It's bad enough for the first kid when a new baby shows up to steal your thunder. But the injustice is compounded when you have to start wearing glasses while your little sibling stays as cute and non-four-eyed as ever. If this sounds familiar, you're not alone: firstborn kids are more likely to be nearsighted. Part of the reason might be that they get more education. A study in the United Kingdom and Israel found that myopia—that's nearsightedness, if you're one of those lucky people who hasn't spent much time at the optometrist's office—is about 10% more common in firstborn children. But that study only looked at subjects between 15 and 22 years old. Is this a new issue, or have older siblings always gotten the short end of the eyesight stick? To find out, researchers in the UK gathered data from nearly 90,000 subjects. These people were part of a huge national project called the UK Biobank, which is tracking the health of about half a million adults. For this vision study, the scientists—Jeremy Guggenheim of Cardiff University and Cathy Williams of the University of Bristol—looked only at white adults between 40 and 69 years old. Overall, about 30% of them were myopic. After controlling for age and sex, the researchers saw that first children were about 12% more likely to be nearsighted than second children. This was similar to the finding of the earlier study with younger subjects. Firstborns were also 21% likelier than second-borns to have severe myopia. And when compared to people born fourth or later, firstborns were almost 40% more likely to be nearsighted. (The researchers counted only children as firstborns too, but the results were the same when they removed only children from the analysis.) Earlier research suggested that parents spend more resources on educating their oldest children than other kids. So Guggenheim and Williams adjusted their analysis for how many years of education people had received. This took away about a quarter of the extra myopia in firstborns. The authors think this means education is a factor. They couldn't specifically study anyone's early childhoods to see how much time they had spent outdoors (which is good for the eyes) versus reading and studying (not so good). But it seems that somewhere along the way to completing their longer educations, oldest siblings were more likely to end up with glasses. It wasn't a large effect. The difference between firstborns and everyone else averaged out to less than –0.25 diopters in a glasses prescription. (Again, for those of you who aren't experts, 0.25 is usually the smallest increment by which an eye doctor adjusts your prescription.) And since accounting for education only removed some of this difference, there must be other factors involved. The authors say these likely include social factors, birthweight, and time spent outside. In many parts of the world, people's eyes have gotten worse in recent decades. This has been especially dramatic in East Asia. Spending a lot of time on "near work," such as reading, is a known risk factor. So is spending less time outdoors. Even among the subjects in this study, who were 40 to 69 years old, myopia was more common in younger people. No matter how much you shun schoolbooks for nature, though, genetics and health factors can help send you to the optometrist's office. You may as well pick out some cool frames and go back to your reading. If any younger siblings give you trouble, remind them that your parents are likely to invest more in your education (or just hold something way up high where they can't reach it).
Image: by Brisbane Falling (via Flickr)
Guggenheim JA, Williams C, & UK Biobank Eye and Vision Consortium (2015). Role of Educational Exposure in the Association Between Myopia and Birth Order. JAMA ophthalmology, 1-7 PMID: 26448589