Geographical variation in the seasonality of suicide

Journal of Affective Disorders, 15 (1988) 163-168 Elsevier

163

J A D 00563

Geographical variation in the seasonality of suicide R . M i c c i o l o 1,2, C h . Z i m m e r m a n n - T a n s e l l a

1, p. W i l l i a m s 3 a n d M . T a n s e l l a 1

i Cattedra e Servizio di Psicologia Medica, Istituto di Psichiatria, UniversitZt di Verona, 37134 Verona, Italy, 2 lstituto di Statistica e Ricerca Operativa, Universith di Trento, 38100 Trento, Italy and -~Institute of Psychiatry, University of London, London SE5 8AF, U.K.

(Received 13 October 1987) (Revision received 9 March 1988) (Accepted 14 March 1988)

Summary This paper reports the results of a study on the seasonal variation of suicides occurred in Italy during the years 1969-1981. We found evidence for a cyclical pattern with one cycle per year for N o r t h / C e n t r a l as well as for South Italy with no difference between the two parts in the variance explained by it. However, urban regions showed a higher seasonality than the rural parts of Italy, independently of their geographical location. To account for these findings, the hypothesis of a strong seasonal related urban rhythm has been put forward.

Key words: Suicide; Seasonal variation; Climatic variables

Introduction There is a general consensus among most published studies that deaths by suicide show a significant seasonal variation in incidence, with the main peak occurring in the spring (for example Eastwood and Peacocke, 1976; Meares et al., 1981; N~iyh~i, 1982; Micciolo et al., 1988). However, there is less certainty about the natural or social environmental factors that cause seasonality or contribute to its timing. The importance of climatic variables in producing the seasonal rhythm is sug-

Address for correspondence: Christa Z i m m e r m a n n Tansella, Ph.D., Associate Professor of Medical Psychology, Cattedra e Servizio di Psicologia Medica, Istituto di Psichiatria, 37134 Verona, Italy.

gested by two observations: first, the seasonal pattern found throughout northern hemisphere studies is reversed in southern hemisphere studies, i.e., the peak occurs during the same season but at opposite times during the calendar year (for example Swinscow, 1951; Parker and Walter, 1982) and second, less seasonal variation occurs where there is less climatic variability throughout the year (Aschoff, 1981). Sunlight or luminance seems to be the most important climatic factor correlated with the peak incidence in spring (Parker and Walter, 1982; N~iyh~i, 1983). At the same time the fact that the amplitudes of the seasonal rhythms have decreased over the past century (Aschoff, 1982) suggests that other, sociocultural, factors are overriding the influence of natural factors. The observed deseasonalization has been attributed to industrialization and urbanization: the seasonal

0165-0327/88/$03.50 ~ 1988 Elsevier Science Publishers B.V. (Biomedical Division)

164

variation of suicide is higher in prevalently rural countries such as Finland or Greece, compared to more industrialized countries such as Denmark, Sweden and Australia (Bolander, 1972; Bazaz et al., 1979; Parker and Walter, 1982), and the spring peak in suicide incidence is more characteristic for agricultural compared to modern occupations (N~iyh~i, 1982). These findings give credit to Durkheim's prediction (1897) that rural communities in which there would be a larger seasonal variation in the pace of life would show a correspondingly larger variation in suicidal rates when compared with big cities. On the other hand, when considering Durkheim's idea of the varying pattern of social interaction over the year as the cause for seasonal variation in suicide, we cannot exclude the influence of seasonal related social changes also in urban areas, in particular in countries with very distinct seasons (e.g., cold winter and long warm summer). One way of separating climatic and sociocultural factors is to compare data collected from regions with marked climatic differences and different degrees of urbanization. Italy, due to its geographical characteristics, has these requisites. It extends over 10 degrees of latitude, and consequently there are significant climatic differences between the n o r t h / c e n t r a l and the southern parts: the seasonal differences are more pronounced in the north/central regions where generally there is a relatively long cold winter and a warm summer, while the south is characterized by a less cold and shorter winter and a very .warm summer season. Moreover, both parts, although to different degrees, have some predominantly rural and some predominantly industrialized urban regions. The aim of this paper is therefore to examine the seasonal variation in the incidence of suicide in rural and urban regions of N o r t h / C e n t r a l and Southern Italy. Our hypotheses are that there is a larger seasonal variation in the north/central part compared to the south and in the rural compared to the urban areas. Method

Data The mean values for some selected climatic variables for the years 1980-1984 were calculated

from the tables published by the Istituto Centrale di Statistica (ISTAT, 1985) for the n o r t h / c e n t r a l and southern part of Italy separately. Monthly data on suicide in each region of Italy (defined according to the certified cause of death) for the years 1969-1981 were obtained from the Istituto Centrale di Statistica (ISTAT) in Rome. First, the regions were grouped according to the customary geopolitical division into n o r t h / c e n t r a l (Piemonte, Val d'Aosta, Lombardia, Trentino Alto-Adige, Veneto, Friuli-Venezia Giulia, Liguria, Emilia Romagna, Toscana, Umbria, Marche and Lazio) and southern (Abruzzi, Molise, Campania, Puglia, Basilicata, Calabria, Sicilia, Sardegna), and analyses performed. Then the n o r t h / c e n t r a l regions were divided into two groups, depending on their location on the urban-rural continuum. This was done by ranking the regions according to the population density (inhabitants per km 2) for 1984 (the midpoint of the survey years) obtained from the Annuario Statistico ltaliano (ISTAT, 1985). The six regions with the highest densities were designated as 'predominantly urban' (Piemonte, Lombardia, Veneto, Liguria, Emilia Romagna and Lazio) while the other six were designated as 'predominantly rural' (Val d'Aosta, Trentino Alto-Adige, Friuli-Venezia Giulia, Toscana, Umbria and Marche) with a cut-off point of _> 148 p e r s o n s / k m 2. The same procedure was applied to the southern regions: those designated as urban were Campania, Puglia, Calabria and Sicilia, while Abruzzi, Molise, Basilicata and Sardegna were categorized as rural, with a cut-off point of > 116 persons/kin 2. Further analyses were then performed on these groups of regions. Analysis The mean values of minimal and maximal temperature, lowest and highest temperature and of the temperature differences, the hours of sunshine and quantity of rain for the n o r t h / c e n t r a l and southern parts of Italy were compared by t-test. The frequency of suicide in each month was plotted and inspected. Then harmonic analyses were performed. To remove all trends, i.e., the mean differences between years, an analysis of variance (ANOVA) approach was used. The G L I M Package was utilized (Backer and Nelder, 1978) fitting to monthly data a generalized linear model,

165

specifying a Poisson error with link identity. The main effect of years was then subtracted from the original data and the residuals so obtained were subjected to harmonic analysis using the method described by Pocock (1974). In this technique, all the harmonics are estimated and then categorized as seasonal or non-seasonal, depending on whether the frequency per annum is an integer. The variance can then be partitioned into seasonal, non-seasonal and random components. The contribution of individual harmonics can also be estimated.

raw rates are reported, for comparison. It can be seen that, for each year, the rates are substantially higher in the n o r t h / c e n t r a l than in the southern regions. Inspection of rates for the urban-rural groupings showed that, in southern regions, suicide rates were higher in the less densely populated regions. In n o r t h / c e n t r a l regions, the rates of the urban and rural groups are similar. The monthly frequencies of suicide in each of the four groups of regions were then plotted and inspected. Cyclical fluctuations in the number of suicides were seen in each group, and they appeared to be consistent across the years. Harmonic analysis was first used to cast light on the n o r t h / s o u t h difference in the seasonality of suicide (Table 3). As can be seen, the proportion of variance accounted for by seasonal harmonics was slightly greater for the n o r t h / central regions than for the southern regions. In both cases, only the first harmonic (one cycle per year) was significant (X 2 = 50.5 and 55.9 respectively; df= 2; P < 0.001). N o difference emerged between the two parts of the country in the variance explained by the first harmonic. Table 3 also shows the results of the harmonic analyses conducted on the four n o r t h - s o u t h / urban rural groups of regions. In each case, only the first harmonic was significant (X 2 ranged from

Results

Table 1 shows the climatic variables for N o r t h / Central and South Italy. It can be seen that with the exception of the difference between mean minimal and maximal temperature all other variables were significantly different for the two parts. Table 2 shows the annual suicide rates ( × 100000 population) for the north/central and southern regions for the 13 years of the study. Sex and age standardized rates (using indirect standardization) were calculated only for the years 1972 1981; for the previous 3 years ISTAT did not provide the distribution of regional population by sex and age. In parentheses the corresponding

TABLE 1 CLIMATIC

MEAN VALUES (1980-1984) FOR NORTH/CENTRAL

AND SOUTH ITALY

Source: I S T A T , 1985. Climatic variables

Minimal temperature(°C) a M a x i m a l t e m p e r a t u r e ( o C) a Differences between m i n i m a l a n d m a x i m a l t e m p e r a t u r e ( ° C) H i g h e s t t e m p e r a t u r e ( ° C) b Lowest temperature (°C) b D i f f e r e n c e b e t w e e n h i g h e s t a n d lowest temperature (°C) Q u a n t i t y of r a i n ( m m ) H o u r s of s u n s h i n e

North/central 13 m e a s u r e m e n t centers

t

8.5_+ 18.0-+

1.7 1.2

11.7+ 20.4 +

2.3 1.8

3.81 * * * 2.87 * *

9.4_+ 35.2 + 6.7+

1.8 2.0 2.6

8.5 + 37.7 + -- 1.2_+

2.3 1.8 3.0

1.01 N S 3.26 ** 4.72 * * *

39.1_+ 2.0 594 + 2 0 7 2357.3_+174.1

2.24 * 3.67 ** 4.82 * * *

42.1+ 3.7 887 + 1 8 0 2014 +160

F o r e a c h y e a r the m e a n s o f d a i l y t e m p e r a t u r e d a t a h a v e b e e n c a l c u l a t e d . b F o r e a c h y e a r the h i g h e s t a n d the lowest t e m p e r a t u r e value h a v e b e e n r e c o r d e d . *P<0.05,**P<0.OI,***P<0.001. a

South 11 m e a s u r e m e n t centers

166 TABLE 2 ANNUAL SEX AND AGE STANDARDIZED SUICIDE RATES IN ITALY 1969-1981 ( x 100000) Raw data are given in parentheses. North/central regions Urban 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981

7.23 6.88 6.59 6.69 6.64 7.58 7.69 8.40 8.48 7.93

(6.18) (6.63) (7.03) (7.02) (6.68) (6.38) (6.50) (6.48) (7.43) (7.63) (8.38) (8.53) (8.04)

Southern regions Rural

6.46 6.59 6.48 6.02 7.21 7.18 7.42 8.25 8.32 7.82

(6.93) (7.47) (7.15) (6.68) (6.82) (6.71) (6.26) (7.52) (7.52) (7.86) (8.78) (8.91) (8.44)

18.8 to 43.7; df= 2; P < 0.001). All s i g n i f i c a n c e levels r e p o r t e d a b o v e m a y b e c o n s i d e r e d s u p e r f l u ous, t a k i n g i n t o a c c o u n t t h a t the a n a l y z e d d a t a d o n o t refer to s a m p l e s b u t to the total p o p u l a t i o n of suicides w h i c h o c c u r r e d in Italy. T h e results are clear. T h e s e a s o n a l v a r i a n c e was h i g h e s t in the two u r b a n g r o u p s ( n o r t h / c e n t r a l a n d s o u t h e r n ) , l o w e s t in the rural s o u t h e r n regions, while the rural n o r t h / c e n t r a l r e g i o n s occ u p i e d an i n t e r m e d i a t e p o s i t i o n . In u r b a n n o r t h / c e n t r a l a n d s o u t h e r n r e g i o n s t h e p e a k of the first h a r m o n i c o c c u r r e d in M a y . R u r a l r e g i o n s i n s t e a d s h o w e d a J u n e p e a k . T h e a m p l i t u d e s (the e x p e c t e d r a n g e s in n u m b e r of suicides in r e l a t i o n to the m e a n n u m b e r ) for the u r b a n a n d rural n o r t h / c e n t r a l r e g i o n s w e r e 28 a n d 32%, a n d for the u r b a n and rural s o u t h e r n r e g i o n s 39 a n d 35%. Fig. 1 s h o w s the e x p e c t e d n u m b e r s of suicides in the

Urban

Rural

-

(4.56) (4.56 (5.27 5.07 (4.63 5.76 (5.27 4.35 (3.99 5.(/4 (4.63 5.91 (5.42 5.89 (5.44 6.03 (5.68 6.23 (5.91 6.62 (6.31 6.43 (6.17

4.24 3.97 3.79 4.39 4.02 4.30 4.65 4.69 5.43 5.06

(3.50) (3.60) (3.62) (3.62) (3.39) (3.24) (3.77) (3.45) (3.70) (4.07) (4.12) (4.79) (4.47)

f o u r g r o u p s of regions, as p r e d i c t e d b y the first h a r m o n i c , t o g e t h e r w i t h m o n t h l y n u m b e r s of suicides a v e r a g e d o v e r the c o n s i d e r e d 1 3 - y e a r period.

Discussion O u r f i n d i n g s c o n f i r m the s e a s o n a l i t y in the i n c i d e n c e of suicide, w i t h o n e cycle p e r y e a r for N o r t h / C e n t r a l a n d S o u t h Italy, b u t t h e r e was n o d i f f e r e n c e b e t w e e n the t w o p a r t s w i t h r e g a r d to the v a r i a n c e e x p l a i n e d by the s e a s o n a l h a r m o n i c s . O u r first h y p o t h e s i s of a g e n e r a l l y l a r g e r s e a s o n a l v a r i a t i o n in the n o r t h / c e n t r a l r e g i o n s is t h e r e f o r e n o t s u p p o r t e d . Also, the s e c o n d h y p o t h e s i s o f a h i g h e r s e a s o n a l v a r i a t i o n in rural r e g i o n s c o u l d n o t be c o n f i r m e d . O n the c o n t r a r y , t h e r e w a s a h i g h e r s e a s o n a l i t y in the u r b a n c o m p a r e d to the

TABLE 3 HARMONIC ANALYSES IN DIFFERENT GEOGRAPHICAL AREAS IN ITALY

Urban Rural All

Percentage of variance explained by all seasonal harmonics

Percentage of variance explained by the first harmonic (1 cycle/year)

North/central regions

Southern regions

North/central regions

Southern regions

61.9 39. l 69.1

52.6 18.9 55.9

43.7 33.0 50.5

41.9 18.8 48.0

167 N o r t h - - C e n t r a l

Urban

South

N

Urban o

0 ..'.

.•',,

160

55

".%

•

.o..

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50 145 45 130 ~0

115

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5,

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7L

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N o r t h - - C e n t r a l o

9'

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a 11

35

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3

12

Rural

i

i

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5

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South

i

8

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I'0

I'I

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Rural

,,"

50

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16

45 o..

"

o.......

lq 0""

~0 35

r

9

~... ••" ~.,.

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.

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Fig. 1. Expected numbers of suicides as predicted by the first harmonic (continuous line) and monthly numbers of suicides averaged over the 13-year period (broken line).

rural regions independent of their north/central or southern location. The only difference obtained regarded the rural north/central and the rural southern parts: the first harmonic (one cycle per year) explained 33% of the variance in the rural north/central regions, and about 19% in the rural southern regions. The rural southern regions had the lowest seasonal variation in suicide; in both rural regional groups the spring peak was postponed to June and the trough was in December, as compared to May and November for the two urban regional groups• These unexpected findings require comment. The first issue is the absent n o r t h / c e n t r a l - s o u t h difference, which suggests that the different climatic pattern of the two parts exerts no in-

fluence on the annual suicide rhythm. There was, however, a difference in the extent of variation, predictable on the basis of previous observations: the amplitudes of the peaks of the first harmonic were lower in the north/central regions than in the southern regions. Thus, the effects of seasonality, in terms of peak incidence, are less marked in the north/central regions, confirming the deseasonalizing influence of the higher general level of industrialization and standard of living characteristic of N o r t h / C e n t r a l Italy compared to the South. The second issue is why the urban regions, contrary to our expectations, show a higher seasonality in suicide variation than the rural parts of Italy. Natural seasonal factors, which should have

168 t h e i r m a x i m a l i m p a c t in r u r a l r e g i o n s , s e e m to b e less p o t e n t t h a n o t h e r s e a s o n a l r e l a t e d c h a n g e s in u r b a n i z e d a r e a s in d e t e r m i n i n g t h e a n n u a l r h y t h m in s u i c i d e i n c i d e n c e . T h i s s u g g e s t s e n v i r o n m e n t a l , non-natural, rhythms coinciding with calendar defined seasons which characterize urban life t h r o u g h o u t I t a l y a n d w h i c h a r e a b s e n t o r less m a r k e d in r u r a l a r e a s . Perhaps these patterns of seasons related c h a n g e s in u r b a n r e g i o n s ( i n c r e a s e d social s t i m u l a t i o n in s p r i n g , e.g., o u t d o o r a c t i v i t i e s , t o u r i s m , s c h o o l a n d u n i v e r s i t y finals, etc.) a r e t y p i c a l f o r I t a l y a n d m a y n o t b e f o u n d in t h e s a m e f o r m in c o u n t r i e s w i t h o t h e r t y p e s o f m o r e o r less s e a s o n a l r e l a t e d social o r g a n i z a t i o n . U n f o r t u n a t e l y our data, which were divided on the basis of the population density of regions, do not permit this h y p o t h e s i s to b e t e s t e d . C o m p a r i s o n o f t h e seas o n a l v a r i a t i o n in s u i c i d e for l a r g e cities w i t h t h a t f o r s m a l l e r p r o v i n c i a l t o w n s w i t h i n t h e s a m e reg i o n s m i g h t c a s t l i g h t o n t h i s issue. F i n a l l y t h e d i f f e r e n c e in t h e s e a s o n a l v a r i a t i o n in s u i c i d e b e t w e e n t h e r u r a l n o r t h / c e n t r a l a n d t h e rural southern regions requires comment. There are two possible explanations: rural north/central c o m p a r e d to r u r a l s o u t h e r n r e g i o n s h a v e h i g h e r s u i c i d e s e a s o n a l i t y b e c a u s e o f t h e i r g r e a t e r seasonal related climatic variability; or rural regions in t h e s o u t h a r e m u c h m o r e r u r a l t h a n t h o s e in t h e n o r t h / c e n t r a l p a r t s , w h i c h are m o r e i n f l u e n c e d b y u r b a n r h y t h m s . F u r t h e r s t u d i e s a r e n e c e s s a r y to cast light on this matter.

Acknowledgement This study was supported by the Consiglio Nazionale delle Ricerche (CNR, Roma), Progetto Finalizzato Medicina Preventiva e Riabilitativa, 1982 1987, C o n t r a c t N o . 8 6 . 0 1 9 6 2 . 5 6 .

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