The human sex ratio at birth is defined as male divided by total births and has been a topic of study for centuries. In humans, male live births slightly exceed females, such that for every 1000 live births there are 515 males and 485 female births (M/F 0.515) as shown in a global analysis (Grech & Mamo, 2020). This ratio (M/F) may be influenced by many factors and a PRISMA search yielded these avenues of study: sex-selective termination, stress, geographical and secular trends and seasonality, coital rates, radiation, and theoretical modelling. This narrative focuses on the effect of acute and chronic stress on M/F (West & Grech, 2020).
The literature indicates that stress applied periconceptually or in early pregnancy culls males. The ultimate cause has been proposed to be the Trivers-Willard hypothesis which suggests that a male who reaches reproductive age in fit mating condition is expected to out-reproduce a sister in similar condition. Conversely, if both are in poor condition, a female is expected to out- reproduce a brother. This is because a weak son would compete poorly with stronger males for mating rights, thus producing no or fewer offspring than a weak female daughter would. This hypothesis implies that natural selection has developed mechanisms by which pregnant females subjected to environmental stressors manipulate M/F by culling male fetuses that are least likely to eventually sire grandchildren. Males are specifically selected for spontaneous termination as a live born male in poor condition is likelier to die before reaching reproductive age than a female in similar condition, despite receiving a greater maternal investment (Trivers & Willard, 1973).
1.2 Acute Stress
Acute stress may transiently decrease M/F. Stress-related acute declines in M/F were linked to several terrorist attacks with a transient drop in M/F including The Troubles in Northern Ireland, the Rodney King riots in Los Angeles in 2012, the Breivik shootings in Oslo in 2011 and the Sandy Hook school shootings in Connecticut in 2012.(Grech, 2015c) These effects are non-trivial. For example, the perinatal mortality rate for the US for 2017 was 5.9/1000 while the Breivik shooting in Oslo led to a transient M/F dip in Norway that corresponded to an equivalent perinatal mortality of 26.4/1000. A meta-analysis of the effect of terrorist attacks on M/F confirmed this effect (Masukume et al., 2017).
A very wide spectrum of stressors have also been associated with transient declines in M/F: sovereignty referendums in Canada (Grech, 2013a, 2014a), riots in France, terrorist attacks in Japan, parliamentary elections in Malta, the assassination of President J. F. Kennedy (Grech, 2015a), the 2007 recession in the U.S.(Grech, 2015d), the death of Princess Diana in 1997 in the UK (Grech, 2015h), famine during China’s Great Leap Forward (Song, 2012), the Eyjafjallajokull volcanic eruption in Iceland in 2010 (Grech & Borg, 2016), and American legislation facilitating immigration from Cuba to the United States so as to allow Cuban citizens to escape from their country (Grech, 2014b). These stress-related M/F dips accord with the Trivers-Willard hypothesis (TWH) (Trivers & Willard, 1973).
It must however be noted that the effects of stress may not be universally generalisable/applicable, and that this is hampered by unavailability of month M/F data. This is best illustrated with a study showed that showed that the assassination of President John F. Kennedy in November 1963 was associated with a significant dip in M/F in the United States for 1964 due to a dip confined to March 1964. No dip was present for 1964 for any country, nor for the amalgamation of European countries, for the Far East, the North American continent, or for the amalgamation of the total. However,data for Malta showed a significant decline in March 1964 (M/F 0.466, p=0.0175), equivalent to a loss of 56 boys (estimated at 6.3/1000 births). The Malta March M/F dip transiently exceeded the expected rate for perinatal mortality, making terrorist attacks potentially important Public Health issues (Grech & Zammit, 2016).
1.3 Chronic stress
In the US, M/F was shown to demonstrate a decreasing trend along the subpopulations Asian/Pacific Islander>White>American Indian/Alaska Native>Black/African American. Race remains the most important determinant of socioeconomic status in the US and lower socioeconomic status is associated with lower M/F, a possible case for the lower baseline M/F of Indian/Alaska Native and Black/African American groups. This is equivalent to a constant loss of 3.5-4/1000 male births when compared to White M/F. Furthermore, a temporal M/F review of a United Nations dataset showed that Least Developed countries exhibit a stable and lowest M/F, possibly due to chronic stress.(Grech, 2018a)
Several metrics indicate individual countries' health and socioeconomic status and in a global overview, all except for the Human Development Index (HDI) correlated with M/F at statistically significant levels.(Grech & Calleja) This was confirmed by a meta-analysis. M/F may therefore serve as a surrogate sentinel health indicator, one that is relatively cheap since this data is automatically collected by national statistics agencies as male and female births.
Evidence is accumulating that observed differences in M/F between populations may be due to socio-economic stress in that this type of chronic stress may be the reason for a lower birth sex ratio at racial, national and global levels. While innate periconceptual and gestational (e.g. hormonal) racial differences have been cited as possible causes for these M/F differences, this may be a mechanism/pathway and not the actual cause for lower M/F. Stress may act by using hormonal pathways to effect the observed M/F disparities, with lower M/F in socially deprived and chronically stressed populations.
Acute events may transiently decrease M/F, or chronically depress M/F in situations of prolonged stress. More research could potentially be done in this field to explicate M/F drops in response to stress but this would only be possible with the availability of monthly (and not just annual) numbers of male and female births in entire populations as well as monthly foetal deaths by sex. Data analysis is complicated by the knowledge that foetal deaths are underreported. M/F indeed remains a fascinating field of study.
|Date of Award||28 Jul 2023|
|Supervisor||Sasee Pallikadavath (Supervisor)|