It is well known in twin households that twins fight a lot. While nontwins may perceive twin pairs as peacefully coexisting in the outside world, at home twins are constantly bickering and driving their parents crazy. Most parents will tell you that any fight is usually over quickly, and the twins resume their playful interaction until the next eruption occurs. Parents of singletons do not usually experience sibling hostilities whose score is settled so quickly.
As twins reach adolescence and young adulthood, they do not want their parents interfering in their squabbles. Their fights are personal and unique, and having an outsider involved only inflames the situation. One woman told me that if her mother tries to get in the middle of an altercation with her sister, she will be even more enraged at her mother. She and her identical twin sister will resolve this dispute on their own terms by themselves. A mom of nineteen-year-old boy/girl twins told me that her children firmly yet politely warn her to stay out of their business because they will arbitrate the matter on their own.
This course of action makes sense because twin pairs regulate and negotiate issues of fairness and competition within their unique dyadic parameters. The bigger issue is how they go about managing the discord rather than what gives rise to the actual argument. For better or worse, they settle conflicts by themselves, knowing the rules and keeping score. This war-and-peace pact acts as a built-in mechanism to keep the twinship running as effortlessly as possible.
Of course, this is not the strategy one would adopt with younger twins, who usually have many more physical altercations that need to be monitored and mitigated with parental intervention. With older twins, however, it is often easier to allow them the freedom to work things out between themselves. This approach avoids the risk of pulling in a parent who may take sides, which can create damaging emotional mishaps and divisions between the twins.