When it comes to siblings, rivalry is rooted in competition: competition for parents' time, attention and love.
In families with more than one child, it's practically unavoidable, experts say, but it's also perfectly normal.
"Sibling rivalry is a natural part of sibling relationships," said Dr. Anita Chandra-Puri, a pediatrician with Northwestern Memorial Physicians Group and a spokeswoman for the American Academy of Pediatrics in Elk Grove Village.
In young children responding to the addition of a new baby to the family, it can manifest itself as regressive behavior. For example, a toilet-trained toddler might suddenly demand a diaper or a bottle. That same toddler might insist on playing with toys he or she has outgrown, or hide toys belonging to the new baby, Chandra-Puri said.
In older children, rivalry is evidenced by siblings tattling on each other, encouraging each other to engage in prohibited or risky behavior or in physical altercations between them, said Chandra-Puri, the mother of four boys.
The good news, says Chandra-Puri is that it "tapers off as children become more confident in themselves and in their relationships with family members."
To some extent, sibling rivalry can actually benefit youngsters by teaching them to negotiate differences and deal with the fact that life isn't fair, she said. The lessons children learn and the communication skills they develop as a result translate to the classroom and eventually the boardroom.
"Learning how to deal with differences is an important skill," Chandra-Puri said, adding it's better still for children to learn those skills in a controlled environment like the home.
In short, sibling rivalry has value.
Of course, managing the strife between their children falls to parents who must recognize that sibling rivalry "is a normal part of children growing up together and finding relationships with each other," Chandra-Puri said.
Parents can help by remaining fair and impartial, refraining from comparing children and keeping the communication lines open. She also suggests giving children the verbal tools to iron out their differences and setting reasonable boundaries for negotiating play.
It's up to the parents to decide how much they will tolerate, says the doctor, who suggests parents make sure strife and squabbling never escalates to physical or violent altercations.
"There will be verbal arguments," said Chandra-Puri. "There will be tears and some sad times as well as happy times. This is what makes their relationship ... It's the glue that holds siblings together and is an important part of their bonding."