The equatorial enigma: Why are more girls than boys born in the Tropics – and what does it mean?

Aristotle once suggested that the sex of a child was determined by the ardour of the man at the time of insemination, whereas other ancient Greek philosophers thought that it had something to do with the left and right sides of the body.

Two millennia later, an 18th-century French surgeon writing under the pseudonym of Procope Couteau took up the idea and advised men wishing to have baby boys to cut off their left testicle – a procedure no more painful than extracting a tooth, he said.

In more recent times, prospective parents wishing for either a boy or a girl have been offered all manner of remedies and food supplements to affect the sex of a baby. But none of these folk recipes – even those involving crystals under the bed – has been able to alter the fundamental biology that determines the 50:50 sex ratio.

A study published yesterday, however, has revealed a new twist to an ancient story. Scientists have found that the probability of giving birth to a baby girl rather than a baby boy increases significantly the nearer the mother lives to the equator. Conversely, the higher the latitude – and the further away from the equator – the greater the chances of a woman having a baby boy.

Kristen Navara of the University of Georgia in Athens studied the sex ratio of newborn boys to girls in 202 countries, from northern Europe to equatorial Africa, and found a clear link between latitude and a skewed sex ratio. The nearer to the equator the greater the probability of baby girls, according to the study published in the journal Biology Letters.

The natural sex ratio at birth is, in fact, slightly biased towards males in humans, with about 106 boys being born to every 100 girls. This sex ratio of 51.5 per cent in favour of boys is believed to be nature's way of balancing the slightly increased risk of premature death in young males, and so bringing the overall sex ratio in the child-rearing age groups nearer to the natural balance of 50:50.

Dr Navara, however, found that this average sex ratio at birth masks an underlying geographical trend. Using data on global birth rates compiled by the Central Intelligence Agency, Dr Navara found that countries in tropical latitudes produced significantly fewer boys – 51.1 per cent males – compared to countries in temperate and subarctic regions, where the sex ratio is 51.3 per cent in favour of boys.

The difference may seem small, but it is nevertheless statistically significant, Dr Navara said. It was even larger between some of the countries in the study. In tropical Central African Republic, for instance, the sex ratio was 49 per cent boys, whereas in more temperate China it was 53 per cent in favour of baby boys, she said.

"We found that this difference was independent of other cultural variables, including socio-economic status. It was an over-arching pattern and this effect remained despite enormous cultural variations between the countries we looked at," she said.

The sex ratio is an important biological factor in evolution and any shift away from the 50:50 norm provokes fierce debate among evolutionists. But the determination of sex itself is not controversial.

Sex in mammals is determined by the type of sperm that fertilises the egg. A sperm carrying the X chromosome of the man will become a female embryo whereas a sperm carrying the Y chromosome will produce a female embryo at conception. In theory, men produce equal numbers of X and Y sperm which means that the sex ratio at birth should be 50:50.

Evolutionary biologists have shown using mathematical models that any movement away from the 50:50 ratio should become unstable, which is why there should be equal numbers of baby boys and baby girls being born in the population. However, there are possible exceptions to this rule.

One exception is if male embryos and newborn boys are more likely to die prematurely. As a result of this increased risk for males, nature has compensated by skewing the birth rate in favour of boys, or so it was believed.

Another could come about if food is at risk of being in short supply. In hard times, it should in theory be more advantageous to give birth to males rather than females because females need more energy than males because of the effort of producing eggs and being pregnant.

A study in Italy has, for instance, found that couples are more likely to conceive a boy in autumn, while those who want a girl should conceive in spring. It was thought that nature favours conception of boys from September to November and girls from March to May. One explanation may be the evolutionary necessity of keeping the overall sex ratio close to the 50:50 norm. Another could be due to seasonal variations in the availability of food.

This underlying biological trend may now be showing itself up more clearly in the latest study on latitude.

Dr Navara said that the difference in the birth sex ratio between higher and lower latitudes may reflect an ancient evolutionary mechanism reflecting the fact that food resources in more northerly regions are more varied than in the tropics.

"This study really reminds us of our evolutionary roots. Despite enormous cultural and socio-economic variability, we continue to adjust reproductive patterns in response to environmental cues, just as we were originally programmed to do," she said.

This biological trend works independently of cultural factors that may work in favour of one sex over another. In some societies in Asia and Africa, for instance, baby boys are favoured over girls and the rise in selective abortions and infanticide has skewed the overall sex ratio in favour of males.

Dr Navara said she took this into account in her study by taking out those countries where selective abortions based on the sex of the foetus are known to occur. "I eliminated some Asian and African countries to get rid of any sex-specific abortion," she said. The trend in favour of women giving birth to girls the nearer they are to the equator was still significant, she said.

But the research does not suggest that simply having a romantic holiday in a tropical country could increase a woman's chances of ending up with a baby girl. The data used in the study only applied to women who were born in the country under consideration.