Deidentification is a cognitive identity-formation process that increases the extent to which one sibling (or both) in a sibling dyad defines his or her identity in terms of difference from other sibling. Although extremely common, not all siblings deidentify. Deidentification, as a process of difference, is in direct competition with processes that cause similarity in siblings, such as modeling and a shared environment. In most sibling relationships, all of these effects will exert influence on identity formation, some causing identification (siblings judge themselves as similar) and some causing deidentification (siblings judge themselves to be different).