The population-based study, the largest of its kind, analyzed 13,613 brain tumor cases in Sweden. It found people with four or more siblings were twice as likely to develop a brain tumor as people with no siblings.

The study also found there was a two to fourfold increase in brain tumor rates among children younger than 15 who had three or more younger siblings compared to children of the same age who had no siblings. The study did not find an association between the number of older siblings and brain tumors.

"Since the size of a family and the number of younger siblings correlate with the incidence of brain tumors, this suggests infectious agents may be causing the disease," said study author Andrea Altieri, DSc, with the German Cancer Research Center in Heidelberg, Germany. "The number of siblings a person has indicates they were exposed at an early age to infections, since children come in close contact with each other and thereby share exposures to many infectious agents."

According to Altieri, the finding that brain tumor rates were higher among people with younger siblings, and not older siblings, suggests infections or re-infections in late childhood may play an important role in causing the disease, while exposure to infections in infancy, birth to five months old, may be beneficial.

Given these findings, researchers say efforts to identify the specific infectious agents that may be causing brain tumors are warranted.

"This study represents the only population-based study providing reliable quantification of the effects of number of siblings on the risk of brain tumors. The two to fourfold increased risks for individuals with a high number of younger siblings are stronger than most established risk factors for the disease," said Altieri.

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Materials provided by American Academy of Neurology. Note: Content may be edited for style and length.