Human Sex Ratio - Factors Affecting Sex Ratio in Humans - Environmental Factors

Environmental Factors

Effects of climate change

Various scientists have examined the question whether human birth sex ratios have historically been affected by environmental stressors such as climate change and global warming. Catalano et al. report that cold weather is an environmental stressor, and women subjected to colder weather abort frail male fetuses in greater proportion thereby lowering birth sex ratios. Cold weather stressors simultaneously extend male longevity thereby raise human sex ratio in its older age bracket. Catalano team finds that a 1°C increase in annual temperature predicts one more male than expected for every 1,000 females born in a year.

Helle et al. have studied 138 years worth of human birth sex ratio data, from 1865 to 2003. They find an increased excess of male births during periods of the exogenous stress (World War II) and during warm years. In the warmest period over the 138 years, the birth sex ratio peaked at about 1.08 in northern Europe.

Effects of gestation environment

Causes of stress during gestation, such as maternal malnutrition generally appear to increase fetal deaths particularly among males, resulting in a lower boy to girl ratio at birth. Also, higher incidence of Hepatitis B virus in populations is believed to increase the male to female sex ratio, while some unexplained environmental health hazards are thought to have the opposite effect.

The effects of gestational environment on human sex ratio are complicated and unclear, with numerous conflicting reports. For example, Oster et al. examined a data set of 67,000 births in China, 15 percent of whom were Hepatitis B carriers. They found no effect on birth sex ratio from Hepatitis B presence in either the mothers or fathers.

Effects of chemical pollution

A 2007 survey by the Arctic Monitoring and Assessment Program noted abnormally low sex ratios in Russian Arctic villages and Inuit villages in Greenland and Canada, and attributed this imbalance to high levels of endocrine disruptors in the blood of inhabitants, including PCBs and DDT. These chemicals are believed to have accumulated in the tissues of fish and animals that make up the bulk of these populations' diets. However, as noted in the Social factors section below, it is important to exclude alternative explanations, including social ones, when examining large human populations whose composition by ethnicity and race may be changing.

A 2008 report provides further evidence of effects of feminizing chemicals on male development in each class of vertebrate species as a worldwide phenomenon, possibly leading to a decline in the sex ratio in humans and a possible decline in sperm counts. Out of over 100,000 recently introduced chemicals, 99% are poorly regulated.

Other factors that could possibly affect the sex ratio include:

• Social status of the mother, known to be a factor in influencing the sex ratio of certain animals such as swine, but apparently not in humans.
• Whether the mother smokes.
• Whether the mother has a partner or other support network, although this correlation is widely considered to be the result of an unknown third factor.
• Latitude, with countries near the equator producing more females than near the poles.

Other scientific studies suggest that environmental effects on human sex ratio at birth are either limited or not properly understood. For example, a research paper published in 1999, by scientists from Finland’s National Public Health Institute, reports the effect of environmental chemicals and changes in sex ratio over 250 years in Finland. This scientific team evaluated whether Finnish long-term data are compatible with the hypothesis that the decrease in the ratio of male to female births in industrial countries is caused by environmental factors. They analyzed the sex ratio of births from the files of Statistics Finland and all live births in Finland from 1751 to 1997. They found an increase in the proportion of males from 1751 to 1920; this was followed by a decrease and interrupted by peaks in births of males during and after World War I and World War II. None of the natural factors such as paternal age, maternal age, age difference of parents, birth order could explain the time trends. The scientists found that the peak ratio of male proportion precedes the period of industrialization or the introduction of pesticides or hormonal drugs, rendering a causal association between environmental chemicals and human sex ratio at birth unlikely. Moreover, these scientists claim that the trends they found in Finland are similar to those observed in other countries with worse pollution and much greater pesticide use.