First-borns may experience fewer parental resources as a relative protection against developing myopia. Photo: Getty Images.

There has been speculation and various research efforts suggesting near-sightedness is more frequent in those born first than born later. However, prevalence of myopia in alpha birth orders between only-children and first-borns is currently unknown. In a recent German-based study, researchers analyzed data from the nationwide, representative German Health Interview and Examination Survey for Children and Adolescents to assess the relationship between birth order and prevalence of myopia.

Included in the analysis was 3,429 individuals aged 11 to 17. Results indicated a significantly higher percentage of self-reported myopia in only-children compared with first-born siblings, with rates of 34.5% and 28.2%, respectively. After using a model with myopia as the dependent variable and only-child status as the independent variable, only-children were independently and positively associated with myopia with an odds ratio of 1.40; this was including the variable being adjusted to age, sex, BMI, socioeconomic status and migration status. Interestingly, no relationship was found for hyperopia.

In addition to only-children having a positive correlation with myopia, self-rated quality of life in adolescents was lower than those without, which has been reflected in prior studies. Parent-rated problem behavior was found to be lower in myopic adolescents when compared with their peers without myopia diagnosis, too.

In their paper, the authors further expound upon why these associations might be observed, citing several possible explanations. In general, they believe it seems to be related to physiologic responses to environmental influences or an interaction between environmental and genetic factors.

One possibility explored in literature is maternal and birth-related effects on myopia development. One report has found increased myopia risk with higher maternal age, with birth order an independent risk factor for myopia. An explanation for this finding may be due to greater maternal age often coinciding with more education, in turn leading to more educational exposure in children.

Another explanation is based on the observation that near-work, like reading time and computer use, are linked to higher educational exposure and leads to greater myopia prevalence. The authors write that, “therefore, a possible explanation for the association of only-child status and myopia could be higher expectations of parents regarding their child’s school achievements and as a result creating a more myopiagenic surrounding with more time spent doing close work and less time spent outdoors.” This may mean less parental attention being focused on the first-born when siblings are born. Another reason may be because first-borns spend more time outside playing with their siblings, consequently decreasing myopia risk.

Looking ahead, the study authors articulate that “further research is needed to decipher the causal mechanisms behind the association between the sibling status and myopia as well as to find out modifiable environmental factors which can lead to preventive measures.”