“Backward” versus “backwards” — is there any difference?

Question

The dictionaries I've looked in don't distinguish between these two words, backward and backwards (at least when used as adverbs). Is there some real historical, grammatical or regional difference between them?

Answer 1

Paul Brians offers this:

As an adverb, either word will do: “put the shirt on backward” or “put the shirt on backwards.” However, as an adjective, only “backward” will do: “a backward glance.” When in doubt, use “backward.”

This appears to be from a book "Common Errors in English Usage".

I know that "toward" is considered US and "towards" is considered UK, so this may be the same. (Incidentally I looked in four reference books and didn't find anything about "backward" and "backwards".)

Answer 2

Backwards denotes direction whereas backward means less developed or slow.