Seasonal Variation by Race in the Male-to-Female Ratio at Birth in the United States

ABSTRACT Objective: In humans, males' births exceed females' births. This ratio is conventionally referred to as M/F and is used to denote male births divided by total births. This ratio is influenced by a large number of factors and has been shown to exhibit seasonality. This study was carried out in order to ascertain whether seasonal variation in M/F exists in the United States of America and whether such variations are influenced by race. Materials and Methods: Data on births by gender and race from 2003 to 2013 were obtained from Centres for Disease Control and Prevention Wonder section as four races: White, Black/African American, Asian/Pacific Islander and American Indian/Alaska Native. ANOVA and ARIMA tests were carried out. Results: This study analysed 45 103 146 live births (M/F 0.51182) over 2003 to 2013. M/F was highest in Asian/Pacific Islander (p < < 0.0001), followed by White (p = 0.002), American Indian/Alaska Native (p = 0.04) and Black/African American. Significant seasonality was present overall, with a peak in June, for Whites more than Black/African American. Conclusion: Parental stress lowers M/F, and lower M/F found in Black/African and American Indian/Alaskan births may be stress related. The dampened seasonality noted in Black/African American births may also be due to this phenomenon. More males were born in spring, as in other species, with interesting inter-racial differences.

Population Stress, Civil Unrest and the Male to Female Ratio at Birth in Chile, Argentina, Australia and Finland.

Introduction: Male live births occur slightly in excess of female births but the ratio is influenced by a very large variety of factors. This ratio is remarkably stable but may exhibit sudden shifts in response to influences such as population stress. A World Health Organisation dataset comprising annual live births by year and country was analysed not only to pinpoint outlier values but also to attempt to identify historical causes for such outliers. Methods: The WHO HFA (Health for All) mortality database was used to identify outlier M/F values. Data was analysed using chi tests and chi tests for trend. Comparison for years of interest were made against baselines of only up to five years before and after such years since M/F exhibits potentially confounding long-term secular variations. Results:  Chile: There was a significant difference in M/F between the period 1967-1971 and 1972 (p<0.0001). There was a decline over 1972-1976 (p<0.0001).  Argentina: Increases were noted in 1978, 1988 and 1994 (p<0.0001).  Australia: M/F rose sharply in 1956 (p=0.0008).  Finland: There were increases in 1962 and 1971 (p<0.0001). Discussion: Stress has been shown to affect M/F. Almost all of the observed outliers were temporally associated with political events or political decisions that influenced the respective countries’ economies. M/F ratios worldwide provide tantalising hints that even transient stress may influence this ratio, making it a sensitive indicator of overall population stress.

Economic Stress and the Secondary Sex Ratio: Ireland, Iceland, Greece and Latvia.

Introduction: Male live births occur slightly in excess of female live births at a ratio of approximately 0.515 (male/total births). Stress has been shown to reduce M/F, including stress engendered by contracting economies. This study was carried out in order to ascertain whether the economic depression caused by the Eurozone recession at the end of the previous decade influenced M/F in the European countries most heavily affected, namely Iceland, Ireland, Greece and Latvia. Methods: Annual data on male and female live births were obtained directly from the World Health Organisation except for Latvia which was obtained from Eurostat. Quarterly data for Ireland was obtained from the Irish Central Statistics Office. Results: There were no significant changes in M/F except in Ireland which showed a sharp and highly significant dip in 2007 due to a fall in M/F in the last quarter only (p<0.0001). Discussion: Darwinian evolution should encourage species to adapt to changing circumstances by altering the odds of having a child of a specific gender. Parents without stress and in good condition should produce sons since these are theoretically capable of producing more offspring, and viceversa. The findings for Ireland support the contention that economic stress is also capable of depressing M/F.

The Japanese Decline in Secondary Sex Ratio Correlates with Percentage Change in GDP/Annum.

Background: Male births occur in excess of female births. The ratio of male:female births is commonly referred to as M/F and is expected to approximate 0.515. Stress has been shown to decrease M/F due to an increased rate of male spontaneous abortions. Since Japan’s economy declined after the mid-1970s, this study was carried out in order to ascertain whether there was any relationship between M/F and annual change in percentage gross domestic product (GDP) in Japan. Methods: Annual male and female live births were obtained from a World Health Organisation Mortality database. GDP data was downloaded as an Excel sheet from the website of the World Bank. Data for both variables was available for the period 1961-2009. Results: This study analysed 71878631 total live births. There was an overall and abrupt decline in M/F which commenced in 1975 (0.5149, from 0.5155 in 1974), one year after the decline in percentage annual GDP growth which fell from 8.0% to -1.22 from 1973 to 1974. M/F correlated with percentage annual GDP growth (r=0.4, p=0.005). Conclusion: M/F is increasing overall in Asia, but decreasing in Japan, as is the trend in developing countries. This country exhibits a rise in GDP change along with M/F in boom years, followed by a progressive fall in both values, which occurs almost in parallel, especially after 1980. To the author’s knowledge, this is the first time that M/F has been linked with percentage annual GDP.

An Analysis of Seasonal Variation and Climatic Influences on the Male to Female Ratio of Live Births in Malta.

Aims: In humans, male births occur in excess of female births. The ratio of male births to total births (MFR) is expected to approximate 0.515. Many factors have been shown to influence MRF and a seasonal pattern has also been described. This study was carried out in order to ascertain whether seasonal variation in MRF exists in Maltese live births and whether environmental influences exert any role. Methods: Monthly live births subdivided by gender, were obtained from official Maltese government publications for the period 1958-2013. Analysis was carried out with SPSS, the Bio- Med-Stat Excel add-in for contingency tables and Demetra, using chi square tests, ANOVA, Freidman and Kruskall-Wallis tests, correlation, regression, and ARIMA models. Mean annual temperatures were available to 1958. Monthly data for maximum and minimum monthly temperatures, relative mean humidity, lowest relative humidity, hours of bright sunshine and rainfall was only available for 2001 to 2012. Results: This study analysed 297254 live births (153652 males and 143602 females, MRF 0.5169, 95% CI 0.5151-0.5187). MRF exhibited a non-significant bimodal pattern (June and December peak). MRF displayed a significant negative correlation between MRF and average mean yearly temperature (p=0.049). There was an association between monthly mean relative humidity and MRF (p = 0.03). Discussion: The nonsignificant seasonal pattern seen accords with that previously documented for other countries but the negative MRF correlation with mean annual temperature goes contrary to that previously documented in other countries. The humidity association has not been previously described. Conclusion: Some seasonal variation may exist in MFR in Malta.

The Male: Female Ratio at Birth in Malta is Decreased by the Soccer World Cup.

Introduction: The ratio of male to female live birth is defined as male births divided by total births (M/T) and is expected to approximate 0.515. This study was carried out in order to ascertain whether M/T in Malta was in any way influenced by the World Cup. Methods: Index months were months in which the World Cup was held (always in June), from 1958 to 2010 (n=14). Live births were analysed for January before the event to nine months after (the following March). Results: This study analysed 297254 live births. The decline from June to February was highly significant (p=0.001). A dip in M/T was noted in the February after the World Cup and this significantly lower when compared to the previous January to January period (p=0.001). M/T in these Februaries was also significantly lower than that in the other Februaries in the rest of the period studied (p=0.0023). Discussion: The majority of fans side with Italian or English national teams for old colonial political reasons. Stress leading up to and following the World Cup may have reduced M/T in Malta, with a rapid recovery when the effect passed.

Conflicts in the Last Fifty Years and Subsequent Effects on the Male: Female Ratio at Birth.

Introduction: Male births occur slightly in excess of female births. In man, the ratio of male divided by total live births (M/F) is expected to approximate 0.515. Warfare has been shown to exert significant effects on M/F. This study was carried out in order to identify any such effects on M/F in belligerent countries in recent conflicts. Methods: Births were obtained from a World Health Organisation Mortality database. Recent wars (post Second World War) were identified. Chi-square tests were used throughout. Male and female births in belligerent countries were compared to the aggregate of the previous and following years. Countries included were those with available data for periods spanning identified wars. Results: This study analysed 260747284 live births. M/F in the United States increased significantly during the Korean (p=0.011) and Vietnam (p=0.011) conflicts but decreased during the Gulf War (p=0.02) and there were no changes during the invasion of Iraq. There was no significant shift in M/F in Greece during the invasion of Cyprus, in England and Argentina in the Falkland war and in the Balkans during the Yugoslavian wars. The South Ossetia and Abkhazia Wars were associated in rises in M/F in the Southern Caucasus (Armenia, Azerbaijan, Georgia) only after the wars, and then serially to the end of the 20th century (probably due to sex-selective terminations of pregnancy in favour of male births). The Portuguese Colonial War lowered Portugal’s M/F during the last year of conflict (1974, p=0.0001). Discussion: This study confirms the observation that M/F rises during lengthy periods of warfare, and may fluctuate even during short periods of conflict. The former has been attributed to increased coital rates which increase conception rates early in the menstrual cycle, skewing M/F in favour of male conceptions. The latter may be due to the known M/F lowering effect of stress.

Tendências seculares das proporções de nascimentos entre gêneros na América do Sul durante a segunda metade do século XX / Secular trends in sex ratios at birth in South America over the second half of the 20th century

OBJETIVOS: Analisar as tendências seculares e gradientes de latitude na proporção masculino-feminina ao nascimento (M/F: masculinos divididos por total de nascidos vivos) no Continente Sul Americano. MÉTODOS: Os dados sobre nascidos vivos masculinos e femininos em âmbito nacional, durante um ano por país, foram obtidos da Organização Mundial da Saúde e analisados com tabelas de contingência. O continente foi arbitrariamente dividido em duas regiões: a primeira: de 10º acima da linha do Equador até 20º abaixo da linha Equador (10º N-20º S), e uma segunda abaixo de 20º da linha do Equador (> 20ºS). RESULTADOS: O estudo analisou 147.773.689 nascidos vivos. Encontrou-se uma tendência crescente M/F na maioria dos países próximos ao Equador (10º N-20º S) (p = 0,004). A análise do período entre 1950 e 1974 não mostrou diferença, porém após este período (1974-1996) encontrou-se um aumento M/F (p = 0,03). Um aumento na proporção M/F foi encontrado em todo o período na região > 20ºS (p < 0,0001). Também foi observado um gradiente de latitude, nascendo mais lactentes masculinos em latitudes mais frias (mais ao sul, > 20ºS) (p < 0,0001). CONCLUSÃO: A proporção M/F está crescendo na América do Sul, diferentemente do presente declínio na Europa e na América do Norte. Este estudo também mostra que os gradientes de latitude para a proporção M/F são semelhantes àqueles anteriormente publicados na América do Norte, nascendo mais bebês masculinos nas latitudes mais frias, ao contrário das tendências publicadas na Europa. É provável o inter-relacionamento de vários fatores ainda pouco entendidos.

OBJECTIVES: Latitude gradients have been found in the male-female ratio at birth (M/F: male divided by total births), which is anticipated to be 0.515. METHODS: Annual national male and female live births by country were obtained from the World Health Organization (WHO) and analysed with contingency tables. The continent was arbitrarily divided into two regions: a region 10º above the Equator to 20º below the Equator, and a second area 20º below the Equator. RESULTS: This study analyzed 147,773,689 live births. An overall increasing trend in M/F was found for the region >20º (p < 0.0001) for the entire period. For the aggregate, a significant decrease was present for the period 1950-74 (p = 0.01) followed by a significant increase thereafter (p < 0.001). A latitude gradient was also noted, with more males being born in cooler (more Southern > 20º S) latitudes (p < 0.0001). There were 3,765,648 male births in excess of what was anticipated. CONCLUSION: The M/F ratio is increasing in South America, unlike the decline present in Europe and North America. This study also showed that M/F latitude gradients are similar to those previously reported in North America, with more males being born in cooler latitudes, contrary to the trends reported in Europe. The interplay of several poorly understood factors is likely.