Department of Social Medicine, University of Bristol
School of Geography, University of Leeds
Department of Social Medicine, University of Bristol, UK
Correspondence: Dr David Gunnell, Senior Lecturer in Epidemiology and Public Health Medicine, Department of Social Medicine, University of Bristol, Canynge Hall, Whiteladies Road, Bristol BS8 2PR, UK. Tel: +44 (0) 117 928 7253; Fax: +44 (0) 117 928 7236; e-mail: D.J.Gunnell{at}Bristol.ac.uk
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ABSTRACT |
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Aims To assess whether cohort effects underlie some of these changes.
Method Graphical displays to assess ageperiodcohort effects on suicide for the period 1950-1999.
Results Successive male birth cohorts born after 1940 carried with them, as they aged, a greater risk of suicide than their predecessors although this effect diminished for the 1975 and 1980 birth cohorts. There was less clear evidence of any increased risk of suicide in post-war female birth cohorts.
Conclusions Succeeding generations of males born in the post-war years have experienced increasing rates of suicide at all ages, an observation in keeping with patterns seen in other countries. If these trends continue into middle- and old-age they will lead to a great increase in overall male suicide rates.
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INTRODUCTION |
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An important public health concern is that birth cohorts could carry with them their increased predisposition to suicide as they age. Such cohort effects could arise because of the exposure of particular generations to factors during their development or early adult life that have a longterm impact on their risk of suicide throughout life. Sixteen years ago an analysis of suicide trends in Britain up to 1980 found no evidence for birth cohort effects (Murphy et al, 1986). This finding contrasted with similar analyses that identified such effects in both Canada and the USA (Murphy & Wetzel, 1980; Solomon & Hellon, 1980). The years covered in the earlier analysis of data for England and Wales included only the first 10 years (1970-1980) of the period over which young male suicide rates increased most rapidly. Subsequent analyses of suicides up to 1985 (Surtees & Duffy, 1989) and 1990 (Charlton et al, 1992) found some evidence of birth cohort effects in younger males and females. In males, each successive 10-year birth cohort, born in the post-war years, experienced higher suicide rates at all ages; in contrast, each successive cohort of females appeared to experience lower suicide rates.
Here we update and refine the earlier assessments of cohort effects in England and Wales to investigate evidence for cohort effects on suicide between 1950 and 1999. As suicide rates in the post-war years have been influenced by changes in the lethality of commonly used methods of suicide and changes in the coding of suicides (period effects: Kreitman, 1976; Charlton et al, 1992; Gunnell et al, 2000), we assessed the effects of both these factors on apparent trends.
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METHOD |
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Age, period and cohort effects
Distinguishing age, period and cohort effects from each other is
problematic because of the linear dependency between these three variables;
this is known as the identification problem. A number of different graphical
and modelling approaches to delineate these effects have been suggested in the
literature (Robertson & Boyle,
1998a,b).
Previous authors have found that ageperiodcohort modelling does
not add greatly to the information obtained from graphical approaches (see,
for example, Allebeck et al,
1996; Newman & Dyck,
1988) and for this reason we have mainly restricted our analysis
to graphical assessments of trends.
Assessment of age and period effects
We assessed the extent to which suicide varies with age by plotting age-
and gender-specific trends in suicide (1950-1999) within six age bands: 15-24;
25-34; 35-44; 45-54; 55-64; and 65+. Period effects refer to the extent to
which living in a particular time period affects disease risk similarly in all
groups in the population, regardless of age and birth cohort. We investigated
period effects by plotting age-specific suicide rates in five different time
(of death) periods: 1955-1959; 1965-1969; 1975-1979; 1985-1989; and 1995-1999.
We omitted the intermediate periods (1950-1954, 1960-1964, etc.) to simplify
the graphical presentation of the data. We also assessed period effects by:
(a) looking for changes in age-specific trends common to all agegender
groups in the graphs of age-specific rates; and (b) by excluding some methods
of suicide and undetermined deaths from our analysis of cohort effects (see
below).
Assessment of cohort effects
We investigated the influence of cohort effects on suicide rates by
plotting suicide rates at age 15-19, 20-24, 25-29, 30-34, 35-39, 40-44, 45-49,
50-54 and 55-59 for those born in nine successive 5-year birth cohorts
i.e. births around 1940, 1945, 1950 and so on up to 1980. Office for National
Statistics mortality data are summarised as the number of deaths, by year,
within 5-year age bands. Each of our estimates of the suicide rate for a
particular birth cohort at a given age, therefore, includes the mortality
experience of people born over a 9-year period centred around the specified
year of birth of the birth cohort (see
Table 1). For example, for the
1940 birth cohort (highlighted), we used the suicide rates for 15- to
19-year-olds between 1955 and 1959 as our estimate of their suicide rates at
age 15-19. Likewise, for our estimate of their suicide rates at age 20-24, we
used the suicide rates of 20- to 24-year-olds between 1960 and 1964, and so
on. As can be seen in Table 1,
deaths in any particular year within an age band never contribute to more than
one birth cohort's estimated mortality rate. Deaths occurring among those born
in the years around the mid-year of each birth cohort contribute to a
diminishing extent as one moves towards the mid-year of the next birth
cohort.
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Assessment of cohort effects with adjustment for possible period
effects
Changes in the toxicity of domestic gas, car exhaust gases and the drugs
most commonly taken in overdose have all influenced patterns of suicide in
England and Wales over the past 50 years
(Kreitman, 1976;
Gunnell et al, 2000;
Amos et al, 2001). To
assess the influence of these period effects on suicide rates, we first
examined cohort effects for all suicides regardless of the method of suicide,
and then for non-overdose, non-gas suicides only. The main methods of
non-overdose, non-gas suicide in England and Wales are hanging, jumping and
drowning (Charlton et al,
1992) and these methods have not changed in lethality or
availability in the post-war years.
Similarly, over the period covered by this analysis there was a change made in the official classification of suicide (see above). The category undetermined deaths (those given a coroner's open verdict) was introduced in 1968. As this change in the categorisation of deaths could have influenced the number of officially recorded suicides subsequent to 1968 (a period effect), we investigated the influence on any apparent birth cohort effects of restricting our analyses to those deaths given a suicide verdict alone (ICD9, E950-959).
To further investigate the presence of birth cohort effects, we assessed the extent to which the ranking of successive birth cohort's suicide rates at age 15-19 were sustained at later ages by calculating Kendall's rank correlations of their rankings using Stata (version 7.0; StataCorp, 2001).
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RESULTS |
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Figure 2 further investigates period effects. Suicide rates in the six age groups are shown for five time (of death) periods. Period effects are present if the suicide rate at all ages in a particular time period is higher or lower than that in other time periods and if there is evidence of parallelism between the lines. There is no strong evidence of period effects in males; the profile of age-specific suicide rates varied markedly between 1955 and 1995. In females, Fig. 2 indicates that there have been year-on-year decreases in suicide rates across all age groups except 15- to 24-year-olds over the time studied, indicative of period effects.
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Cohort effects
Figure 3 shows that in every
5-year age band up to age 30-34, suicide rates were higher in each successive
birth cohort. This indicates a possible birth cohort effect on youth suicide.
This observation is supported by the finding of significant correlations
(generally >0.75) between the ranking of each birth cohort's suicide rates
at age 15-19 with the ranking at subsequent ages
(Table 2). There is evidence,
however, that suicide rates for the 1975 and 1980 birth cohorts at age 15-19
and 20-24 (1975 birth cohort only) are similar to those of the 1970 birth
cohort. For the earlier born (pre-1960) birth cohorts there is some evidence
that suicide rates peak at successively younger ages: 45-49 years for 1940 and
1945 birth cohorts, and 40-44 for the 1950 birth cohort, indicating a possible
period effect as peaks occur in the same time period (around 1990).
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In Fig. 3b suicides by overdose and gassing are excluded. Similar birth cohort effects were observed, but in contrast to those shown in the full analysis, there was no evidence that rates peaked earlier in later born cohorts, supporting the suggestion that this was a period effect because of the popularity and lethality of a particular method of suicide. As this occurred towards the end of each cohorts' period of follow-up, the effect could reflect the recent decrease in suicides because of the reduced toxicity of car exhaust gases (Amos et al, 2001). Exclusion of undetermined deaths in Fig. 3c does not alter these conclusions.
In females (Fig. 4a), unlike males, there is no clear evidence that successive birth cohorts have experienced monotonic increases in their suicide rates at all ages. The statistical analysis in Table 2 supports this finding. In the analysis excluding overdose and gassing (Fig. 4b) and undetermined deaths (Fig. 4c), which together account for over half of all female suicides, there is a suggestion that the 1975 and 1980 birth cohorts experienced higher suicide rates than previous birth cohorts, but because of the limited number of data points it is not possible to reach a firm conclusion about this.
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DISCUSSION |
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Strengths and limitations
The main limitation of our analysis is the rather short period of follow-up
for the cohorts that have experienced some of the highest levels of suicide
before the age of 30. It is possible that as they age these birth cohorts will
no longer continue to experience such high death rates. Indeed, there is
evidence of such a diminution in the cohorts born before 1960.
We did not attempt to undertake ageperiodcohort modelling in view of the recognised limitations of these approaches (Newman & Dyck, 1988; Lee & Lin, 1995). Our graphical displays allow us to investigate age and period effects, but because of their linear dependency assessment of their separate effects is problematic. The most consistent evidence of a period effect was the decrease in suicides in all agegender-groups in the late 1960s (Fig. 1). The most likely explanation for this is the detoxification of the domestic gas supply and the subsequent reduction in both domestic gas and overall suicides (Kreitman, 1976). The rise in suicides in most age gendergroups in the late 1970s/early 1980s coincides with the rise in unemployment at that time (Gunnell et al, 1999) and could reflect the effects of economic recession on suicide.
As far as we could, we controlled for the period effects of changes in the availability/lethality of common methods of suicide (domestic gas poisoning and overdose) by carrying out an analysis restricted to those suicides using other methods. Although this approach takes no account of possible method transfer to methods other than overdoses and gassing, there is some epidemiological evidence that people who use another method of suicide rather than gassing tend to use overdose, and this method was also excluded from this restricted analysis (Gunnell et al, 2000). Furthermore we investigated the effects of changes in the official coding of suicide deaths by inspecting figures based on suicide deaths alone, excluding undetermined deaths (ICD8 and 9 E980E989). The change in coding did not appear to influence the patterns we observed. By restricting our analysis to post-war deaths we ensured the well-recognised effects of war on national suicide rates (Charlton et al, 1992) did not influence observed trends. We were, however, unable to control for other possible period effects, such as the economic recession in the 1980s and associated raised levels of unemployment (Gunnell et al, 1999). Our analysis of period effects in males (Fig. 2) revealed no strong evidence of such effects. Thus, the year-on-year rises in young male suicide are more likely to be cohort effects or reflect the influence of year-on-year changes in risk factors for suicide, which have varying effects on different age gender groups.
Cohort effects on suicide reported in other countries
Analyses of national mortality data up to the early 1990s for the USA
(Murphy & Wetzel, 1980),
Canada (Solomon & Hellon,
1980), Italy (La Vecchia
et al, 1986), New Zealand
(Skegg & Cox, 1991), Spain
(Granizo et al, 1996),
Sweden (Allebeck et al,
1996) and Belgium (Moens
et al, 1987), using a range of different analytical
approaches, have reported that suicide rates in successive male birth cohorts
have increased in the post-war years. In contrast, an analysis of successive
Danish birth cohorts, up to 1971 births, found no evidence of such an effect
(Bille-Brahe & Jessen,
1994). Similarly in Australia, successive birth cohorts followed
up to 1979 showed little evidence of cohort effects, except in the youngest
(1955 and 1960) birth cohorts (Goldney
& Katsikitis, 1983). A more recent analysis of Australian data
up to 1999, however, reports post-war patterns of male suicide consistent with
those seen in our analysis (Snowdon &
Hunt, 2002).
Findings for female suicides are less consistent than those for males. In Britain declines in the suicide rates of successive post-war cohorts have been reported (Surtees & Duffy, 1989), whereas there have been increases in successive female birth cohorts in the USA (Murphy & Wetzel, 1980), Canada (Solomon & Hellon, 1980), Belgium (Moens et al, 1987) and, to a lesser extent, Australia (Goldney & Katsikitis, 1983). No effects were seen in New Zealand (Skegg & Cox, 1991), Denmark (Bille-Brahe & Jessen), Italy (La Vecchia et al, 1986) or Sweden (Allebeck et al, 1996).
Differences between studies could reflect real variations in national patterns of suicide, differences in the period studied, the analytical approaches used, or study power. Analyses of suicide trends in small countries and in women, where the numbers of suicides are relatively low, are unable to give precise estimates of suicide rates within a particular age periodcohort grouping. Detection of cohort effects in this context is difficult because of fluctuations in rates based on few events. The different findings could also reflect the varying influence of period effects in different countries and the capacity of the analytical approaches used to assess and control for such effects. Goldney & Katsikitis (1983), for example, suggested that differences in the findings in Australia compared with the USA and Canada could have arisen through the period effects of restrictions on the prescribing of barbiturates in Australia introduced in 1967. More recent analyses of Australian and Canadian suicide trends suggest similar birth cohort effects to those seen in this analysis of data in England and Wales (Mao et al, 1990; Snowdon & Hunt, 2002).
Possible explanations for birth cohort effects
Taken together, the evidence from a range of industrialised countries
suggests that, at least up until the 1970 birth cohort, each successive
post-war cohort of males has experienced a higher suicide rate than its
predecessor. Findings for females are less clear-cut. There are a number of
possible explanations for the cohort effects observed in males. First, cohort
effects could result from the long-term impact of changing levels of exposure
in childhood or early adulthood to some environmental influence on long-term
mental health. Such factors might include increasing levels of parental
separation (Agid et al,
1999), increased substance misuse
(Christophersen et al,
1998), or changes in the labour market such as increased female
participation (Platt & Hawton,
2000). Second, it is possible that increased media portrayals of
idealised lifestyles in televised drama and advertising materials results in
young people having increased, but unrealistic, expectations from life. Such
mismatches between expectation and reality could lead to increases in suicide
risk. Similarly, changes in the media portrayal of suicide (as relatively
common-place and acceptable) could permanently influence the attitudes of
young viewers regarding its cultural normality, and hence their likelihood of
committing suicide in times of crisis. Last, the effects could conceivably
result from changes in the popularity of particular methods of suicide in
successive generations. Such an effect could occur if an individual's
preferred method of suicide is based on early life experiences (e.g. media
portrayal) and remains the same thereafter. In the UK, for example, it is
possible that the increased suicide rates in successive male birth cohorts
could reflect a greater preference for hanging as a method of suicide; this
method is more usually fatal than the other commonly used methods such as
overdose and gassing (Pounder,
1993).
Reasons why these effects should influence female suicide rates to a lesser extent are unclear, although there is a suggestion that similar patterns could be beginning to emerge in younger female generations in England, and in the analyses based on large populations such as the USA and Australia (see above). The main social change among women in the post-war years has been their increased engagement in the labour market and greater equality with men.
Public health implications
If younger birth cohorts carry their increased suicide rates through into
later life then the recent falls in suicide rates in England and Wales will be
reversed (Kelly & Bunting,
1998). Possible explanations for the observed effects require
clarification to inform suicide prevention activities.
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Clinical Implications and Limitations |
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LIMITATIONS
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REFERENCES |
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Received for publication May 8, 2002. Revision received October 7, 2002. Accepted for publication October 21, 2002.
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