First borns are more likely to be myopic than their siblings: 12% more likely, to be precise. The reason? A tendency to receive more education. That’s according to a study published this month in JAMA Ophthalmology, involving 89,000 adults who participated in the UK Biobank health project. The study also found that first borns face a 20% higher risk of being severely nearsighted. That doesn’t mean that first borns have a particularly high risk of myopia, though. In fact, in the current study, the effect averaged out to less than -0.025 diopters in a glasses prescription (-1.5 to -7 diopters means mild to moderate myopia). An earlier study of younger participants in Israel and the UK found similar results, suggesting that this is fairly robust over time.

Meanwhile, separate research has linked birth order to other (seemingly unrelated) differences between first borns and their younger siblings, such as measured intelligence, school performance and earnings.

In August this year, researchers at the University of Illinois examined the IQ scores as well as personality traits of 377,000 American high school students, seeking the famed first-born intelligence advantage. The study found that eldest siblings actually did score higher on IQ tests on average – but only by one point. An earlier, 2007 study published in Science found that among 241,310 young Norwegian men born from 1967 to1976, first borns had a more practically significant, three-point higher IQ than younger siblings.

The difference between the two studies is curious. If the IQ difference were somehow due to biological factors, we might expect it to be more consistent across large studies. Sadly for first borns, though, scientists and psychologists say it likely doesn’t have anything to do with differences in inherent intelligence. To rule out the possibility, though, the authors of the Norwegian study looked at the IQ scores of second-born siblings whose elder sibling had died, leaving them first borns in practice – and found that the first-born advantage held for them as well.

They concluded that the academic advantage must be something to do with how first borns are treated in comparison to their siblings. (Contrary to a once widely held belief, intelligence isn’t immutable: use of the brain actually alters it physically – for example, in the case of London cab drivers. IQ tests are also arguably not the best means of measuring it.)

Cue the ‘undivided-attention theory’, which posits that first borns benefit from initially getting more parental attention; the ‘lazy-parent theory’ that says parents are simply less strict the second time round; and, the out-of-the-box proposal that older children benefit from the consolidation of knowledge that comes from teaching their younger siblings. Another possible explanation – which also helps to explain why younger siblings are more adventurous and unconventional on average – is that younger siblings, loath to live in the shadow of the eldest, try to differentiate themselves in different ways, leaving first borns to accumulate academic accolades.

Consistent with the idea that parental investment tends to be greater the first time round, research suggests that older kids tend to receive slightly longer educations than their younger brethren. Researchers at Cardiff University hypothesized a link with myopia. Fortunately, they had data on the education of the participants and so were able to test their theory. When they added the number of years in fulltime education to their control variables (which also included age and sex), they found the difference in the risk for myopia between first borns and their siblings narrowed by a quarter.

“Greater educational exposure in earlier-born children may expose them to a more myopiagenic environment, for example, more time doing near work and less time spent outdoors,” the researchers explain, adding that other social factors are probably also at play, which would account for the remaining gap.

If environmental factors account for IQ and myopia, could they also account for another physical difference relating to birth order – the differing risk of being overweight in adulthood? A recent study published in the Journal of Epidemiology & Community Health found that first-born women are 29% more likely to be overweight and 40% more likely to be obese than their sisters. (The findings support previous findings among men.) Although the study was purely observational, lead author Professor Wayne Cutfield told CBS news that one possible reason relates to the fact that the blood vessels leading to the placenta tend to be narrower in first pregnancies. This means lower blood and nutrient supply, which might affect how first borns store fat and regulate glucose later on in life.

An appealing social explanation relates back to the idea that parents are simply fussier with first borns: they are more likely to worry about the child getting enough to eat, leading to a higher chance that the eldest child is over-fed. (Or maybe, once again, the higher risk comes with more intense education and thus possibly less exercise.) The authors conclude that shrinking family sizes might be one lesser-considered factor contributing to the observed increase in adult body mass index worldwide.

As intriguing as they are, many of these observed differences between first borns and their siblings are often practically insignificant, researchers warn. Whatever the actual advantages of being the first born, they’re pretty meager compared to the days of old that inspired tales like Macbeth. In any case, I’d rather be the adventurous one. DM

Photo: The two brothers, Britain’s Prince William, Duke of Cambridge (L) and Prince Harry (R) walk through a sea of red poppies inside the moat at the Tower of London in London, Britain, 05 August 2014. EPA/WILL OLIVER.