Complex sentence: When informants associate a particular language variety with a particular group, the presumed social attributes of the group are transferred to the dialect itself; that dialect is then associated with those attributes even when informants cannot correctly identify the source of the dialect.
Split into simpler sentences: When informants associate a particular language variety with a particular group, the presumed social attributes of the group are transferred to the dialect itself. That dialect is then associated with those attributes even when informants cannot correctly identify the source of the dialect.