders considered here are significantly and positively associated with MDE in the total sample in both linear (results not shown, but available on request) and logistic regression models. () ORs are in the range 3.6-161.9. Consistent with previously-reported NCS-R results (Kessler et al., 2005
), the conditional 12-month prevalence of comorbid disorders among respondents with MDE is lowest among those 65+ years old for 10 of the 14 comorbid disorders (6 significantly so at the .05 level). Associations (odds-ratios) of MDE with comorbid mental disorders vary significantly with age for six of the 14 comorbid disorders. In three of these six the OR is highest in the 65+ age group (bipolar disorder, panic disorder, generalized anxiety disorder). In the other three, which involve substance disorders, the OR is missing in the 65+ age group because no elderly respondents (with or without MDE) have the comorbid disorder but the OR is highest in the next oldest age group (50-64). The OR is also highest in the 65+ age group for three other disorders (social phobia, PTSD, intermittent explosive disorder) even though the age difference is not statistically significant.
Twelve-month comorbidity (odds-ratios) of DSM-IV/CIDI MDE with other 12-month DSM-IV/CIDI disorders by age1
As noted above in the section on analysis methods, ORs describe multiplicative relationships, which mean ORs can be large even when the proportions of depressed and non-depressed people who have a particular comorbid disorder are small. For example, when the conditional prevalence of a comorbid disorder is 3% among people with depression and 2% among people without depression, the difference is small (1%) but the odds-ratio is large (3.1). It is consequently useful to examine patterns of comorbidity based on linear regression equations. When we did this (detailed results are available on request), we found that age-related variations in prevalence differences for comorbid disorders are less consistent than age-related variations in ORs. In four cases the prevalence difference is highest among the elderly (dysthymia, PTSD, alcohol abuse, drug dependence, the latter two involving respondents in the 50-64 age group because the comorbid disorder did not occur among those in the 65+ age group) and in five other cases lowest among the young (bipolar disorder, panic disorder, adult separation anxiety, drug abuse, alcohol dependence, with the last three referring to the 50-64 age group), but only two of these nine are statistically significant (drug abuse and dependence). In two other cases the prevalence difference is lowest among the elderly (agoraphobia, specific phobia), while in the remaining three cases there is no clear age trend in the prevalence differences.
Comorbidity of 12-month MDE with chronic physical disorders
As with the comorbid mental disorders, all 14 physical disorders are significantly comorbid with MDE in linear models in the total sample. In 11 of these 14 the comorbid disorder is more prevalent among people with than without MDE and in the other three less prevalent. (Detailed results are available on request.) Only 6 of the 14, in comparison, are significantly comorbid with MDE in logistic regression models. () All six ORs are positive (1.4-3.8). The association between age and the conditional prevalence of the comorbid physical disorders is also for the most part positive, with conditional prevalence either increasing monotonically with age (hypertension, stroke, arthritis), being highest in the two oldest age groups (cancer, heart disease, chronic lung disease, chronic pain), or being lowest in the youngest age group (low back pain, ulcers, heart attack, diabetes). However, the age gradient is not consistent, as age is significantly related to prevalence for only eight of the 14 disorders and prevalence is highest in the 65+ age group for only three of these eight (hypertension, stroke, arthritis). Prevalence is highest in the next oldest age group (50-64) for the majority of comorbid disorders. Nonetheless, the proportion of depressed respondents with one or more comorbid physical disorders is considerably higher in the 65+ age group (94.6%) than in any of the younger age groups (69.8-89.6%).
Twelve-month comorbidity (odds-ratios) of DSM-IV/CIDI MDE with chronic physical disorders by age1
Associations of MDE with comorbid disorders have a general tendency to decrease with age. In the logistic regression models, the ORs decrease with age for six disorders (heart attack, heart disease, asthma, lung disease, cancer, diabetes), are lowest in the elderly for two others (allergies, ulcers), and are highest in the young for one other (hypertension), although only three of these nine are statistically significant (heart attack, lung disease, hypertension). In the linear regression models, the MDE prevalence difference deceases monotonically with age for seven comorbid disorders and is lowest among the elderly for five others (low back pain, headaches, chronic pain, diabetes, ulcers). Seven of these 12 age trends are statistically significant. An age trend is absent for only the two remaining disorders (hypertension, stroke).
Age-related differences in treatment of 12-month MDE
Somewhat more than half (57.7%) of NCS-R respondents with 12-month MDE reported treatment for emotional problems in the year before interview. () The proportion in treatment is lowest in the 65+ age group (48.3% vs. 51.2-64.0) due to low proportions of the elderly than younger respondents in specialty (18.3% vs. 29.9-38.2%) and CAM (3.9% vs. 7.3-12.3%) treatment. More detailed analysis shows that these low treatment proportions are not due to the lower severity of MDE, as the proportion in treatment among the severely depressed also varies significantly with age (χ23 = 9.5, p = .023) and is lowest among the 65+ years olds (33.6% vs. 63.3-69.9%). Proportional treatment in the general medical (GM) sector also varies significantly with age, as depressed patients in the 65+ age group receive a higher proportion of their treatment in the GM sector (78.8%) than do younger respondents (47.8-69.5%).
Past year treatment of emotional problems among respondent with 12-month DSM-IV/CIDI MDE by age
Age-related socio-demographic correlates of MDE prevalence and treatment
We examined four socio-demographic correlates of MDE and treatment: sex, family income, employment status (employed/self-employed, student, homemaker, retired, disables/unemployed), and marital status (married, previously married, never married). All four are significantly related to MDE in the total sample. Elevated risk of MDE is related to being female, poor, disabled/unemployed, and unmarried, with ORs of 1.7-2.8. (Detailed results are available on request.) The strength of these associations (ORs), though, varies significantly with age. The higher odds of MDE among women than men is especially pronounced among respondents 65+ years old (3.2 vs.1.5-20). The elevated odds of MDE associated with being poor (in the lowest quartile of the income-per-family-member distribution) is also especially pronounced among respondents 65+ years old (3.9 vs. 1.2-2.7). Employment status, in comparison, is unrelated to MDE among the elderly although it is significant at earlier ages. The elevated odds of MDE among the previously married, finally, is somewhat higher among respondents 65+ years old than those 50-64 years old (2.3 vs. 1.7). Sex and marital status are the only two socio-demographic variables studied that are significantly related to 12-month treatment of emotional problems among respondents with 12-month MDE. Women have higher odds of treatment than men (1.5) and the previously married have higher odds of treatment than the married (1.6). The association of sex with treatment is significantly weaker among depressed people who are 65+ years old (1.0) than younger (1.3-2.2). (Detailed results are available on request.)