Older siblings in large families are at risk for household chores and babysitting duties and, according to a recent article in Neurology, several cancers as well. Altieri et al. examined over 13,000 cases of nervous system tumors, including astrocytomas, meningiomas and neuroblastomas, in the Swedish Family-Cancer Database and found that the incidence of brain cancers was twice as high in people with four or more siblings than in only children. In children under the age of 15, having three or more younger siblings increased a child’s risk of having brain cancers by two to four times relative to only children.

As any parent with a child in daycare knows, the more children that play in one room, the more colds and coughs any one child will get. According to the authors, because crowded houses are linked to increased rates of infections, their data suggest that cancers of the nervous system may be caused by viruses. They think that young immune systems may be strengthened by certain infections, while teenage immune systems are more vulnerable, and the younger siblings may re-expose older siblings to the viruses.

There are several interesting alternative theories. Helen Pearson suggests that the latter-born children may be exposed to different factors in the womb than their older siblings that affect immune system function. Paul Graham Fisher told Yahoo! Health News that the data may simply indicate that it is common for Swedish people to have more children after one child has been ill.

Because brain tumors are rare in children (brain tumors strike 3 children out of 100,000), it is unclear what the small increase in incidence in older siblings of large families means, let alone what impact the present study will have. Considering that cervical cancer is caused by human papillomavirus, it is tempting to pursue viral causes for brain tumors and imagine a world in which all children are immunized against possible infection. Perhaps if other cancers were shown to have viral causes, the public would stop focusing on the sexual transmission of human papillomavirus and start focusing on how to protect their children from cancer.