A study that was presented a week ago at the annual meeting of the American Academy of Allergy, Asthma, and Immunology in San Francisco reported a greater prevalence of allergies in firstborn children than laterborns. The study, conducted in Japan, looked at 13,000 children ages 7 to 15 years, and the relationship between their birth orders and their experiences (as reported by the parents) of a variety of allergic diseases including: food allergies, allergic rhinitis, skin sensitivities, allergic conjunctivitis, and asthma.
What they found were relationships between birth order and the prevalence of food allergies, rhinitis, and conjunctivitis such that increasing birth order was associated with decreasing allergies (for these three conditions). For food allergies, the prevalence was 4% for firstborns, 3.5% for secondborns and 2.6% for other laterborns. Interesting for those collecting studies of birth order effects but also to parents of kids with allergies and those interested in the hygiene hypothesis.
The hygiene hypothesis, in its simplest form, suggests that a lack of exposure to infectious agents, microorganisms and parasites during early childhood has a negative impact on the development of the immune system. And in fact, this new Japanese study is not the first to suggest that family size plays a role in allergic response. David Strachan published a study in the British Medical Journal (BMJ) in 1989 that used the hygiene hypothesis to account for the fact that hay fever and eczema are less common in children from large families than in children with no siblings. The logic here is that children in big families are exposed to more infections and parasites through their siblings and, as a result, their immune system learns to respond accordingly.
This hypothesis has also been used to explain the greater burden of allergic and autoimmune disease that is seen in industrialized countries and often attributed to improved hygiene, vaccination, and use of antibiotics. The rapid increase in such allergies certainly seems to point the finger at environmental factors playing a role. It is also true that family size is typically much smaller in industrialized countries than in developing ones. Their hygiene, water quality, and medical care are poorer and their family sizes are often much larger. More exposure to pathogens and parasites all round.
Of course, it's likely that it's not just about family size but also about firstborns specifically and the way parents often treat them. Parents are typically nervous with the birth of a first child and are often hyper concerned about hygiene and the safety of the infant. By the time there are several children in the home, parents are often much more relaxed and the firstborn's soother that was always washed (read sterilized!) after being dropped to the floor might end up with a later child's being wiped off quickly or inspected for significant contamination before being popped back into baby's mouth. It sounds like a bad deal for the later child but it might be a blessing in disguise. That little one might be better off for not having the floor washed before he's allowed to crawl on it and for letting the dog lick his face.
It's also true that it might not be about the hygiene at all. It could be prenatal conditions in the womb influencing the immune system. The fraternal birth order effect
, originally posited by Ray Blanchard, suggests that the likelihood of developing a homosexual orientation in males increases with each older brother (though not with older sisters). A number of studies have demonstrated the effect, including those where the focal individual was not raised with his brothers, indicating the possibility that the effect is due to in-utero environmental conditions and perhaps some sort of maternal immune response. The prenatal environment could also play a role in the relationship between birth order and allergies although this does not at first pass explain why the increasing rates of allergies over time.
Regardless, they are interesting results, both to the birth order researcher and those interested in the possible relationship between hygiene and allergies.