A New Zealand study has confirmed that summer is the favourite time of year for couples to try and conceive.
But the research has put a dampener on overseas work claiming that more boys are born in hot years.
Biologists at Victoria University have studied the sex ratios of new babies in New Zealand in every year since 1876 and found no relationship to the mean annual temperature either in the year of conception or in the year of birth. But their study, presented at this week's Population Association conference in Auckland, seems unlikely to stop research on the causes of varying sex ratios.
Researchers have found slight increases in the proportion of boys born in warmer years in cold European countries, including Finland, Sweden, Norway, Denmark and Germany, where records date back 150 years.
In contrast, American researcher Kristen Navara found slightly higher rates of boys born in temperate and subarctic countries (51.3 per cent) than in the tropics (51.1 per cent).
Both findings appear to be related to stress. Researchers have found that fewer boys have been born after their parents were exposed to a variety of stresses just before or during pregnancy, including earthquakes, poverty, extreme heat or extreme cold.
"Data is very mixed," said Dr Diane Ormsby, who presented the New Zealand study this week.
"We were interested to know, because New Zealand is such a long, thin country with variable climate and very different regions, whether or not we would show anything here ..."
Her team did find a clear summer "mating season" in New Zealand, as in other countries.
Births varied between a low point of about 4400 in February, nine months after winter, to a peak of about 5000 births in October, nine months after the summer holidays.
"So yes, summer is a good time to make a baby because people have more time to be with each other, or perhaps they are more relaxed," she said.
But the proportion of boys born fluctuated between about 51.2 per cent and 51.5 per cent through the year with no apparent relationship to the weather.
Over the whole period since 1876 the mean average temperature taken at seven places between Auckland and Dunedin varied between 11.04C and 13.33C, with a slight upward trend linked to global warming.
But the proportion of male babies jumped around between 50.4 per cent and 52 per cent, again with no relationship to the temperature in either the year of birth or the previous year.
"New Zealand is not the same as Scandinavia or Germany," Dr Ormsby concluded.
Scientists are still not sure what evolutionary factors may explain why baby boys slightly outnumber baby girls in all human societies, let alone why the ratio appears to vary with stress levels.
But boys generally have a higher death rate than girls during their childhood and youth because of accidents, violence and other factors. Researchers have suggested that a woman may have fewer boys in more stressful times because her sons are then even less likely than usual to survive and give her grandchildren.
BOYS BY WHISKER
Boys as percentage of all births:
Asia... 51.4 per cent
Europe... 51.4 per cent
Oceania... 51.3 per cent
South America... 51.2 per cent
North America... 51.0 per cent
Africa... 50.8 per cent
All subarctic... 51.3 per cent
All temperate... 51.3 per cent
All tropical... 51.1 per cent
Source: Kristen Navara, University of Georgia, 2009