First-born children are more likely to be more nearsighted than their younger siblings, according to new research published in JAMA Ophthalmology. Investigators at Cardiff University’s Eye Clinic in the United Kingdom used data from the British Biobank longitudinal survey to study nearly 90,000 adults between the ages of 40 and 69. Participants had a vision assessment, reported no history of eye disorders and shared potential confounders such as highest educational qualification through questionnaires. The researchers defined myopia and high myopia as autorefraction of −0.75D or less and −6.00D or less, respectively. 

Using this information, they found that firstborns were 10% more likely to develop myopia and 20% more likely to show signs of severe myopia. “The risk for myopia became progressively lower for later birth orders, suggesting a dose response,” the researchers wrote in the study.

The investigators further found that adjusting for education changed the results considerably: the dose response disappeared once they took each child’s education into account. 

While the researchers are leery of claiming a direct correlation, they say the results support “a role for reduced parental investment in education of children with later birth orders in their relative protection from myopia.” 

Guggenheim JA, Williams C, UK Biobank Eye and Vision Consortium. Role of educational exposure in the association between myopia and birth order. JAMA Ophthalmol. 2015 Oct 8:1-7. [Epub ahead of print].