Happy birthday? Depends when

Many people dismiss the Zodiac signs, but scientific studies have found that there are in fact correlations between them and important aspects of their lives.

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"Age ain't nothing but a number," sang the 14-year-old multi-platinum-selling songstress, actress and diva known to the world during her short lifetime as Aaliyah. But age, or more specifically, your date of birth, is far from an inconsequential set of numbers. Your birth date can play a hugely influential, and at times seemingly mysterious role, in shaping your life and perhaps even your death. Take, for example, the numerous studies reporting links between female reproductive success (fertility) and birth month. One Canadian study based on historical records reports that women born in early summer had significantly more offspring than those born in early autumn. This particular study, published by the Royal Society for Biological Sciences, followed the reproductive history of the women across two generations. The women born in June - a good birth month - averaged seven more grandchildren than those born in October, which was the worst birth month in terms of future reproductive success.

In addition to predicting progeny, life expectancy also appears to be influenced by the numbers we call a birth date. In one study of life expectancy, researchers at the Max Planck Institute in Germany discovered that in northern hemisphere nations (Denmark and Austria) people born in the autumn months (October to December) lived longer than those born in the spring (April to June). This pattern was totally reversed in southern hemisphere nations such as Australia, where life span peaked for those born from April to June, and troughed for those born in October to December. As for first-generation immigrants to Australia from the northern hemisphere, their life span followed the pattern of their birth hemisphere, suggesting it is both where and when you are born that counts.

Stranger still, researchers have even identified the month of birth as a risk factor in suicide. One study in the United Kingdom examined all the suicides in England and Wales over a 22-year period, some 27,000 in all. They found that a significant and disproportionately large number of the people who would later take their own lives were born in April, May and June. These findings were even more pronounced in females, for whom there was a 29 per cent increase in the risk of suicide for women born in the peak months compared with those born in the low-risk autumn months.

All of these fairly bizarre birth date-related findings are explicable in terms of what scientists call the foetal origins theory. The basic idea is that our early environment makes us more or less vulnerable to certain major illnesses later on in life. Our environment changes from month to month in terms of temperature, hours of sunlight, the types of food available and the types of pathogens in the air. It is these natural seasonal variations in environment that are thought to account for these seemingly bizarre birth month-related peaks and troughs.

However, the findings of a recent study undertaken at Zayed University in Abu Dhabi are not so easily explained in terms of the foetal origins theory. This study explored birth month as a predictor of academic performance in female students. While there was no clear link between birth month and academic performance, a significantly larger number of the students were born in January than any other birth month. In fact, while the numbers are fairly similar for students born in other months, with no significant differences between them, in January there is a very obvious spike.

So, why so many January-born students at Zayed University? The answer lies not in the natural environment but rather in educational policy. Historically, for government schools in Abu Dhabi the school age cut-off date for admission to KG1 was December 31; this means that if you are born in January, you are typically going to be the oldest in the class, perhaps a whole 11 months older than a December-born classmate. This concept, termed "relative age", has been well explored, and the bulk of research suggests that older children outperform younger children in KG1 and well into their school careers. One recent US study reported that the youngest students in the class were significantly less likely to make it to college compared with their older classmates.

Given that entrance to Zayed University is based on high-school performance, we would expect more of the relatively older girls, especially those born in January, to make the grades required to secure a place there. This is exactly what we are seeing: a significant spike of January-born students, arguably reflecting the early advantage conferred upon them by being born just the right side of the school age cut-off date.

But what about those born just before the cut-off date - the relative babies of the class? There is no doubt that at least early on they are educationally disadvantaged. Furthermore there is emerging evidence that the relatively younger are also at greater risk of developing a psychological disorder. In some nations, parents who realise these disadvantages have opted to hold their children back a year so that they enter school later with an enviable relative age. Also some schools with two or more sections of a class place students in such a way as to minimise age differences. Age is far more than just a number, and I'm sure the relative-age issue has been factored into Abu Dhabi's ongoing educational reform.

Justin Thomas is professor in the department of natural sciences at Zayed University in Abu Dhabi