Believe it or not, the number of brothers and sisters you have, especially younger ones, could predict your chances of developing a brain tumor, according to a new study.

The population-based study 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 four-fold 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. Interestingly, the study found no 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 actually be beneficial.

Given these findings, published in the December 12, 2006, issue of Neurology, the scientific journal of the American Academy of Neurology, researchers say efforts are needed to identify the specific infectious agents that may be causing brain tumors.