Although the average person typically thinks of sibling rivalry as school-age children arguing over toys, sharing a bedroom, and who gets more of mom's and dad’s attention, the feuds and jealousy often spill into individuals’ adult lives. In fact, these rivalries are hardly innocent, as they can rip apart families once considered loving and intact.

While little evidence has been shown to prove a genetic link to sibling rivalry, individuals may begin the rivalry early on. Judy Dunn, a pioneer in sibling studies, observed siblings in their home environment and gathered information regarding sibling interactions. She noted that even at 18 months, younger siblings observed and reacted to the way their mothers interacted with older siblings. Furthermore, by age three, most children understand societal norms and can use rules for their own benefit.

Parents play a huge role in creating jealousy among siblings. Jeremy Boyle, research associate at Brigham Young University, categorizes parental influence into three categories: expectations, labels and favoritism. Parents may place expectations on their children based on their own inadequacies. Children's failure to live up to these expectations can have devastating consequences, especially when another sibling has been able to meet these expectations. Similarly, parents may label their children when comparing them to each other. Children resent being referred to as "the lazy kid," especially when their sibling is considered the “overachiever.” This can also lead to favoritism. The child referred to as "lazy" may feel as though the overachieving sibling gets more of his parents’ attention, time and money. As a result, those feelings linger and follow him into adulthood.

We typically regard siblings as our first friends and closest relatives. Typically, siblings have known each other the longest and hit many milestones in life together or around the same time. When siblings grow up and begin their adult lives, jealousy may intensify. The issues from childhood could linger, especially as the siblings choose different lifestyles and reach different accomplishments. There may be competition to see who gets married first, who buys a home first, or who gets promoted first.

Adult siblings may find it difficult to accept that their siblings have established lives of their own. Oftentimes, when a sibling marries or has a child, the other sibling feels that the bond is threatened. This is especially true if one of them does not approve of the other sibling’s partner. She may feel as though she has to vie for her sibling’s time. To her, the relationship has changed dramatically overnight.

While siblings cannot change what happened in the past, they also cannot change how their parents treated -- or continue to treat -- them. However, they can work together to overcome jealousy and rivalry. Clear communication is vital to maintaining a strong bond. Avoid mixed signals that may occur as a result of sarcasm and misinterpretation. Instead, stick to the facts. Share your views on the matter at hand, but also take the time to really listen to your sibling. If you don’t understand something your sibling said, ask for clarification, but try to remove judgment from your questioning. For example, “Why would you do that?” often comes off judgmental, but “Were you near the bank or the car wash when you lost your keys?” sounds neutral. Your relationship may not be salvaged overnight, but starting with clear communication will help.