More baby girls in cold weather? More boys when it's warm? Not in New Zealand

New Zealand bucks the trend when it comes to links between temperature at the time of conception and the sex of babies.

In places like Germany and Scandinavia, colder years tend to produce fewer boys than normal, while warmer years produce a higher number of males.

But Victoria University researchers have shown that’s not the case in New Zealand.

A team of four, led by Dr Barnaby Dixson, analysed data on sex ratios of births and temperature fluctuations in New Zealand since 1876 and found no relationship between the gender split and the weather.

Dr Dixson says the global average for male births is 51.7 per cent while the average in New Zealand over the last 135 years has been relatively stable at 51.3 per cent.

"Colder years or seasons had no influence on this percentage," he says.

Dr Dixson, from Victoria’s School of Biological Sciences, says researchers have several theories to explain their findings.

"The chances of a fertilised egg being male or female are, in theory, equal. But previous research has shown that male foetuses are relatively fragile and sensitive to the cold so if the temperature is low, or the environment is harsh, it is thought that the male foetus is less likely to survive.

"It’s possible the temperatures in New Zealand are not sufficiently cold or harsh to have an impact on developing males, or perhaps New Zealand males aren’t as delicate as others."

The research has appeared in the prestigious scientific journal PLoS One, an international, peer reviewed publication put out by the Public Library of Science.

Dr Dixson says the team also did a regional analysis, comparing data for Southland and Northland, although this is not covered in the PLoS paper.

"The two areas have dramatically different ambient temperatures but there was no difference in the likelihood of having a male or a female baby."

The research team, made up of Barnaby Dixson, Diane Ormsby and Phil Lester from Victoria’s School of Biological Sciences and John Haywood from the School of Mathematics, Statistics and Operations Research, is now turning its attention to Australia.

They have started analysing historic population and weather data to see if Australia also contradicts current thinking on the correlation between temperature and the proportion of boy births.

"Australia has much greater temperature variations than New Zealand and, in some areas, is hot enough that heat stress may impact on the gender of babies," says Dr Dixson.