The new study suggests more parental investment in education for first-borns results in higher incidence of nearsightedness.
Researchers from the study, published in JAMA Opthalmology, say nearsightedness - also known as myopia - is increasing in younger generations.
It is an important public health issue, because it is a cause of visual impairment and blindness - either directly, through myopic chorioretinal atrophy, or indirectly, through a predisposition to cataract, glaucoma and retinal detachment.
Though major risk factors for nearsightedness include genetic background, time spent outdoors and time spent doing "near work" - such as reading and writing - previous research has suggested nearsightedness is more common in first-born children.
The researchers of the latest study, led by Jeremy A. Gugenhenheim, PhD, of Cardiff University in the UK, wondered whether a potential reason for this association could be educational investment.
According to the team, it was previously reported that parents tend to put more resources into first-born children, resulting in better educational achievements in first-born children, compared with subsequent siblings.
Growing evidence for education's role in myopia
To investigate whether parents might subject their first-born children to a more "myopia-predisposing environment," the researchers carried out an analysis of over 89,000 UK Biobank participants who were aged 40-69 years, had a vision assessment and no history of eye disorders.
They found that first-born individuals were about 10% more likely than later-born individuals to be myopic, and first-borns were 20% more likely to have high myopia - a more severe form.
The results also show that the association was larger before adjusting for two measures of educational exposure, which suggests that parents investing fewer resources in the education of children born later may be partly responsible.
Commenting on their results, the authors write:
"Our findings that statistical adjustment for indices of educational exposure partially attenuated the magnitude of the association between birth order and myopia, and completely removed the evidence for a dose-response relationship, therefore support the idea that reduced parental investment in children's education for offspring of later birth order contributed to the observed birth order vs. myopia association and produced the observed dose-response relationship."
Although their results add to evidence supporting education's role in increasing myopia, the authors note that "a causal relationship cannot be confirmed using observational data."
Myopia is a growing public health issue worldwide
The large sample size and availability of information on a wide range of potential confounders were among the study's many strengths, but there were also some limitations.
For example, the self-reported data used to exclude participants with cataracts, as well as the wide age range of the sample, increased risk for bias "due to confounding between myopia and changing demographic variables."
Additionally, the two measures of education used - highest educational qualification and age at completion of full-time education - may not have fully captured all important aspects of the education experience.
Furthermore, the study did not include information on time the participants spent outdoors during childhood, so any role this exposure played in the link between birth order and myopia could not be included.
Still, the researchers say their results "suggest that the association between birth order and myopia is not due to a new environmental pressure in the last 30-40 years" and "supports a role for reduced parental investment in education of children with later birth orders in their relative protection from myopia."
Given that myopia and other vision troubles are increasing around the world, in 2009, Joshua Silver, a physicist from the UK, introduced his self-adjustable glasses, which are low-cost, flexible and one-size-fits-all.
They work by turning dials on the side of the frame that are attached to syringes that inject or remove fluid to adjust the curve of the lens. Since users look through the lenses while adjusting them, they are able to find the best possible prescription. After adjusting it, the users seal off the valve and remove the fluid mechanism, leaving a regular - adjustable - pair of glasses.
So far, over 30,000 glasses have been distributed in poorer countries. A video illustrating how the glasses work can be seen below: