Within a single clause, a pronoun normally refers back to an antecedent. This is called anaphora.
John shaved himself.
John took his children to the zoo.
John wondered why Mary hated him.
The reflexive 'himself' necessarily means 'John'; but the other two pronouns can refer to John or to someone else. Even when we assume they refer back to John, is it because they're interpreted as a constant 'John', or as a bound variable co-indexed with 'John'? This makes a difference in some complex sentences, which can have two different readings. The reading where 'him'/'his' is strictly identical to John is called the strict reading, and the one where it's a variable that happens to be assigned a co-referential interpretation is called the sloppy reading.
John took his children to the zoo and so did Bill.
John wondered why Mary criticized him and Bill did too.
Depending on the context, these can have either reading. Either 'so did Bill' means 'Bill also took John's children to the zoo' or it means 'Bill also took his own children to the zoo'. Mary might have been criticizing just John, or both John and Bill.

These sentences are created by ellipsis (omission) of a repeated verb phrase. Normally the ellipsed material is easy for the hearer to reconstruct, because it has to be identical to a verb phrase that's still present. But not always precisely identical: some grammatical manipulation is allowed, as in the second of the following two:

Mary went to the zoo yesterday and so did John.
Mary went to the zoo yesterday and John will tomorrow.
Just as in that you can't actually reconstruct the word 'went' but have to use the underlying form 'go' ('John will go tomorrow'), so in the case of strict and sloppy anaphors you have to consider not simply the words to be repeated but their underlying grammar. There are two possible reconstructions: one where the pronoun already has the same constant reference as its antecedent (such as John), and one where it's a variable bound to its antecedent, and not receiving an interpretation until processing of the sentence gets to Logical Form.

The pronouns 'him' and 'his' always optionally have non-bound readings referring to some other party. The anaphor 'himself' necessarily binds to its antecedent, and the only available reading is the sloppy one. Or at least, the strict reading is much degraded in acceptability.

John blamed himself and so did Bill. (= Bill blamed Bill.)
??John blamed himself and so did Bill. (= Bill blamed John.)
Just as 'went' can be reconstructed as 'go' by discarding the tense feature, so the pronouns can be stripped of their gender feature and just reconstructed as a variable, giving sloppy identity. This is, however, not as clear-cut, and I leave you to wonder how acceptable the following are in your speech under the sloppy reading. (As always, example sentences on the page might sound different if given a reasonable prior context to facilitate the interpretation.)
John took his children to the zoo and so did Mary.
John blamed himself and so did Mary.

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