Being a twin may include benefits that extend beyond having a playmate from birth. According to a report in the British Medical Journal, twins are less prone to asthma than singletons. Researchers identified all twins born in Scotland from 1981 to 1984 and tracked subsequent respiratory illness-related hospital admissions up until 1994. They found that although twins were more likely to be admitted for acute bronchitis and bronchiolitis, they were less than half as likely to be admitted for asthma.

The reasons for this so-called twin effect remain somewhat mysterious. Previous studies linked low birthweight to an increased risk of asthma, yet twins tend to weigh less at birth than singletons. A more plausible explanation, the authors suggest, is that the reduced risk of asthma associated with twinship represents a special case of the protective effect against allergic disease that large families can bestow. This model holds that kids with many siblings more often avoid such ailments simply because they are exposed to more infection during childhood. The findings, the authors conclude, may provide insight into the early developmental influences on asthma and, as such, warrant further investigation.