There are some facts that we do know that could lead us to better understanding the relationship between siblings and thereby uncover the key to understanding what makes rivalry such a constant. Approximately, 82% of people in Western countries have at least one sibling. Siblings generally spend more time with each other when they are children, versus the time that they spend with their parents. The relationship between siblings dictates the ferocity of their rivalry. Relationship factors include anything from parental treatment, birth order, and personality, to people and experiences outside the family unit. All of these experiences leave a unique impression on a child and can greatly affect how he or she handles conflicts with siblings.

Sibling rivalry can range in severity. Some siblings might fight constantly as children but be very close in adulthood. Some children rarely fight with their siblings while others seem to be constantly at each others thoughts. In an ideal familial situation, it can be difficult to pin point the cause of sibling rivalry. It becomes easier to theorize where feelings of aggression come from in more dysfunctional relationships or abusive relationships, keeping in mind that abuse can take on the form of being both physical and emotional.

Children with siblings that receive more attention because they poses a particular talent or seem to be favored more by a parent can be the recipients of a lot of hard feelings from jealous and hurt siblings. Experts estimate that as early as age three, children have the ability to start comparing themselves to others and begin to act accordingly in the family. Even in perfectly healthy families feelings of jealousy can arise from something as basic as bringing home a new baby. Especially in the case of an oldest child welcoming home their first sibling, it can be difficult to give up the sole attention of the parents, thus harboring ill from the new sibling's infancy.

It is in adolescence that sibling rivalry seems to be especially unpleasant as siblings are at an age where they are physically and intellectually better able to really get under each others skin. The pressures and awkwardness of one's teenage years provide more than adequate ammunition for siblings to really let loose on each other if so inclined. Add to that the increased emotional insecurities of one's teenage years and you have a recipe for some of the most potent sibling rivalry there is. It is between the ages of 10 and 15 that siblings are reported as having the highest level of competition.

While it is true that many siblings that were rivals in childhood, can grow to be friends in adulthood, there still remains a surprising amount of childhood rivals who hold on to their discontent for each other far into their adult lives. In fact, approximately one-third of adults describe their relationship with siblings as rivalries or distant. It is not until one reaches the age of sixty or over that these numbers start showing a marked decrease and siblings begin to let go of their life-long rivals.