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March 3, 2021

Global warming might mean fewer baby boys

By ALLISON CHEN | February 7, 2019



The ratio of male to female births may be affected by temperature.

One unexpected impact of climate change may be an altered ratio of male to female children born, a recent study suggests. Research published by a team of Japanese and Danish scientists in Fertility and Sterility indicated that there is a statistically significant relationship between temperature differentials and the male-to-female birth ratio. 

According to the World Health Organization, the worldwide sex ratio at birth is around 105 males for every 100 females. There are a number of theories as to why more males are born, with one major potential factor being that men have a higher risk of dying at an earlier age. In order to maintain a balanced sex ratio at reproductive age, there will have to be more males at birth. 

This differential, however, may not be present at conception. As suggested by a study published in Significance, a magazine published by the American Statistical Association and the Royal Statistical Society, the sex ratio is 50:50 at conception, with more female deaths in utero resulting in the ultimate ratio at birth.  

The study in Fertility and Sterility analyzed the yearly mean temperature differences in Japan, as well as the ratio of male to female births and male to female fetal deaths, in order to evaluate whether there was an association between temperature and either of the ratios. 

Researchers found correlations between temperature fluctuations and the ratio of fetal deaths, as well as between temperature and the sex ratio at birth. Researchers discovered that as temperature differences rose, more males died spontaneously in utero, and fewer male infants were born. 

The researchers acknowledged that there were some confounding factors and other considerations involved in the results. The same associations found in the data from Japan were not found in data from Finland or New Zealand, although the scientists noted that differences in temperature were not as large in those two countries as in Japan. 

The researchers also recognized that other environmental elements, such as air pollution or toxic chemical contamination, might impact sex ratios at birth.

Data that examined the influence of stress on the ratio of male to female infants supported the study however. After the devastating 2011 Tohoku earthquake killed almost 20,000 people in Japan, the sex ratio at birth in three affected prefectures lowered significantly, with more female infants being born approximately a year after the disaster. 

This indicates that stress has a detrimental effect on male fetuses and/or Y-bearing sperm cells. Causes of stress can include not only earthquakes but other natural disasters, and the more extreme temperature swings predicted by climate change models.

When asked about the scale of these sex ratio changes, scientists responded that impacts were more likely to be local than global. 

“[Changes] may depend on different environmental factors of each place,” Misao Fukuda, lead author of the Fertility and Sterility study, wrote in an email to CNN

Researchers anticipated that, instead of broad temperature changes having an effect, a greater influence would come from the local environmental events in an area, such as droughts, forest fires and floods, which result from climate change. 

Some scientists also did not expect changes to last for a long time after an environmental stressor. In a study published on the Tohoku earthquake, Fukada showed that sex ratios at birth did not remain low after the drop but returned to normal within one to two months.  

While the ultimate list of factors that might impact sex ratios, and the scale and length of alterations, has not yet been fully determined, researchers agree that we will be seeing changes in coming years. Only time will tell how temperature changes or associated environmental events will impact the population and the composition of future generations. 

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