Complex sentence: However, the social identity approach assumes that there is a hydraulic relationship between personal and social identities, such that increases in the salience and influence of one diminishes the salience and influence of the other.
Split into simpler sentences: However, the social identity approach assumes that there is a hydraulic relationship between personal and social identities. That is, the increases in the salience and influence of one identity diminishes the salience and influence of the other.