GERNSBACHER, M.A. (1997). Generic pronominal anaphora: The case of the English singular they. Verbum, 19, 67-84.
A conventional feature of all languages is that they provide devices for referring to concepts that were previously introduced in a text or discourse. Virtually all languages have devices called pronouns. For example, after we have introduced the man with the black hat, we can refer to that man as he. Or after we have introduced the woman with the red dress, we can refer to that woman as she. Certain rules apply to when and how we may use pronouns, or if we do not like the concept of rules, we can call these restrictions, “constraints.” Presumably, these constraints are used by native comprehenders to understand pronominal anaphora.