Surprising new research indicates that those who have a younger brother or sister before preschool have lower risks of becoming obese — and scientists are scratching their heads as to why.

In fact, children who don’t have a sibling were three times more likely to be obese by the time they graduated kindergarten, according to a statement from the University of Michigan Health System.

The caveat is the child must have a younger sibling by the time he or she reaches first grade, or when the child is between two to four years of age.

Why is this the case? Scientists aren’t sure, noting that only a link has been found, not a cause.

Some theories have been put forward. Perhaps children who have siblings are more likely to engage in active play and spend less time in front of television screens. Or, it may be because parents are more health conscious and change dietary habits in a way that indirectly results in the other child keeping a lower body mass index (BMI).

The finding could be critical in the battle against the obesity epidemic in children in America. More research could uncover more explanations as to why this phenomenon exists, and therefore allow doctors to recommend courses of treatment or habits that could solve obesity.

The study involved 697 children, and will be published in the journal Pediatrics.

“Research suggests that having younger siblings – compared with having older or no siblings – is associated with a lower risk of being overweight. However, we have very little information about how the birth of a sibling may shape obesity risk during childhood,” senior author Julie Lumeng, M.D., a developmental and behavioral pediatrician at U-M’s C.S. Mott Children’s Hospital, said in the statement. “This study is believed to be the first to track subsequent increases in BMI after a child becomes a big brother or sister.”