Sporting tournaments and changed birth rates 9 months later: a systematic review

Search strategy

A search strategy and search terms were developed for this study. Without regard to language, a systematic literature search was conducted. Using the search strategy (Table 1), potentially relevant studies were looked up from database inception through to 7 November 2022 in the electronic databases PubMed and Scopus. The first author carried out all searches. The following search term syntax was used:

PubMed: “Sports”[Mesh] AND (“Birth Rate”[Mesh] OR “Sex ratio”[MESH] OR “Fertility”[MESH])

Scopus: KEY (“sports”) AND KEY (“birth”) OR KEY (“birth rate”) OR KEY (“fertility”)

Using Covidence systematic review software (www.covidence.org), we completed the title/abstract and full-text screening for this study. This was performed independently by two authors (GM and MR). The reference lists of the studies that were included in the search were manually searched as an add-on. The ‘cited by’ function of Google Scholar was also used to find articles citing the included studies.

Eligibility

Table 2 lists the inclusion and exclusion criteria that were employed in this study.

Data extraction and analysis

Data were extracted from the included studies by two reviewers (GM and MR). Author name and year, country or region of the tournament, sport and change in birth pattern (total births/SRB) were extracted. Extraction differences were discussed by these two authors and were resolved. Due to the variety of sports, eras and geographic locations, a narrative synthesis was done in accordance with the Synthesis Without Meta-analysis (SWiM) guidelines (Campbell et al., 2020).

Quality assessment

As far as we are aware, there is no instrument that has been validated for use in evaluating the quality of ecological studies (Betran et al., 2015). However, the group exposure (sporting tournament) was assessed according to how the population that experienced the tournament defined it; in other words, there was common understanding that the sporting tournament was significant, and the studies characterized it as such. There was minimal concern about outcome bias because the human live birth statistics of the entire population (and not just a sample) was available and the danger of misclassifying sex at birth is low (Davis, Gottlieb & Stampnitzky, 1998).

Principle findings

In this study, we found that five major sporting tournaments were linked to noticeably increased birth metrics 9(±1) months later. The Super Bowl (increased US birth sex ratio in multiple years (Grech & Zammit, 2019), however, from 2004 to 2013 there were no observable birth increases in winning counties and losing was not associated with a changed number of births (Hayward & Rybińska, 2017)), the 2009 UEFA Champions League (16% increase in Solsonès and Bages births in Spain (Montesinos et al., 2013)), the 2010 FIFA World Cup (increased birth sex ratio (Masukume & Grech, 2015) and over 1,000 extra births in South Africa (Masukume, Grech & Scherb, 2016)), the 2016 UEFA Euros (2% increase in Northern Ireland births (McKenna, Znaczko & Morrison, 2019)) and the 2019 Rugby World Cup (increased birth sex ratio in some Japanese prefectures (Inoue & Mizoue, 2022)). Nine months after the most popular provincial la Liga soccer teams unexpectedly lost matches, there were 0.8% fewer births in those provinces from 2001 to 2015; the number of births were unaffected by unexpected wins (Bernardi & Cozzani, 2021). After the 1998 FIFA World Cup a changed sex ratio at birth was not witnessed nine months on (Masukume & Grech, 2016). Nine months following a performance improvement of one standard deviation by a European national soccer team at the FIFA World Cup or UEFA Euro Championships, from 1960 to 2016, there was a 0.3% decline in births nine months on (Fumarco & Principe, 2021).

Comparison with other studies

Some studies have linked a surge in births nine months later to several occasions, such as carnivals, religious events and secular holidays like Christmas and New Year’s Day (January 1). This increase has been attributed to the holiday celebrations, which promoted greater conception through increased sexual activity among the public (Jukic et al., 2013; Kadhel et al., 2017; Macfarlane et al., 2019; Régnier-Loilier & Rohrbasser, 2011). Although exceptions exist, the principle finding of this systematic review, of baby booms 9(±1) months after winning or hosting major sports tournaments, across different sports and continents, is consistent with the mechanism of celebratory sexual intercourse (Hayward & Rybińska, 2017). After a team wins, it has been hypothesized that a person’s happiness coincides with a hormonal shift that elevates their desire for sexual activity (Casto & Edwards, 2016). Large-scale human emotions can therefore have a significant impact on population demographic changes.

Despite the fact that there was no general increase in live births after the Super Bowl (Hayward & Rybińska, 2017), the ratio of male to female live births increased nine months later (Grech & Zammit, 2019). Heightened alcohol and tobacco consumption during celebratory periods in the first trimester following Super Bowl victories has been associated with reduced birth weight nine months later (Brian, Hani & Daniel, 2016). Celebratory behaviour can manifest in various ways, encompassing both inebriation and heightened sexual activity (McKenna, Znaczko & Morrison, 2019). Thus reduced birth weights are not incompatible with increased male to female ratios at birth. While both the number of births and the male to female live birth ratio increased after the 2010 FIFA World Cup. This indicates that it is useful to consider both the overall number of live births and the births’ male to female ratio (SRB).

Although England, Northern Ireland, Scotland and Wales are separate countries with their own national soccer teams, the United Kingdom was treated as a single entity in one investigation of this phenomenon (Fumarco & Principe, 2021). According to this investigation, there was a 0.3% drop in births nine months after a national team performance increase of one standard deviation. The 2016 UEFA tournament in which Northern Ireland participated, had a 2% increase in NI births nine months later, which appears to be in conflict with this investigation (McKenna, Znaczko & Morrison, 2019). The findings of this investigation (Fumarco & Principe, 2021) should thus be handled cautiously given how the United Kingdom was treated as a single nation.

The context in which baby booms take place appears to be important, such as when a nation hosts big international sporting events for the first time (e.g., South Africa 2010 FIFA World Cup (Masukume & Grech, 2015; Masukume, Grech & Scherb, 2016), Japan 2019 Rugby World Cup (Inoue & Mizoue, 2022)), or when it first qualifies for one and does relatively well (Northern Ireland 2016 UEFA Euros (McKenna, Znaczko & Morrison, 2019)). It is conceivable that during these inaugural tournaments, public excitement is at its peak, increasing the frequency of celebratory sexual intercourse. This may help to explain why France, despite winning the tournament, did not experience a baby boom nine months after the 1998 FIFA World Cup as it was the country’s second time hosting the tournament and it had previously qualified (Masukume & Grech, 2016). Unexpected defeats in the Spanish top division la Liga, when fewer births were recorded nine months later, are another contextual element (Bernardi & Cozzani, 2021). An unexpected defeat is more likely to have a negative impact on mood than an expected defeat, with probable aftereffects including a reduction in the number of sexual encounters and subsequent births nine months later.

That a change in the number of babies born or in the sex ratio at birth occurred circa nine months after major sporting tournaments across different sports and continents suggests a true effect and a common underlying mechanism. This mechanism suggests that euphoria or sadness experienced when a sports team wins or loses coincides with a hormonal change that increases or decreases a person’s desire for sexual activity, which then affects birth rates nine months later. It appears that major multi-sport events like the Olympic Games do not lead to changed birth rates nine months later (McCartney et al., 2010). In contrast, this systematic review suggests that major single-sport events, such as those in soccer, rugby, and football league, featuring a singular apex game or tournament, likely generate more focused attention and heightened excitement, contributing to observable effects on birth rates. Baby booms post major single-sport events can potentially shape social norms, influencing family planning and creating unique cultural narratives, prompting future exploration.

Strengths and limitations

A noteworthy strength is the methodology, which is transparent and replicable, and does not impose any language restrictions on the search. Our investigation was constrained because we could have overlooked important articles by not searching gray literature. Also, potential selective reporting of significant findings remains a concern. This study has the same drawbacks as ecological studies, such as its restricted capacity to draw conclusions at the individual level (the ecological fallacy) (Björk et al., 2021). However, it is probable that ecological research is one of the best ways to assess how a population event affects a population outcome (Pearce, 2011).