The researchers from Cardiff University suggested that the cause was “parental investment in education” because parents may have a tendency to put more pressure on first-borns. They theorized that parents may be more demanding that first-borns do more "near" activities, such as reading, which may impact their eyesight. Previous studies have shown a strong link between time spent outdoors and a diminished risk of myopia, and it may stand to reason that children who spend more time on studies may be spending less time outdoors.
Jeremy Guggenheim, a doctoral student, and colleagues wrote that while there's no way to make a definitive causal link, their study found that when they adjusted for a proxy for educational exposure — the highest educational degree or age at completion of full-time education — they saw a less dramatic association between near-sightedness and birth order.
"These data suggest that the association between birth order and myopia is not due to a new environmental pressure in the last 30 to 40 years," Guggenheim wrote. "The attenuated effect size after adjusting for educational exposure supports a role for reduced parental investment in education of children with later birth orders in their relative protection from myopia."
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