Seasonal Variation In Births Research Articles

Temporal, Seasonal, and Regional Differences in Births and Deaths in Åland (Finland)

A good knowledge of the seasonal variation during normal years is of fundamental importance for analyses of the effects of wars, famines, epidemics, or similar privations on births and deaths. In this study we consider data from the Aland Islands (Finland) for 1650-1950. During the period 1650-1793 there are subperiods with missing data for all parishes, and consequently the total data for the Aland Islands for this period have to be estimated using available data. For the period 1794-1950 the registered data seem to be complete and reliable, but the war year 1809 shows a marked deficit of births. During the last decades of the 19th century the number of births increases markedly and after that shows a strong decrease. After the 1930s births increase again. To allow seasonality comparisons between the Aland Islands as a whole and its subregions, we base our studies on seasonal indexes. There is a markedly decreasing temporal trend in the seasonal variation of births for the Aland Islands as a whole, but the general pattern remains mainly the same, having two peaks, one in March-April and one in September-October. For the period 1901-1950 the seasonal variation almost disappeared. The strength of the seasonal variation in births shows regional differences, but the general pattern is mainly the same. The outermost parish, Kökar, an isolate of its own, shows the strongest seasonal variation in births. The annual number of deaths shows some marked peaks, especially in the war year 1809. For both sexes there are marked peaks in 1809, indicating that the deaths were mainly caused by epidemic diseases rather than by killing in battles. For mortality a decreasing trend in the seasonal variation is observed during 1650-1750, but after 1751-1800 the strength of seasonality shows an increasing trend and a sinusoidal pattern.

Temporal, Seasonal, and Regional Differences in Births and Deaths in �land (Finland)

A good knowledge of the seasonal variation during normal years is of fundamental importance for analyses of the effects of wars, famines, epidemics, or similar privations on births and deaths. In this study we consider data from the Åland Islands (Finland) for 1650–1950. During the period 1650–1793 there are subperiods with missing data for all parishes, and consequently the total data for the Åland Islands for this period have to be estimated using available data. For the period 1794–1950 the registered data seem to be complete and reliable, but the war year 1809 shows a marked deficit of births. During the last decades of the 19th century the number of births increases markedly and after that shows a strong decrease. After the 1930s births increase again. To allow seasonality comparisons between the Åland Islands as a whole and its subregions, we base our studies on seasonal indexes. There is a markedly decreasing temporal trend in the seasonal variation of births for the Åland Islands as a whole, but the general pattern remains mainly the same, having two peaks, one in March-April and one in September-October. For the period 1901–1950 the seasonal variation almost disappeared. The strength of the seasonal variation in births shows regional differences, but the general pattern is mainly the same. The outermost parish, Kökar, an isolate of its own, shows the strongest seasonal variation in births. The annual number of deaths shows some marked peaks, especially in the war year 1809. For both sexes there are marked peaks in 1809, indicating that the deaths were mainly caused by epidemic diseases rather than by killing in battles. For mortality a decreasing trend in the seasonal variation is observed during 1650–1750, but after 1751–1800 the strength of seasonality shows an increasing trend and a sinusoidal pattern.

Variation in season of birth in singleton and multiple births concordant for autism spectrum disorders

Patterns of seasonal variation in births in some neuropsychiatric conditions have been found in previous research; however, no study to date has examined these disorders for seasonal variation in singletons and multiple births separately. This study aimed to determine whether the birth date distribution for individuals with autism spectrum disorders (ASD), including singletons and multiple births, differed from the general population. Two ASD case groups were studied: 907 singletons and 161 multiple births concordant for ASD. Two control groups were obtained from registered births of singletons and multiples. Results of the non-parametric time-series analyses, where day of birth was used, suggested there were three peaks in ASD singletons and ASD concordant multiple births. Roughly, the peaks were in April, June and October for singletons and about 2-4 weeks earlier in multiples. Results from multivariable Poisson regression, where month of birth was used, indicated that ASD concordant multiple births in males tended to be higher than expected in March, May and September (with borderline statistical significance), but were 87% less in December (P < 0.05), as compared with January. Overall, the patterns of relative risk estimates from Poisson regression are similar to findings from the non-parametric time-series approach, but are not exactly congruent. It is important to note that indications of seasonality may be sensitive to the selection of time cut-points and therefore an arbitrary binning of time can either mask existing trends or falsely indicate the presence of a trend. The presence of seasonal trends in ASD singletons and concordant multiple births suggests a role for non-heritable factors operating during the pre- or perinatal period, even among cases with a genetic susceptibility.

Variation in Incidence of Neurodevelopmental Disorders With Season of Birth

The etiologies of autism spectrum disorder and many neurodevelopmental disorders are largely unknown. The detection of a seasonal variation of birth of children diagnosed with a certain disorder could suggest etiological factors that follow a seasonal pattern. We examined the seasonal variation of births of children diagnosed with any of 4 common childhood neuropsychiatric disorders: autism spectrum disorder, hyperkinetic disorder, Tourette syndrome, and obsessive-compulsive disorder. The study cohort consisted of all children born in Denmark from 1990 through 1999 identified in the Danish Medical Birth Register (n = 669,995). Outcome data consisted of both inpatient and outpatient diagnoses reported to the Danish National Psychiatric Registry from 1995 through 2004 using the International Classification of Diseases, 10th edition, diagnostic coding system. Logistic regression combined with spline (a smoothing method) was used to estimate the variation with season of birth for each disorder. Estimates of risk of each disorder with season of birth were adjusted for differences in follow-up time and change in incidence over time. No convincing variations in season of birth were observed for any of the 4 disorders, or for the autism-spectrum-disorder subtypes. Although we cannot rule out the possibility of seasonal variation of birth for a range of childhood neurodevelopmental disorders, we find little evidence that seasonal environmental factors are related to these disorders.

Season of Birth in Danish Children with Language Disorder Born in the 1958–1976 Period

Two preliminary studies have indicated a variation in season of birth in severely language-disordered children. In the current study, the season of birth in 472 Danish children with language disorder born between 1958 and 1976 was compared with the season of birth of all Danish live-born children in the same period. For some part of the period (1964–1969), an excess of boys born in November was found. Particular attention was given to the inconsistent findings also found in language-related disorders like infantile autism and dyslexia and the choice of statistical method to determine seasonality.

Seasonality of births is associated with seasonality of marriages in Malta.

This study was carried out to quantify secular trends in seasonal variation in births in Malta, a small Mediterranean country where the vast proportion of births occur in wedlock due to a predominantly Roman Catholic population. It also related such variations to seasonal variation in marriages. Annual seasonal peaks of marriages and births were analysed over the period 1950-1996 by X11 ARIMA. A significant peak in marriages (n = 111,932) in the third quarter of the year was found for almost the entire period under study. This was paralleled by a peak in births (n = 299,558) for the period 1970-1996, which lagged after the peak in marriages by 13-14 months. For the period 1994-1996, when monthly data for monthly pregnancies were available by pregnancy order, the peak in births was caused by first pregnancies only. Seasonal patterns in births occur almost universally due to cultural and/or biometeorological factors. The best known patterns include those of the southern United States, where births decline in April and May, and in northern Europe, where births peak in March and April. In Malta, the late summer peak in births appears to be due to a practical and planned approach by Maltese couples to contraceptive planning, probably influenced by the Roman Catholic ethos and social pressures, with unprotected intercourse occurring only after marriage. In Malta, birth control, albeit by so-called natural methods, was introduced in the 1960s. Prior to this period, births peaked towards the beginning/end of the year, and this may be the more natural seasonality of births in Malta.

The seasonality of live birth is strongly influenced by socio-demographic factors.

The number of births varies markedly by season, but the causes of this variation are not well understood. The proposed explanations include temperature or photoperiod (affecting hormonal concentrations, sperm quality or sexual activity), seasonal variation in pregnancy loss, or cultural factors. In this paper we examined whether birth seasonality is influenced by socio-demographic factors. We used data on all live births registered in the Czech Republic in 1989-1991 (n = 387 496). Differences in the degree of seasonality between socio-demographic groups (defined by maternal age, marital status, education and birth order) were examined by inspection of curves, by comparing coefficients of variations of monthly numbers of births, and by calculating the ratios of the number of births in the 3 peak months (March to May) to the number of births in the 3 lowest months (October to December). We found large differences in the size of the seasonal variation in births by socio-demographic factors. The seasonal variation was highly pronounced in mothers who were 25-34 years old, had higher education, were married, and were pregnant with their second or third child. By contrast, birth seasonality was weak in mothers who were < or =19 years or > or =35 years old, unmarried, had low education, and expected their first or fourth or higher order birth. In a multivariate model, all four socio-demographic variables contributed significantly to seasonal variation. These results suggest that the seasonality of births is, at least in this population, strongly influenced by socio-demographic factors.

Season of birth among patients with schizophrenia and their siblings: evidence for the procreational habits hypothesis.

The birth rate of patients with schizophrenia during the winter and spring months is 5%-8% higher worldwide than the birth rate of the general population in the winter and spring months. Seasonal variation of births among the unaffected siblings of patients with schizophrenia has not been studied with adequate sample sizes. The authors investigated the seasonal variation of births among siblings of patients with schizophrenia in a large, nationwide, representative patient and sibling population. Finnish patients with schizophrenia born from 1950 to 1969 (N=15,389) were identified from three nationwide health care registers. Unaffected siblings of these patients born in the same time period (N=37,819) were identified from the Finnish National Population REGISTER: The seasonal variation of births among patients and siblings were examined by using a log-linear model. Explanatory variables were sex, year of birth categorized into four 5-year groups, and seasonal variation, which was analyzed by fitting a short Fourier series to the monthly birth data. The odds for having been born during the winter-spring months were slightly higher among both siblings and patients in all birth-year groups. However, patients born from 1955 to 1959 showed prominent seasonal variation of births, but the magnitude of this variation remained unchanged among siblings. Seasonal variation of births among patients with schizophrenia may consist of two factors: 1) parental procreational habits causing a slight excess of births of both patients and unaffected siblings during the winter-spring months and 2) irregular environmental factors that considerably increase the magnitude of the seasonal variation of births among patients but not their siblings.

Regional, temporal, and seasonal variations in births and deaths: the effects of famines.

A knowledge of the seasonal variation in births and deaths during normal years is important for analyses of the effects of wars, famines, epidemics or similar privations on these two variables. In studies of seasonality, multiple trigonometric regression models are more flexible than the simple sine curve. The seasonal variation in mortality in Iceland, 1856-1990, shows a strong secular decrease, and a connection between this and the epidemiological transition is considered. As a consequence of the severe famine in Finland in 1867-68, the mortality for the whole year 1868 was almost four times as high as during normal years, and the seasonality of the mortality was even more accentuated. The birth rate in Finland during 1868 was about 70 percent of that during normal years and showed an aberrant seasonality, with a strong trough from October 1868 to February 1869 (fewer conceptions between January and May 1868, when the food shortage was severe).

Seasonal variation of livebirths, stillbirths, extramarital births and twin maternities in Switzerland.

A study was made of the seasonal variation in all births, and births according to marital status, multiplicity and birth status (live and still) in Switzerland recorded between 1876 and 1990. To obtain seasonal variation in as pure as possible form, our analyses are based on rates. When comparing the seasonality in data sets showing markedly different levels, standardised indices were used. Assuming the length of pregnancies with twins to be about one month shorter than for pregnancies with singletons, lagged twinning rates were calculated but, in comparison with actual twinning rates, the general seasonal variation remained. Therefore, this study was based on actual twinning rates. A monotonic increase in the amplitude of the seasonal variation in general births was noted for the period 1876-1930, with strong seasonal variation holding for 1921-1980. After that, a marked decline in the amplitude can be observed. Seasonality of both all births and twin maternities showed very similar pattern for the periods 1876-1930 and 1969-1990, with maxima in the spring (March-May) and troughs in late autumn (October-December). Twin maternities showed a strong seasonality for the period 1876-1930, being about 20% higher in March than in October. The twinning rate in the period 1876-1930 was about 2.6 per thousand units higher than in the period 1969-90. For twin maternities there was also a stronger seasonal variation during the earlier period than during the later one. The pattern of the seasonal variation for extramarital births, showing a maximum in February (conceptions in May-June) and a minimum in August (conceptions in November-December) with a difference of no less than 24% was more marked than for the marital births. It seems likely that this seasonality of extra-marital maternities was due mainly to seasonal variation of coital rates and multiple ovulation in the early summer months coinciding with optima of light, temperature and food supply. A strong reduction in the rate of stillbirths (gestational age more than 29 weeks) was observed during the twentieth century. The stillbirth rate declined from about 40 per 1000 in the 1870s to fewer than 5 per 1000 in the 1980s. Irrespective of this strong decline in the stillbirth rate, the same seasonal rhythm was noticed throughout the period with high stillbirth rates among births around March and low rates during the summer and autumn.

Seasonal variation of births in rural West Bengal: magnitude, direction and correlates.

This paper examines seasonal variation of births in a rural community of West Bengal, India, by exploring data from the 1992-93 National Family Health Survey. Suitable time series analyses were used to determine the seasonal pattern of births and to estimate peaks. The trigonometric regression technique was used to carry out this objective. The study attempted to link the results of the regression analysis to the atmospheric temperature of the region during 1987-91, the distribution of respondents' husbands' occupations and the marriage pattern of the community. It was found that, in the study population, conceptions were numerous in the first quarter of a calendar year and the distribution of conceptions over calendar months was negatively associated with the average monthly temperature. In addition, the marriage pattern of the community and the occupational distribution of the fathers also had a significant effect on the distribution of births over calendar months. It is hoped that the findings will boost the development of needs-based maternal and child health (MCH) and family planning programmes in the community.

Seasonal variation of livebirths, stillbirths, extramarital births and twin maternities in Switzerland

A study was made of the seasonal variation in all births, and births according to marital status, multiplicity and birth status (live and still) in Switzerland recorded between 1876 and 1990. To obtain seasonal variation in as pure as possible form, our analyses are based on rates. When comparing the seasonality in data sets showing markedly different levels, standardised indices were used. Assuming the length of pregnancies with twins to be about one month shorter than for pregnancies with singletons, lagged twinning rates were calculated but, in comparison with actual twinning rates, the general seasonal variation remained. Therefore, this study was based on actual twinning rates. A monotonic increase in the amplitude of the seasonal variation in general births was noted for the period 1876–1930, with strong seasonal variation holding for 1921–1980. After that, a marked decline in the amplitude can be observed. Seasonality of both all births and twin maternities showed very similar pattern for the periods 1876–1930 and 1969–1990, with maxima in the spring (March–May) and troughs in late autumn (October–December). Twin maternities showed a strong seasonality for the period 1876–1930, being about 20% higher in March than in October. The twinning rate in the period 1876–1930 was about 2.6 per thousand units higher than in the period 1969–90. For twin maternities there was also a stronger seasonal variation during the earlier period than during the later one. The pattern of the seasonal variation for extramarital births, showing a maximum in February (conceptions in May–June) and a minimum in August (conceptions in November–December) with a difference of no less than 24% was more marked than for the marital births. It seems likely that this seasonality of extra-marital maternities was due mainly to seasonal variation of coital rates and multiple ovulation in the early summer months coinciding with optima of light, temperature and food supply. A strong reduction in the rate of stillbirths (gestational age more than 29 weeks) was observed during the twentieth century. The stillbirth rate declined from about 40 per 1000 in the 1870s to fewer than 5 per 1000 in the 1980s. Irrespective of this strong decline in the stillbirth rate, the same seasonal rhythm was noticed throughout the period with high stillbirth rates among births around March and low rates during the summer and autumn. Twin Research (2000) 3, 189–201.

Season of birth and bulimia nervosa.

Previous studies suggest season of birth variation in eating disorders akin to those of psychoses. We studied season of birth variation in bulimia nervosa. Season of birth variation in 935 patients was examined after adjustment for population trends. Variation was also examined for subgroups by age and previous anorexia nervosa. Season of birth did not differ significantly from population norms among bulimics (p >.30), contrasting with studies of other eating disorders. With a history of anorexia nervosa (n = 227), peak season of birth was in March (p <.05). This is consistent with previous studies and also with seasonal birth variation for psychoses. Overall, we find no evidence of season of birth variation in bulimia nervosa, and suggest any positive findings be treated with caution. We discuss a number of confounding influences and argue that one explanation remains shared trait vulnerability between anorexia nervosa and psychoses.

Decreasing seasonal variation of births in schizophrenia.

Patients with schizophrenia have a winter-spring excess of births compared with the general population, the cause of which is unresolved. Fluctuations in the magnitude of the seasonal variation may provide clues to its aetiology. All Finnish patients with schizophrenia born between 1950 and 1969 (N = 15892) were identified from two nationwide health-care registers. Their background demographic information was obtained from the Population Register Centre, which also provided monthly numbers of births in each municipality of Finland as multidimensional tables, with sex and year, month and place of birth as marginals. The incidence of schizophrenia was modelled using Poisson regression analysis, with sex, onset age, birth cohort, place of birth (urban/rural), trend and seasonal variation as explanatory variables. We also constructed a monthly time series and decomposed it into three components--seasonal, trend and remainder. Seasonal variation of births among patients born in the 1950s, especially between 1955 and 1959, was marked, but decreased among patients born in the 1960s. No interaction between place of birth or sex and seasonal variation was observed. The incidence was higher among the rural-born than the urban-born, but declined more slowly among the urban-born than the rural-born. The intensity of the factor causing the seasonal variation of births in schizophrenia may be decreasing. Urban birth may be emerging as a risk factor for schizophrenia in Finland, as elsewhere.

Changes in birth-weight of Hospital-delivered neonates in Addis Ababa

A retrospective study was conducted to establish recent birth-weight changes in singleton live births at Tikur Anbessa Teaching Hospital, Addis Ababa, Ethiopia for about 20 years period, September 1976 September 1996. The overall mean birth-weight was 3126g (SD=502). A decline in mean birth-weight was observed from 3162g (95% CI 3137-3186) and 3162g (95% CI 3149-3176) in the 70's and the 80's, respectively, to 3058g (95% CI 3041-3075) in the 90's. The over all proportion of low birth weight was 8.4%, increasing from 5.8% (95% CI 4.6%-7.3%) in the 70's to 7.1% (95% CI 6.4%-7.8%) in the 80's and to 11.3% (95% CI 10.2%-12.3%)in the 90's. When birth-weight was controlled for sex and maternal characteristics, a significant decrease on birth- weight was observed in the 90's, by about 81g (95% CI 44.3 - 118.5g) as compared to the 1970's. The odds of low birth-weight in the 90's was about 52% higher compared to the 70's: (OR=1.52, 95%CI=1.04-2.22). Similarly important changes in the distribution of pre-term delivery, maternal age, parity, sex ratio, and ante-natal follow-up that might have favored the decrease in birth weight were noted between the 90's and the earlier decades. Mean birth-weight was observed to peak in the months of August, September, and October ranging from 3143g to 3173g and fall between March and July. The proportion of low birth weight negatively correlated with the mean birth-weight was highest (10%) in April and observed to fall between August and October, with the lowest proportion of 6.2% in September. However, there was no considerable seasonal variation in birth-weight. In conclusion, the study has shown that mean birth-weight has declined in the 90's as compared to the earlier two decades and the proportion of low birth- weight increased in Addis Ababa, despite a continuing increase in the socioeconomic status. However, there was no considerable seasonal variation in birth- weight. (Ethiopian Journal of Health Development, 2000, 14(2): 169-176)

Seasonal variations in sexual activity and their implications for sexual health promotion.

Although seasonal variations in births are observed in all human populations, the links between calendar events and sexual activity have received little attention in relation to health promotion and service provision. We have plotted various relevant data--routinely collected data for births within and outside of marriage, abortions, sexually transmitted infections, human immunodeficiency virus tests and condom sales figures--by calendar period. The trends point consistently to an increase in sexual activity and unsafe sex occurring at or around the Christmas period, and a longer but less pronounced subsidiary period of increased sexual activity and unsafe sex coinciding with the summer vacation. We conclude that seasonal patterns of sexual activity have implications for provision of sexual health services and for the timing and targeting of sexual health promotional interventions.

Epidemiologic Study of Infants With Biliary Atresia

To the Editor. Yoon et al1 present an epidemiologic study of 57 infants with biliary atresia born over a 25-year period in the Atlanta area. This data was analyzed using seven variables including month of birth, from which a predilection for the winter months December through March was shown.King's College Hospital, London, is a tertiary referral unit for hepatobiliary diseases predominantly but not exclusively from the south of England. We have specialized in the treatment of biliary atresia since the 1970s and our experience has been reported recently both in general2 and specifically with the biliary atresia splenic malformation syndrome (or sequence).3 Since 1973, we have treated 380 infants with biliary atresia (nonsyndromic, n = 346; syndromic, n = 34) and have data on their month of birth. Using Yoon's own criteria for season (April through July, August through November, and December through March), we found neither predilection nor trend for season (χ2 = 0.56, df = 2:P = 0.75) in these two groups. The Figure illustrates our cases by month of birth in the two types of biliary atresia.Although our series is not from a well-defined area, is heterogenous (but predominantly white), and has no data on the population from which the cases arose, it has the irrefutable advantage of being large. We feel that it is too easy in such studies to infer statistical relationships where none actually exist. A statistical test for seasonality was applied to an average of only 4.7 cases per month and this is clearly a major weakness. Indeed, the wide confidence intervals quoted support this interpretation. The evidence for a viral etiology of biliary atresia is contradictory and, it is true, would be advanced if seasonality could be shown. However, this is not the study that has shown it.In Reply. Using data from a hospital-based case series of patients with biliary atresia, Drs Davenport and Dhawan show that there is no seasonal variation in the births of these patients with biliary atresia. However, in order to meaningfully examine seasonal variation, one needs to look at variation in birth prevalence rates because seasonal variation in births has been shown to exist in most populations.1-1 Drs Davenport and Dhawan may find that because seasonal variation is likely in the population (denominator), the lack of seasonality in the births of their case infants (the numerator) will result in prevalence rates for biliary atresia that are seasonal. In our study,1-2 we used the Walter and Elwood test for seasonality of events with a variable population at risk1-3 to show that there was seasonal variation in the birth rates of infants in metropolitan Atlanta with biliary atresia. To determine the magnitude of the seasonal variation and whether the difference we found was significant, we divided the year into three seasons and used these in a multivariate logistic regression model. Drs Davenport and Dhawan suggest that because the number of cases we studied was small and our confidence limits were wide, we should not infer a statistical relationship between the occurrence of biliary atresia and season. However, we would like to point out that confidence intervals account for the statistical uncertainty, which is related to sample size and any result with a confidence interval that excludes 1.0 is by definition statistically significant.1-4 Our study, which clearly showed that the occurrence of biliary atresia in metropolitan Atlanta was 2.6 times greater from December through March than from April through July, had a 95% confidence interval of 1.2 to 5.9 even when we controlled for other covariates.We would also like to point out that in addition to geographic and climatic variation, the metropolitan Atlanta population differs considerably from the British population with respect to race and other socioeconomic factors. All of these factors could affect the susceptibility of fetuses and infants to biliary atresia or to an environmental agent that might precipitate biliary atresia. Although much can be said for large sample sizes, population-based studies such as we conducted have a number of advantages over hospital-based studies, particularly when it comes to using study estimates to make inferences about the population at large. In summary, although the data presented by Drs Davenport and Dhawan are interesting, we believe the findings from our study support further investigation of the association between biliary atresia and environmental agents such as viruses.

Seasonality of births in Homo sapiens in pre‐industrial Finland: maximisation of offspring survivorship?

AbstractWe examined the seasonal variation in human birth and infant survival rates in pre‐modern Finland. If survival probabilities of children born during different seasons of the year differ and if timing of reproduction has been affected by natural selection, periodic variation in environment could have led to reproduction during the season of best infant survival expectations. Significant seasonal variation in both birth rate and survival probability was found, but the monthly birth and survival rates of newborn were uncorrelated. Hence, if there was any tendency to maximise the reproductive success, increase in some other component of fitness than the infant survival was probably targeted. The effect of major holidays on the birth rate was proved to be notable, suggesting that although the basis for seasonal variation in birth rate was biological, sociocultural factors had an impact on the timing of reproduction in humans. The overall mortality of infant boys exceeded that of infant girls in all seasons. This difference was smallest during the time of best food supplies, indicating that the development of males was less buffered against environmental disturbances than that of females.

Seasonal variation in human reproduction: environmental factors.

Almost all human populations exhibit seasonal variation in births, owing mostly to seasonal variation in the frequency of conception. This review focuses on the degree to which environmental factors like nutrition, temperature and photoperiod contribute to these seasonal patterns by acting directly on the reproductive axis. The reproductive strategy of humans is basically that of the apes: Humans have the capacity to reproduce continuously, albeit slowly, unless inhibited by environmental influences. Two, and perhaps three, environmental factors probably act routinely as seasonal inhibitors in some human populations. First, it seems likely that ovulation is regulated seasonally in populations experiencing seasonal variation in food availability. More specifically, it seems likely that inadequate food intake or the increased energy expenditure required to obtain food, or both, can delay menarche, suppress the frequency of ovulation in the nonlactating adult, and prolong lactational amenorrhea in these populations on a seasonal basis. This action is most easily seen in tropical subsistence societies where food availability often varies greatly owing to seasonal variation in rainfall; hence births in these populations often correlate with rainfall. Second, it seems likely that seasonally high temperatures suppress spermatogenesis enough to influence the incidence of fertilization in hotter latitudes, but possibly only in males wearing clothing that diminishes scrotal cooling. Since most of our knowledge about this phenomenon comes from temperate latitudes, the sensitivity of spermatogenesis in both human and nonhuman primates to heat in the tropics needs further study. It is quite possible that high temperatures suppress ovulation and early embryo survival seasonally in some of these same populations. Since we know less than desired about the effect of heat stress on ovulation and early pregnancy in nonhuman mammals, and nothing at all about it in humans or any of the other primates, this is an important area for future research. Third, correlational data suggest that there may be some degree of regulation of reproduction by photoperiod in humans at middle to higher latitudes. Populations at these latitudes often show a peak in presumed conceptions associated with the vernal equinox. On the other hand, evidence gathered by neuroendocrinologists tends to argue against reproductive photoresponsiveness in humans.

Aetiology of Schizophrenia

For most of this century the causes of schizophrenia have been largely unknown. However, the application of new techniques in genetics, neuroimaging, psychopharmacology and epidemiology are beginning to disclose the aetiology of the disorder. It is hoped that elucidation of the causes of schizophrenia will lead to new and more effective treatments. Family and twin studies indicate that genetic factors are important, although single major genes for susceptibility have not yet been identified. If the genes are determined, gene therapy may become a viable option for the treatment of schizophrenia. Environmental factors are also important and may explain the seasonal variation in births of individuals with schizophrenia and regional variations in the incidence of the disorder. The contributions of obstetric complications and maternal influenza virus infection to schizophrenia are currently being evaluated. There are observed differences in the presentation of schizophrenia between males and females, and these need to be accounted for by aetiological theories. Several observations support the theory that schizophrenia results from processes occurring during neurodevelopment. These processes may be responsible for structural and functional abnormalities that have been detected in the brains of patients with schizophrenia. Abnormalities in the temporal lobe have been related to hallucinations, delusions and thought disorder, while abnormalities in the frontal lobe have been associated with negative symptoms. Observed differences between schizophrenic patients and healthy individuals in the size of the corpus callosum and subcortical structures may also have neurodevelopmental origins. At the neurochemical level, dysfunction of central dopamine systems may mediate symptoms of schizophrenia. Traditional antipsychotics are antagonists of dopamine D2-receptors, but some of the new atypical antipsychotics appear to have a different action and drugs that interact with newly discovered receptor subtypes are under investigation. Schizophrenia has significant psychosocial consequences for the patient, and psychosocial factors such as adverse life events and high expressed emotion in the family have been observed to increase the likelihood of relapses of the disorder.