The observation that more boys were born than girls was noted in the late 1660’s. Recent studies suggest the secondary sex ratio (SSR) is declining in industrialized countries. SSR is proposed as a sentinel for reproductive health. Declining SSR may reflect environmental factors or other influences of reproductive outcomes. We evaluated maternal age, birth order and SSR in the Old Order Amish (OOA), a homogenous sub-group with large family sizes. We used data from the Anabaptist Genealogy Database consisting of records for live births from 1696-2003. We used t-tests to compare mean maternal age and birth order by offspring sex, ANOVA to evaluate whether SSR has changed over time, and logistic regression for multivariable models. We evaluated clustering of SSR within families using random effects models and likelihood ratio tests of random effects. Maternal age was not associated with SSR (OR=1.003 [95% CI, 0.995-1.010), even after adjusting for birth order (AOR=1.000 [95% CI, 0.989-1.012). Similarly, we did not find an association between birth order and SSR in both unadjusted models (OR=1.007 [95% CI, 0.991-1.022), and those adjusted for maternal age (AOR= 1.006 [95% CI, 0.982-1.032]). The proportion of male births varied, however, there was no significant trend overtime. Lastly, we found a significant random effect (P<0.05), which may provide indication that having male births is heritable in families. Conclusions: Neither maternal age nor birth order is associated with the sex of an offspring. These findings suggest that decreases in SSR are unrelated to demographic factors, and rather may be related to other factors such as environmental exposures or other xenobiotic chemicals. These results may be relevant in providing information to the leading indicators to the decline in SSR.

Background: The human secondary sex ratio has been the subject of long-standing medical, environmental and social scientific curiosity and research. A decline in male birth proportion in some industrialised countries is linked to endocrine disruption and is validated by some empirical studies. Increasing parental age and population stress and associated decreases in sex ratio have also been demonstrated. A thorough literature review of 123 relevant and diverse studies provides context for these assessments. Methods: A spatial-temporal investigation of birth sex ratio in Scotland and potential determinants of endocrine disruptor pollution, socio-economic factors including neighbourhood stress, deprivation, smoking, and maternal age, was conducted. This involved review of national and regional sex ratio time trends, and stratified/spatial analysis of such factors, including the use of GIS tools. Secondary data were sourced from Scottish Government web portals including Scottish Neighbourhood Statistics and the Scottish Environmental Protection Agency. Results: Regional differences in sex ratio between 1973 and 2010 are observed which likely lever the national male birth proportion downwards, with the region of poorest air quality from industrial emissions, the Forth Valley, displaying the greatest sex ratio reduction. Further analysis shows significant upwards skewing in sex ratio for the population cohort experiencing the least and 2nd most deprivation. Localised reductions in sex ratio for areas of high modelled endocrine disruptor pollution within the Central Region in Scotland are also displayed. Discussion: Limitations of the analyses include the danger of ecological fallacy in interpreting from area-based measurement and the simplified pollution modelling adopted. Despite this, and given elevated incidence of testicular cancer in Scottish regions mirrors the study’s results, tentative confirmation of the endocrine disruptor hypothesis can be substantiated. Further, elucidation on advanced parental age as a contributory factor to secondary sex ratio change is also given. Recommendations are made with respect to environmental monitoring and health protection, and preventative health strategies in Scotland.

The thesis is mainly focused on the analysis and evaluation of the effects influencing the sex ratio of live births. After studying of the foreign language literature, main factors influencing the sex ratio are defined. Data used in the thesis (births by the sex) are retrieved from The Human Mortality Database, database of United Nations Statistics Division and from particular statistical offices. Based on the results of the exploratory data analysis, countries with unreliable data (data skewed due to the manipulation or due to the not appropriate standard of the registration of born) are specified. Subsequently, the countries with abnormal values of the sex ratio at birth resulting from the cultural traditions or technological development are defined. For countries with not evident influence of the factors mentioned the analysis of the relations and dependencies between sex ratio at birth and variables chosen (groups of the geographical, socioeconomics and demographical impacts). On the basis of this analysis it has been proved that the sex ratio at birth depends primarily, but not exclusively, on the total fertility rate and life expectancy.