Introduction

Males are almost invariably born in excess of females (James, Reference James1987), and the ratio of male-to-female live births is often (albeit technically incorrectly) expressed as the ratio of male to total births. This convention will be used in this paper. A wide variety of factors have been shown to influence the sex ratio at birth (SR), and these include secular and geographical variation (Pavic, Reference Pavic2015), as well as seasonal trends (Melnikov, Reference Melnikov2015). However, toxins, including exogenous chemicals (James, Reference James1987), as well as endogenous hormones produced by stress, invariably lower the sex ratio at birth (James, Reference James2004). This accords with the Trivers–Willard hypothesis, which maintains that evolution should have favoured parents with the ability to influence SR according to conditions surrounding conception and during pregnancy. This is because in polygynous species, a robust son who is conceived under favourable environmental conditions has greater reproductive opportunities than an equivalent daughter who is constrained by pregnancy and lactation. On the other hand, under adverse conditions, a male fetus (which requires greater resources in order to be carried to term) will be less likely to survive pregnancy. If it does, such a frail male may not survive to reproductive age, and if so, would compete poorly with more robust males for mating privileges. However, a frail female is more likely to survive and reproduce. Hence, under unfavourable conditions, the parental passage of genes is favoured if fewer males are produced through the culling of weaker males (Trivers & Willard, Reference Trivers and Willard1973).

Terrorist attacks have been shown to reduce the sex ratio at birth, presumably due to the above-mentioned process. This was famously noted in relation to the September 11 attacks, after which fewer males were born, not only in New York (Catalano et al., Reference Catalano, Bruckner, Marks and Eskenazi2006), but also across the country in California (Catalano et al., Reference Catalano, Bruckner, Gould, Eskenazi and Anderson2005). This event generated significant population stress (Schlenger et al., Reference Schlenger, Caddell, Ebert, Jordan, Rourke and Wilson2002), and it has been proposed that the SR dip was stress induced (Catalano et al., Reference Catalano, Bruckner, Marks and Eskenazi2006). The mechanism was shown to be that of excess male fetal loss, with a decline in male births three months after September 11 (Bruckner et al., Reference Bruckner, Catalano and Ahern2010) and with different ethnic responses (Grech, Reference Grech2015a).

Other stressful events have been shown to cause a transient dip in the sex ratio at birth. For example, in Northern Ireland during the sectorial violence colloquially known as ‘The Troubles’ (1969–1998), SR was lower than during the period beforehand (p=0.0006). There was also a very sharp dip in 1978 (p≤0.004; Table 1) during this particular year of renewed violence and heavy civilian attacks (Grech, Reference Grech2015b). Similarly, the 1992 Los Angeles Riots (also known as the Rodney King Riots) constituted six days of widespread civil rioting following the acquittal of white police officers who had been videotaped brutally attacking Rodney King, an African-American man, after a high-speed pursuit. Video footage clearly showed King being beaten by police officers while lying on the ground. In Los Angeles, SR dipped significantly in August 1992, four months after the riots (p=0.044; Table 1) (Grech, Reference Grech2015c). An analogous result occurred in Norway following the terrorist attacks of Breivik, a Norwegian right-wing extremist who exploded a car bomb in the executive government quarter of Norway. This killed eight and injured another 209, twelve seriously. The assailant moved to Utøya Island, 30 km north of Oslo, and in a shooting spree killed 69 people with an additional 110 injuries, 55 seriously so. In Norway, SR dipped significantly in December 2011, five months after the affair (p=0.004; Table 1) (Grech, Reference Grech2015c). The Sandy Hook Shooting in Connecticut occurred at the Sandy Hook Elementary School after a lone gunman killed his mother, and then 26 people at the school, including 20 first-grade children aged 6–7 years. The SR dipped significantly in Connecticut in April 2013, four months after the event (p=0.009; Table 1) (Grech, Reference Grech2015c).

Table 1

Summarized results of sex ratio at birth of previous studies for stressful events

^a Northern Ireland 1969–1998; source: WHO, N. Ireland Statistics & Research Agency.

^b California, US, late April 1992; source: CDC Wonder, CDC Vitalstats.

^c Norway, 22^nd July 2011; source: Befolkningsstatistikk, Statistisk Sentralbyrå.

^d Connecticut, US, 14^th December 2012; source: CDC Wonder, CDC Vitalstats.

Modified from Tables 1 and 2 of Grech (Reference Grech2015c). Data not seasonally adjusted.

The year 1968 was a momentous one for France. Unrest began in January due to student protests, who were then supported and joined by workers. Universities were closed, many hundreds were arrested and several marches and huge demonstrations were violently quelled by the police. The Mouvement du 22 Mars was a French student movement founded at the University of Nanterre, which carried out a prolonged occupation of the university’s administration building. The police were called in and a public scuffle ensued, which garnered national media attention. Cars were burned by protestors, and there was also significant other property damage. Large sections of Paris temporarily closed, with an eventual ban on public demonstrations that was ignored, leading to violent confrontations between the protesters and the police. The violence continued to escalate with barricades being erected by the protestors, and clashes between the population and the police culminating in the launching of Molotov cocktails by the former and tear gas by the latter. The events were broadcast on radio and television. The Parti Communiste Français (PCF) participated with the major union federations, resulting in a one-day general strike. On the 13th May, over a million people marched through Paris and the Sorbonne was occupied by students. Several other worker strikes occurred, the National Theatre in Paris was seized by demonstrators and several factories were occupied by workers, such that by May ten million workers (two-thirds of the entire French workforce) were on strike. The Paris Stock Exchange was also set on fire by protestors. The crisis eventually ended in June and while the insurgents and campaigners were not frank terrorists, the events precipitated led to very similar results (Prialux & Ungar, Reference Priaulx and Ungar1969).

Aum Shinrikyo is the former name of a Japanese religious cult that is now known as ‘Aleph’. In 1992, the cult’s founder (Shoko Asahara) declared himself Japan’s only enlightened master and outlined a doomsday prophecy that included a global nuclear Third World War. On the 20^th March 1995 in Tokyo, the world’s largest city, members of the cult released sarin (isopropyl methylphosphonoflouridate) in a terrorist attack on the civilian populace. Sarin is an organophosphorus ester that produces potent and irreversible inhibition of cholinesterase. This chemical is toxic to the nervous system and is considered a chemical warfare agent. The toxin was released during the rush hour in five simultaneous co-ordinated attacks on five different trains, many kilometres apart, converging on the centre of Tokyo at the Kasumigaseki station of the subway system. Kasumegaseki is home to most of Tokyo’s government offices and is considered to be the city’s power and nerve centre. By the end of the day, fifteen stations in the world’s busiest subway system had been affected. The attack was devastating, killing twelve people, severely injuring 50 and causing temporary vision problems for nearly 1000 others. On this day, ambulances transported 688 patients to hospitals and nearly 5000 more reached hospitals by other means. In total 5510 patients were seen in hospitals, of which seventeen were critical, 37 severely and 984 moderately ill with vision problems. This remains the most serious attack to occur in Japan since the end of the Second World War (Olson, Reference Olson1999).

This study was carried out to assess the impact of the 1968 French riots and the Aum Shinrikyo sarin Tokyo subway attack on the sex ratio at birth in France and Japan, respectively. Based on the above-mentioned studies, any impact on SR from the events described in France and Japan would be expected to manifest as a dip in the ratio some 3–5 months after the events, and definitely >1 month and<9 months afterwards.