The term "sibling rivalry" was coined by David Levy in 1937 in relation to the common aggressive response of an older sibling to a new baby in the family. It is also used to describe competition or antagonism between children of the same parents.
It has various causes. Freud thought it was connected to the Oedipus complex whereby sibling brothers would compete for their mother's love, or between sisters for their father's attention.
Kyla Boyse from the University of Michigan suggests it stems from a child's need to define himself or herself as an individual and to separate from a sibling.
Alfred Adler proposes that sibling rivalry is based on siblings "striving for significance" within their family.
Most psychologists believe that it stems from an innate desire to attain parental attention achieved through competing with the sibling.
Whatever the cause, the manifestation can be ugly.