Are you big sister or big brother to a bulging brood of siblings? If so, a new study shows that you could be more at risk for developing a brain tumor.

Only about one percent of people will have to face a brain tumor diagnosis in their lives. But German researchers who studied more than thirteen-thousand Swedish cancer cases found that people who grew up with four or more younger siblings were twice as likely to develop these tumors as only children were.

Experts don’t know exactly what is causing the phenomenon but they think germs play a role. Germs pass easily among children, especially when they’re together all the time and fighting over the same Barbie doll.

Because researchers didn’t find a link between having older brothers and sisters and brain tumors they think infections may actually offer protective benefits to infants. But in older children, the same infections may be setting the groundwork for brain tumors later in life.

Infections have been linked to some cancers and certain viruses can spark cell changes that could lead to tumors. But infections have not yet been linked to brain or spinal cord cancers, and experts say prior exposure to certain neurological infections actually seems to help some patients battle brain tumors.

But being the Marcia or Greg Brady of your family doesn’t mean you’ll get cancer. The chances of developing a brain tumor are small, even with the added risk.