Babies born in winter are bigger, brighter and more successful

The weather may be gloomy but the outlook is sunny for boys and girls who arrive now, major new research reveals

For centuries astrologers have sworn that the time of year a baby is born plots the course its life will take. Now extensive research conducted over a seven-year period appears to prove that babies born in the winter are more likely to grow into big, bright and successful adults than their summer counterparts.

The study, which will be published this week, was led by scientists at Harvard University and monitored the development of 21,000 boys and girls worldwide. It shows that there were large seasonal variations when it came to weight, length, height, head size and mental ability.

Researchers believe that the effects on the pregnant mother and the growing foetus of seasonal variations in diet, hormones, temperature, exposure to sunlight and viruses and other infections may influence a baby's characteristics.

The American and Australian psychiatrists and anthropologists from Harvard and Queensland universities measured the children and carried out mental and motor tests at birth, at eight months, and at four and seven years.

Compared to summer births, those born in winter were significantly longer at birth, and were heavier, taller and had larger head circumference at age seven. They also had higher scores in a series of intelligence exercises. By the age of seven, winter- and spring-born children were 210g heavier, 0.19cm taller, and had head circumferences significantly larger than summer and autumn-born children. The results also show that babies born in the winter were the longest, while winter- and spring-borns weighed the most at the age of seven and were also the tallest.

The researchers, whose work appears this week in the medical journal Schizophrenia Research, conclude: "The overall pattern of findings is that winter/spring babies are both 'bigger' on the anthropometric variables and 'smarter' on the selected neurocognitive variables."

The new study is the latest - and largest - in a series of projects worldwide aimed at evaluating the effect of the seasons on human health, longevity and physical and intellectual development. In 2002, scientists at Germany's prestigious Max Planck Institute for Demographic Research announced that people born in the autumn live longer than those born in spring, and do not become as ill in old age.

Studying census data from Denmark, Austria and Australia, the institute also revealed a seasonal link to life expectancy for those older than 50. In Austria, for example, it was found that adults born between October and December lived some seven months longer that those born between April and June.

Dietary changes and seasonal infections are thought to be at the heart of the phenomenon. "A mother giving birth in spring spends the last phase of her pregnancy in winter, when she will eat fewer vitamins," said Gabriele Doblhammer, one of the scientists who carried out the research. "When she stops breast-feeding and starts giving her baby normal food, it is in the hot weeks of summer - when babies are prone to infections of the digestive system."

The season of birth can also influence whether a person is an optimist or pessimist. Yet it is the summer's babies that have a brighter outlook than winter-born grumblers.

The American and Australian researchers offer a number of explanations for such differences in the early years of life. One theory is that foetal exposure to changing seasonal factors such as temperature, rainfall and ultraviolet radiation may be responsible.