One day soon after my little brother started 1st grade, I heard on the playground that he had spent all of his lunch money on candy. Ever the protective big sister, I dragged him back to the candy counter to get a refund on the uneaten portion so he could buy a sandwich.

Older siblings try to help their brothers and sisters, and studies have shown that younger siblings usually benefit from this attention in many ways. But a new study shows that for children with autism, the protective behavior of older siblings may impede their ability to navigate social situations.

Typically developing children usually develop a working theory of mind — the ability to identify the mental states of others — between 3 and 5 years of age. Previous studies have shown that having an older sibling accelerates the development of theory of mind in healthy children.

Some studies have found that birth order is immaterial — just having another sibling between 1 and 12 years of age leads to higher scores on tests of theory of mind.

Curious about whether these results would also apply to children with autism, researchers asked 60 children with the disorder, mostly boys, aged 3 to 12 years to take six tests commonly used to assess theory of mind. These included a false-belief test in which a marble is moved to a new hiding place, and pretend-play tasks in which a banana is used to mimic a telephone.

The researchers found that children with an older unaffected sibling score significantly lower on the tests compared with those who either have no siblings or have only younger ones. The results did not change with their age, autism severity or verbal ability.

Younger children with autism may not get the same amount of attention from parents as only children, which is understandable. Also, the more children parents have, the less money they have to spend on educational opportunities for their children with autism.

Like me, older siblings may also try to help their younger brothers and sisters navigate social situations, buffering them from the kinds of mistakes that lead to social learning.

In trying to protect their younger siblings, and by avoiding the kind of complex interactions typical among siblings — teasing, arguing, joking — that enhance theory of mind in healthy children, older children may be unwittingly blocking the social development of their siblings with autism.

The children in the study are at the more severe end of the autism spectrum, and it’s possible that higher-functioning children might not show the same effects.

Still, this study suggests that there’s a fine line between helping others and hindering their growth.

My brother would have learned that candy is not a satisfying lunch even without my well-meant meddling, and children with autism might well learn to pick up some social cues without their siblings running interference.